[PATCH] Improve amcheck to also check UNIQUE constraint in btree index.

From abaa141811273ebcc511218ec3f40713717a282c Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Mon, 8 Feb 2021 12:26:08 +0400
Subject: [PATCH v2] Make amcheck checking UNIQUE constraint for btree index.
 On index with unique constraint ake check that only one table entry for the
 equal keys (including all posting list entries) is visible. Report error if
 not and show all index entries violating the constraint under warning level.

Authors: Anastasia Lubennikova <a.lubennikova@postgrespro.ru>, Pavel Borisov <pashkin.elfe@gmail.com>
---
 contrib/amcheck/expected/check_btree.out | 138 +++++++++++
 contrib/amcheck/sql/check_btree.sql      |  24 ++
 contrib/amcheck/verify_nbtree.c          | 278 ++++++++++++++++++++++-
 3 files changed, 436 insertions(+), 4 deletions(-)

diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 13848b7449b..875ac1355ae 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -177,11 +177,149 @@ SELECT bt_index_check('toasty', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+CREATE TABLE bttest_unique(a varchar(50), b varchar(1500), c bytea, d varchar(50));
+CREATE UNIQUE INDEX bttest_unique_idx ON bttest_unique(a,b);
+UPDATE pg_catalog.pg_index SET indisunique = false
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+INSERT INTO bttest_unique
+	SELECT 	i::text::varchar,
+			array_to_string(array(
+				SELECT substr('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', ((random()*(36-1)+1)::integer), 1)
+			FROM generate_series(1,1300)),'')::varchar,
+	i::text::bytea, i::text::varchar
+	FROM generate_series(0,1) AS i, generate_series(0,30) AS x;
+UPDATE pg_catalog.pg_index SET indisunique = true
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+DELETE FROM bttest_unique WHERE ctid::text='(0,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,3)';
+DELETE FROM bttest_unique WHERE ctid::text='(9,3)';
+SELECT bt_index_check('bttest_unique_idx', true);
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 0 and posting 2 (point to heap tid=(0,1) and tid=(0,3)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 2 and posting 3 (point to heap tid=(0,3) and tid=(0,4)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 3 and posting 4 (point to heap tid=(0,4) and tid=(0,5)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 4 and tid=(1,3) posting 0 (point to heap tid=(0,5) and tid=(0,6)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 0 and posting 1 (point to heap tid=(0,6) and tid=(1,1)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 1 and posting 2 (point to heap tid=(1,1) and tid=(1,2)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 2 and posting 3 (point to heap tid=(1,2) and tid=(1,3)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 3 and posting 4 (point to heap tid=(1,3) and tid=(1,4)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 4 and tid=(1,4) posting 0 (point to heap tid=(1,4) and tid=(1,5)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 0 and posting 1 (point to heap tid=(1,5) and tid=(1,6)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 1 and posting 2 (point to heap tid=(1,6) and tid=(2,1)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 2 and posting 3 (point to heap tid=(2,1) and tid=(2,2)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 3 and posting 4 (point to heap tid=(2,2) and tid=(2,3)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 4 and tid=(1,5) posting 0 (point to heap tid=(2,3) and tid=(2,4)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 0 and posting 1 (point to heap tid=(2,4) and tid=(2,5)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 1 and posting 2 (point to heap tid=(2,5) and tid=(2,6)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 2 and posting 3 (point to heap tid=(2,6) and tid=(3,1)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 3 and posting 4 (point to heap tid=(3,1) and tid=(3,2)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 4 and tid=(1,6) posting 0 (point to heap tid=(3,2) and tid=(3,3)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 0 and posting 1 (point to heap tid=(3,3) and tid=(3,4)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 1 and posting 2 (point to heap tid=(3,4) and tid=(3,5)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 2 and posting 3 (point to heap tid=(3,5) and tid=(3,6)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 3 and posting 4 (point to heap tid=(3,6) and tid=(4,1)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 4 and tid=(2,2) posting 2 (point to heap tid=(4,1) and tid=(4,4)) page lsn=0/4DAD7E0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 2 and posting 3 (point to heap tid=(4,4) and tid=(4,5)) page lsn=0/4DBD070.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 3 and posting 4 (point to heap tid=(4,5) and tid=(4,6)) page lsn=0/4DBD070.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 4 and tid=(2,3) (point to heap tid=(4,6) and tid=(5,1)) page lsn=0/4DBD070.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 0 and posting 1 (point to heap tid=(5,2) and tid=(5,3)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 1 and posting 2 (point to heap tid=(5,3) and tid=(5,4)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 2 and posting 3 (point to heap tid=(5,4) and tid=(5,5)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 3 and posting 4 (point to heap tid=(5,5) and tid=(5,6)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 4 and tid=(4,3) posting 0 (point to heap tid=(5,6) and tid=(6,1)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 0 and posting 1 (point to heap tid=(6,1) and tid=(6,2)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 1 and posting 2 (point to heap tid=(6,2) and tid=(6,3)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 2 and posting 3 (point to heap tid=(6,3) and tid=(6,4)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 3 and posting 4 (point to heap tid=(6,4) and tid=(6,5)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 4 and tid=(4,4) posting 0 (point to heap tid=(6,5) and tid=(6,6)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 0 and posting 1 (point to heap tid=(6,6) and tid=(7,1)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 1 and posting 2 (point to heap tid=(7,1) and tid=(7,2)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 2 and posting 3 (point to heap tid=(7,2) and tid=(7,3)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 3 and posting 4 (point to heap tid=(7,3) and tid=(7,4)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 4 and tid=(4,5) posting 0 (point to heap tid=(7,4) and tid=(7,5)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 0 and posting 1 (point to heap tid=(7,5) and tid=(7,6)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 1 and posting 2 (point to heap tid=(7,6) and tid=(8,1)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 2 and posting 3 (point to heap tid=(8,1) and tid=(8,2)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 3 and posting 4 (point to heap tid=(8,2) and tid=(8,3)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 4 and tid=(4,6) posting 0 (point to heap tid=(8,3) and tid=(8,4)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 0 and posting 1 (point to heap tid=(8,4) and tid=(8,5)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 1 and posting 2 (point to heap tid=(8,5) and tid=(8,6)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 2 and posting 3 (point to heap tid=(8,6) and tid=(9,1)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 3 and posting 4 (point to heap tid=(9,1) and tid=(9,2)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness may be violated for index "bttest_unique_idx": Index tid=(5,1) doesn't have visible heap tids and key is equal to the tid=(4,6) posting 4 (points to heap tid=(9,2)). Cross-page unique constraint violation can be missed. Vacuum the table and repeat the check.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,2) and tid=(5,3) (point to heap tid=(9,4) and tid=(9,5)) page lsn=0/4DC77A8.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,3) and tid=(5,4) (point to heap tid=(9,5) and tid=(9,6)) page lsn=0/4DC77A8.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,4) and tid=(5,5) (point to heap tid=(9,6) and tid=(10,1)) page lsn=0/4DC77A8.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,5) and tid=(5,6) (point to heap tid=(10,1) and tid=(10,2)) page lsn=0/4DC77A8.
+ERROR:  index "bttest_unique_idx" is corrupted. There are tuples violating UNIQUE constraint
+DETAIL:  Details are in the previous log messages under WARNING priority
+VACUUM bttest_unique;
+SELECT bt_index_check('bttest_unique_idx', true);
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 0 and posting 1 (point to heap tid=(0,1) and tid=(0,3)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 1 and posting 2 (point to heap tid=(0,3) and tid=(0,4)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 2 and posting 3 (point to heap tid=(0,4) and tid=(0,5)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 3 and tid=(1,3) posting 0 (point to heap tid=(0,5) and tid=(0,6)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 0 and posting 1 (point to heap tid=(0,6) and tid=(1,1)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 1 and posting 2 (point to heap tid=(1,1) and tid=(1,2)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 2 and posting 3 (point to heap tid=(1,2) and tid=(1,3)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 3 and posting 4 (point to heap tid=(1,3) and tid=(1,4)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 4 and tid=(1,4) posting 0 (point to heap tid=(1,4) and tid=(1,5)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 0 and posting 1 (point to heap tid=(1,5) and tid=(1,6)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 1 and posting 2 (point to heap tid=(1,6) and tid=(2,1)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 2 and posting 3 (point to heap tid=(2,1) and tid=(2,2)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 3 and posting 4 (point to heap tid=(2,2) and tid=(2,3)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 4 and tid=(1,5) posting 0 (point to heap tid=(2,3) and tid=(2,4)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 0 and posting 1 (point to heap tid=(2,4) and tid=(2,5)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 1 and posting 2 (point to heap tid=(2,5) and tid=(2,6)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 2 and posting 3 (point to heap tid=(2,6) and tid=(3,1)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 3 and posting 4 (point to heap tid=(3,1) and tid=(3,2)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 4 and tid=(1,6) posting 0 (point to heap tid=(3,2) and tid=(3,3)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 0 and posting 1 (point to heap tid=(3,3) and tid=(3,4)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 1 and posting 2 (point to heap tid=(3,4) and tid=(3,5)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 2 and posting 3 (point to heap tid=(3,5) and tid=(3,6)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 3 and posting 4 (point to heap tid=(3,6) and tid=(4,1)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 4 and tid=(2,2) posting 0 (point to heap tid=(4,1) and tid=(4,4)) page lsn=0/4DC9D58.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 0 and posting 1 (point to heap tid=(4,4) and tid=(4,5)) page lsn=0/4DC9D98.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 1 and posting 2 (point to heap tid=(4,5) and tid=(4,6)) page lsn=0/4DC9D98.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 2 and tid=(2,3) (point to heap tid=(4,6) and tid=(5,1)) page lsn=0/4DC9D98.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 0 and posting 1 (point to heap tid=(5,2) and tid=(5,3)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 1 and posting 2 (point to heap tid=(5,3) and tid=(5,4)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 2 and posting 3 (point to heap tid=(5,4) and tid=(5,5)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 3 and posting 4 (point to heap tid=(5,5) and tid=(5,6)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 4 and tid=(4,3) posting 0 (point to heap tid=(5,6) and tid=(6,1)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 0 and posting 1 (point to heap tid=(6,1) and tid=(6,2)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 1 and posting 2 (point to heap tid=(6,2) and tid=(6,3)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 2 and posting 3 (point to heap tid=(6,3) and tid=(6,4)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 3 and posting 4 (point to heap tid=(6,4) and tid=(6,5)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 4 and tid=(4,4) posting 0 (point to heap tid=(6,5) and tid=(6,6)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 0 and posting 1 (point to heap tid=(6,6) and tid=(7,1)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 1 and posting 2 (point to heap tid=(7,1) and tid=(7,2)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 2 and posting 3 (point to heap tid=(7,2) and tid=(7,3)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 3 and posting 4 (point to heap tid=(7,3) and tid=(7,4)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 4 and tid=(4,5) posting 0 (point to heap tid=(7,4) and tid=(7,5)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 0 and posting 1 (point to heap tid=(7,5) and tid=(7,6)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 1 and posting 2 (point to heap tid=(7,6) and tid=(8,1)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 2 and posting 3 (point to heap tid=(8,1) and tid=(8,2)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 3 and posting 4 (point to heap tid=(8,2) and tid=(8,3)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 4 and tid=(4,6) posting 0 (point to heap tid=(8,3) and tid=(8,4)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 0 and posting 1 (point to heap tid=(8,4) and tid=(8,5)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 1 and posting 2 (point to heap tid=(8,5) and tid=(8,6)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 2 and posting 3 (point to heap tid=(8,6) and tid=(9,1)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 3 and posting 4 (point to heap tid=(9,1) and tid=(9,2)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 4 and tid=(5,1) (point to heap tid=(9,2) and tid=(9,4)) page lsn=0/4DC4CD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,1) and tid=(5,2) (point to heap tid=(9,4) and tid=(9,5)) page lsn=0/4DC9DD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,2) and tid=(5,3) (point to heap tid=(9,5) and tid=(9,6)) page lsn=0/4DC9DD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,3) and tid=(5,4) (point to heap tid=(9,6) and tid=(10,1)) page lsn=0/4DC9DD0.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,4) and tid=(5,5) (point to heap tid=(10,1) and tid=(10,2)) page lsn=0/4DC9DD0.
+ERROR:  index "bttest_unique_idx" is corrupted. There are tuples violating UNIQUE constraint
+DETAIL:  Details are in the previous log messages under WARNING priority
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 97a3e1a20d5..8df296431fb 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -115,11 +115,35 @@ INSERT INTO toast_bug SELECT repeat('a', 2200);
 -- Should not get false positive report of corruption:
 SELECT bt_index_check('toasty', true);
 
+-- UNIQUE constraint check
+CREATE TABLE bttest_unique(a varchar(50), b varchar(1500), c bytea, d varchar(50));
+CREATE UNIQUE INDEX bttest_unique_idx ON bttest_unique(a,b);
+UPDATE pg_catalog.pg_index SET indisunique = false
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+INSERT INTO bttest_unique
+	SELECT 	i::text::varchar,
+			array_to_string(array(
+				SELECT substr('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', ((random()*(36-1)+1)::integer), 1)
+			FROM generate_series(1,1300)),'')::varchar,
+	i::text::bytea, i::text::varchar
+	FROM generate_series(0,1) AS i, generate_series(0,30) AS x;
+UPDATE pg_catalog.pg_index SET indisunique = true
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+
+DELETE FROM bttest_unique WHERE ctid::text='(0,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,3)';
+DELETE FROM bttest_unique WHERE ctid::text='(9,3)';
+SELECT bt_index_check('bttest_unique_idx', true);
+VACUUM bttest_unique;
+SELECT bt_index_check('bttest_unique_idx', true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index b8c7793d9e0..4519b1e0400 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -83,6 +83,13 @@ typedef struct BtreeCheckState
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking.
+	 * Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -148,8 +155,21 @@ static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -187,6 +207,7 @@ static ItemId PageGetItemIdCareful(BtreeCheckState *state, BlockNumber block,
 static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 													  IndexTuple itup, bool nonpivot);
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
+static bool errflag; /* Output ERROR at the end of amcheck */
 
 /*
  * bt_index_check(index regclass, heapallindexed boolean)
@@ -449,6 +470,15 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->indexinfo = BuildIndexInfo(state->rel);
+	/*
+	 * We need a snapshot it to check uniqueness of the index
+	 * For better performance, take it once per index check.
+	 */
+	if (state->indexinfo->ii_Unique)
+		state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+	else
+		state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -632,7 +662,16 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
+
+	if (errflag == true)
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				errmsg("index \"%s\" is corrupted. There are tuples violating UNIQUE constraint",
+						RelationGetRelationName(state->rel)),
+				errdetail_internal("Details are in the previous log messages under WARNING priority")));
 }
 
 /*
@@ -1006,6 +1045,152 @@ bt_recheck_sibling_links(BtreeCheckState *state,
 								btpo_prev_from_target)));
 }
 
+/* Check visibility of the table entry referenced from nbtree index */
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+							  tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print error message for unique constrain violation in the btree
+ * index under WARNING level and set flag to report ERROR at the end of check
+ */
+static void bt_report_duplicate(BtreeCheckState *state,
+				 ItemPointer tid, BlockNumber block, OffsetNumber offset,
+				 int posting,
+				 ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+				 int nposting)
+{
+	char	   	*htid,
+				*nhtid,
+				*itid,
+				*nitid = "",
+				*pposting = "",
+				*pnposting = "";
+
+	errflag = true;
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(nexttid),
+					ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+		ereport(WARNING,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) "
+					"page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					(uint32) (state->targetlsn >> 32),
+					(uint32) state->targetlsn)));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+		BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+		OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool has_visible_entry = false;
+
+	/*
+	 * Current tuple has posting list. If TID of any posting list entry is
+	 * visible, and lVis_tid is already valid report duplicate.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid (*lVis_tid))
+				{
+					bt_report_duplicate(state,
+											*lVis_tid, *lVis_block,
+											*lVis_offset, *lVis_i,
+											tid, targetblock,
+											offset, i);
+				}
+				/*
+				 * Prevent double reporting unique violation between the posting
+				 * list entries of a first tuple on the page after cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid (*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list.
+	 * If TID is visible, save info about it for next comparisons in the loop in
+	 * bt_page_check(). If also lVis_tid is already valid, report duplicate.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid (*lVis_tid))
+			{
+				bt_report_duplicate(state,
+											*lVis_tid, *lVis_block,
+											*lVis_offset, *lVis_i,
+											tid, targetblock,
+											offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+									   *lVis_block != targetblock)
+		ereport(WARNING,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			errmsg("index uniqueness may be violated for index \"%s\": "
+					"Index tid=(%u,%u) doesn't have visible heap tids and key "
+					"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+					"Cross-page unique constraint violation can be missed. "
+					"Vacuum the table and repeat the check.",
+					RelationGetRelationName(state->rel),
+					targetblock, offset,
+					*lVis_block, *lVis_offset, psprintf(" posting %u", *lVis_i),
+					ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+					ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+}
+
 /*
  * Function performs the following checks on target page, or pages ancillary to
  * target page:
@@ -1026,6 +1211,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint check that only one of table entries for
+ *   equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1047,6 +1235,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber offset;
 	OffsetNumber max;
 	BTPageOpaque topaque;
+	/* last visible entry info for checking indexes with unique constraint */
+	int			 lVis_i = -1; /* the position of last visible item for posting
+							   * tuple. for non-posting tuple (-1)
+							   */
+	ItemPointer	 lVis_tid = NULL;
+	BlockNumber	 lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
 
 	topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
@@ -1446,6 +1641,39 @@ bt_target_page_check(BtreeCheckState *state)
 										(uint32) state->targetlsn)));
 		}
 
+		/*
+		 * If the index is unique, verify entries uniqueness by checking
+		 * heap tuples visibility.
+		 */
+		if (state->indexinfo->ii_Unique && P_ISLEAF(topaque))
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+					&lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+				 OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+			/* Invalidate scankey tid to make _bt_compare compare only keys
+			 * in the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple).
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+						OffsetNumberNext(offset)) != 0)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid; /* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1463,12 +1691,14 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1503,6 +1733,43 @@ bt_target_page_check(BtreeCheckState *state)
 											(uint32) (state->targetlsn >> 32),
 											(uint32) state->targetlsn)));
 			}
+
+			/*
+			 * If index has unique constraint check that not more than one found
+			 * equal items is visible.
+			 */
+			if (state->indexinfo->ii_Unique && rightkey && P_ISLEAF(topaque))
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* First key on next page is same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  state->targetblock + 1);
+					topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+							break;
+
+					itemid = PageGetItemIdCareful(state, state->targetblock + 1,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, state->targetblock + 1, rightfirstoffset,
+									&lVis_i, &lVis_tid, &lVis_offset,
+									&lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1548,9 +1815,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1713,6 +1982,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
-- 
2.28.0

From 1dcf0206cabcaf967de28bc95b28f8587d36ffaa Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Mon, 8 Feb 2021 12:26:08 +0400
Subject: [PATCH v3] Make amcheck checking UNIQUE constraint for btree index.
 On index with unique constraint ake check that only one table entry for the
 equal keys (including all posting list entries) is visible. Report error if
 not and show all index entries violating the constraint under warning level.

Authors: Anastasia Lubennikova <a.lubennikova@postgrespro.ru>, Pavel Borisov <pashkin.elfe@gmail.com>
---
 contrib/amcheck/expected/check_btree.out | 138 ++++++++++++
 contrib/amcheck/sql/check_btree.sql      |  24 ++
 contrib/amcheck/verify_nbtree.c          | 275 ++++++++++++++++++++++-
 3 files changed, 433 insertions(+), 4 deletions(-)

diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 13848b7449b..fe4a958b1b0 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -177,11 +177,149 @@ SELECT bt_index_check('toasty', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+CREATE TABLE bttest_unique(a varchar(50), b varchar(1500), c bytea, d varchar(50));
+CREATE UNIQUE INDEX bttest_unique_idx ON bttest_unique(a,b);
+UPDATE pg_catalog.pg_index SET indisunique = false
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+INSERT INTO bttest_unique
+	SELECT 	i::text::varchar,
+			array_to_string(array(
+				SELECT substr('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', ((random()*(36-1)+1)::integer), 1)
+			FROM generate_series(1,1300)),'')::varchar,
+	i::text::bytea, i::text::varchar
+	FROM generate_series(0,1) AS i, generate_series(0,30) AS x;
+UPDATE pg_catalog.pg_index SET indisunique = true
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+DELETE FROM bttest_unique WHERE ctid::text='(0,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,3)';
+DELETE FROM bttest_unique WHERE ctid::text='(9,3)';
+SELECT bt_index_check('bttest_unique_idx', true);
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 0 and posting 2 (point to heap tid=(0,1) and tid=(0,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 2 and posting 3 (point to heap tid=(0,3) and tid=(0,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 3 and posting 4 (point to heap tid=(0,4) and tid=(0,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 4 and tid=(1,3) posting 0 (point to heap tid=(0,5) and tid=(0,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 0 and posting 1 (point to heap tid=(0,6) and tid=(1,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 1 and posting 2 (point to heap tid=(1,1) and tid=(1,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 2 and posting 3 (point to heap tid=(1,2) and tid=(1,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 3 and posting 4 (point to heap tid=(1,3) and tid=(1,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 4 and tid=(1,4) posting 0 (point to heap tid=(1,4) and tid=(1,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 0 and posting 1 (point to heap tid=(1,5) and tid=(1,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 1 and posting 2 (point to heap tid=(1,6) and tid=(2,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 2 and posting 3 (point to heap tid=(2,1) and tid=(2,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 3 and posting 4 (point to heap tid=(2,2) and tid=(2,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 4 and tid=(1,5) posting 0 (point to heap tid=(2,3) and tid=(2,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 0 and posting 1 (point to heap tid=(2,4) and tid=(2,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 1 and posting 2 (point to heap tid=(2,5) and tid=(2,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 2 and posting 3 (point to heap tid=(2,6) and tid=(3,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 3 and posting 4 (point to heap tid=(3,1) and tid=(3,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 4 and tid=(1,6) posting 0 (point to heap tid=(3,2) and tid=(3,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 0 and posting 1 (point to heap tid=(3,3) and tid=(3,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 1 and posting 2 (point to heap tid=(3,4) and tid=(3,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 2 and posting 3 (point to heap tid=(3,5) and tid=(3,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 3 and posting 4 (point to heap tid=(3,6) and tid=(4,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 4 and tid=(2,2) posting 2 (point to heap tid=(4,1) and tid=(4,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 2 and posting 3 (point to heap tid=(4,4) and tid=(4,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 3 and posting 4 (point to heap tid=(4,5) and tid=(4,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 4 and tid=(2,3) (point to heap tid=(4,6) and tid=(5,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 0 and posting 1 (point to heap tid=(5,2) and tid=(5,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 1 and posting 2 (point to heap tid=(5,3) and tid=(5,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 2 and posting 3 (point to heap tid=(5,4) and tid=(5,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 3 and posting 4 (point to heap tid=(5,5) and tid=(5,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 4 and tid=(4,3) posting 0 (point to heap tid=(5,6) and tid=(6,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 0 and posting 1 (point to heap tid=(6,1) and tid=(6,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 1 and posting 2 (point to heap tid=(6,2) and tid=(6,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 2 and posting 3 (point to heap tid=(6,3) and tid=(6,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 3 and posting 4 (point to heap tid=(6,4) and tid=(6,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 4 and tid=(4,4) posting 0 (point to heap tid=(6,5) and tid=(6,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 0 and posting 1 (point to heap tid=(6,6) and tid=(7,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 1 and posting 2 (point to heap tid=(7,1) and tid=(7,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 2 and posting 3 (point to heap tid=(7,2) and tid=(7,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 3 and posting 4 (point to heap tid=(7,3) and tid=(7,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 4 and tid=(4,5) posting 0 (point to heap tid=(7,4) and tid=(7,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 0 and posting 1 (point to heap tid=(7,5) and tid=(7,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 1 and posting 2 (point to heap tid=(7,6) and tid=(8,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 2 and posting 3 (point to heap tid=(8,1) and tid=(8,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 3 and posting 4 (point to heap tid=(8,2) and tid=(8,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 4 and tid=(4,6) posting 0 (point to heap tid=(8,3) and tid=(8,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 0 and posting 1 (point to heap tid=(8,4) and tid=(8,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 1 and posting 2 (point to heap tid=(8,5) and tid=(8,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 2 and posting 3 (point to heap tid=(8,6) and tid=(9,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 3 and posting 4 (point to heap tid=(9,1) and tid=(9,2)).
+WARNING:  index uniqueness may be violated for index "bttest_unique_idx": Index tid=(5,1) doesn't have visible heap tids and key is equal to the tid=(4,6) posting 4 (points to heap tid=(9,2)). Cross-page unique constraint violation can be missed. Vacuum the table and repeat the check.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,2) and tid=(5,3) (point to heap tid=(9,4) and tid=(9,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,3) and tid=(5,4) (point to heap tid=(9,5) and tid=(9,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,4) and tid=(5,5) (point to heap tid=(9,6) and tid=(10,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,5) and tid=(5,6) (point to heap tid=(10,1) and tid=(10,2)).
+ERROR:  index "bttest_unique_idx" is corrupted. There are tuples violating UNIQUE constraint
+DETAIL:  Details are in the previous log messages under WARNING priority
+VACUUM bttest_unique;
+SELECT bt_index_check('bttest_unique_idx', true);
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 0 and posting 1 (point to heap tid=(0,1) and tid=(0,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 1 and posting 2 (point to heap tid=(0,3) and tid=(0,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 2 and posting 3 (point to heap tid=(0,4) and tid=(0,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 3 and tid=(1,3) posting 0 (point to heap tid=(0,5) and tid=(0,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 0 and posting 1 (point to heap tid=(0,6) and tid=(1,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 1 and posting 2 (point to heap tid=(1,1) and tid=(1,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 2 and posting 3 (point to heap tid=(1,2) and tid=(1,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 3 and posting 4 (point to heap tid=(1,3) and tid=(1,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 4 and tid=(1,4) posting 0 (point to heap tid=(1,4) and tid=(1,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 0 and posting 1 (point to heap tid=(1,5) and tid=(1,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 1 and posting 2 (point to heap tid=(1,6) and tid=(2,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 2 and posting 3 (point to heap tid=(2,1) and tid=(2,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 3 and posting 4 (point to heap tid=(2,2) and tid=(2,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 4 and tid=(1,5) posting 0 (point to heap tid=(2,3) and tid=(2,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 0 and posting 1 (point to heap tid=(2,4) and tid=(2,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 1 and posting 2 (point to heap tid=(2,5) and tid=(2,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 2 and posting 3 (point to heap tid=(2,6) and tid=(3,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 3 and posting 4 (point to heap tid=(3,1) and tid=(3,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 4 and tid=(1,6) posting 0 (point to heap tid=(3,2) and tid=(3,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 0 and posting 1 (point to heap tid=(3,3) and tid=(3,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 1 and posting 2 (point to heap tid=(3,4) and tid=(3,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 2 and posting 3 (point to heap tid=(3,5) and tid=(3,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 3 and posting 4 (point to heap tid=(3,6) and tid=(4,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 4 and tid=(2,2) posting 0 (point to heap tid=(4,1) and tid=(4,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 0 and posting 1 (point to heap tid=(4,4) and tid=(4,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 1 and posting 2 (point to heap tid=(4,5) and tid=(4,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 2 and tid=(2,3) (point to heap tid=(4,6) and tid=(5,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 0 and posting 1 (point to heap tid=(5,2) and tid=(5,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 1 and posting 2 (point to heap tid=(5,3) and tid=(5,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 2 and posting 3 (point to heap tid=(5,4) and tid=(5,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 3 and posting 4 (point to heap tid=(5,5) and tid=(5,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 4 and tid=(4,3) posting 0 (point to heap tid=(5,6) and tid=(6,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 0 and posting 1 (point to heap tid=(6,1) and tid=(6,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 1 and posting 2 (point to heap tid=(6,2) and tid=(6,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 2 and posting 3 (point to heap tid=(6,3) and tid=(6,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 3 and posting 4 (point to heap tid=(6,4) and tid=(6,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 4 and tid=(4,4) posting 0 (point to heap tid=(6,5) and tid=(6,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 0 and posting 1 (point to heap tid=(6,6) and tid=(7,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 1 and posting 2 (point to heap tid=(7,1) and tid=(7,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 2 and posting 3 (point to heap tid=(7,2) and tid=(7,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 3 and posting 4 (point to heap tid=(7,3) and tid=(7,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 4 and tid=(4,5) posting 0 (point to heap tid=(7,4) and tid=(7,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 0 and posting 1 (point to heap tid=(7,5) and tid=(7,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 1 and posting 2 (point to heap tid=(7,6) and tid=(8,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 2 and posting 3 (point to heap tid=(8,1) and tid=(8,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 3 and posting 4 (point to heap tid=(8,2) and tid=(8,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 4 and tid=(4,6) posting 0 (point to heap tid=(8,3) and tid=(8,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 0 and posting 1 (point to heap tid=(8,4) and tid=(8,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 1 and posting 2 (point to heap tid=(8,5) and tid=(8,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 2 and posting 3 (point to heap tid=(8,6) and tid=(9,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 3 and posting 4 (point to heap tid=(9,1) and tid=(9,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 4 and tid=(5,1) (point to heap tid=(9,2) and tid=(9,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,1) and tid=(5,2) (point to heap tid=(9,4) and tid=(9,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,2) and tid=(5,3) (point to heap tid=(9,5) and tid=(9,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,3) and tid=(5,4) (point to heap tid=(9,6) and tid=(10,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,4) and tid=(5,5) (point to heap tid=(10,1) and tid=(10,2)).
+ERROR:  index "bttest_unique_idx" is corrupted. There are tuples violating UNIQUE constraint
+DETAIL:  Details are in the previous log messages under WARNING priority
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 97a3e1a20d5..8df296431fb 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -115,11 +115,35 @@ INSERT INTO toast_bug SELECT repeat('a', 2200);
 -- Should not get false positive report of corruption:
 SELECT bt_index_check('toasty', true);
 
+-- UNIQUE constraint check
+CREATE TABLE bttest_unique(a varchar(50), b varchar(1500), c bytea, d varchar(50));
+CREATE UNIQUE INDEX bttest_unique_idx ON bttest_unique(a,b);
+UPDATE pg_catalog.pg_index SET indisunique = false
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+INSERT INTO bttest_unique
+	SELECT 	i::text::varchar,
+			array_to_string(array(
+				SELECT substr('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', ((random()*(36-1)+1)::integer), 1)
+			FROM generate_series(1,1300)),'')::varchar,
+	i::text::bytea, i::text::varchar
+	FROM generate_series(0,1) AS i, generate_series(0,30) AS x;
+UPDATE pg_catalog.pg_index SET indisunique = true
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+
+DELETE FROM bttest_unique WHERE ctid::text='(0,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,3)';
+DELETE FROM bttest_unique WHERE ctid::text='(9,3)';
+SELECT bt_index_check('bttest_unique_idx', true);
+VACUUM bttest_unique;
+SELECT bt_index_check('bttest_unique_idx', true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index b8c7793d9e0..6136cf1a22b 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -83,6 +83,13 @@ typedef struct BtreeCheckState
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking.
+	 * Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -148,8 +155,21 @@ static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -187,6 +207,7 @@ static ItemId PageGetItemIdCareful(BtreeCheckState *state, BlockNumber block,
 static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 													  IndexTuple itup, bool nonpivot);
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
+static bool errflag; /* Output ERROR at the end of amcheck */
 
 /*
  * bt_index_check(index regclass, heapallindexed boolean)
@@ -449,6 +470,15 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->indexinfo = BuildIndexInfo(state->rel);
+	/*
+	 * We need a snapshot it to check uniqueness of the index
+	 * For better performance, take it once per index check.
+	 */
+	if (state->indexinfo->ii_Unique)
+		state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+	else
+		state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -632,7 +662,16 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
+
+	if (errflag == true)
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				errmsg("index \"%s\" is corrupted. There are tuples violating UNIQUE constraint",
+						RelationGetRelationName(state->rel)),
+				errdetail_internal("Details are in the previous log messages under WARNING priority")));
 }
 
 /*
@@ -1006,6 +1045,149 @@ bt_recheck_sibling_links(BtreeCheckState *state,
 								btpo_prev_from_target)));
 }
 
+/* Check visibility of the table entry referenced from nbtree index */
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+							  tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print error message for unique constrain violation in the btree
+ * index under WARNING level and set flag to report ERROR at the end of check
+ */
+static void bt_report_duplicate(BtreeCheckState *state,
+				 ItemPointer tid, BlockNumber block, OffsetNumber offset,
+				 int posting,
+				 ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+				 int nposting)
+{
+	char	   	*htid,
+				*nhtid,
+				*itid,
+				*nitid = "",
+				*pposting = "",
+				*pnposting = "";
+
+	errflag = true;
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(nexttid),
+					ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+		ereport(WARNING,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s).",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid)));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+		BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+		OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool has_visible_entry = false;
+
+	/*
+	 * Current tuple has posting list. If TID of any posting list entry is
+	 * visible, and lVis_tid is already valid report duplicate.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid (*lVis_tid))
+				{
+					bt_report_duplicate(state,
+											*lVis_tid, *lVis_block,
+											*lVis_offset, *lVis_i,
+											tid, targetblock,
+											offset, i);
+				}
+				/*
+				 * Prevent double reporting unique violation between the posting
+				 * list entries of a first tuple on the page after cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid (*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list.
+	 * If TID is visible, save info about it for next comparisons in the loop in
+	 * bt_page_check(). If also lVis_tid is already valid, report duplicate.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid (*lVis_tid))
+			{
+				bt_report_duplicate(state,
+											*lVis_tid, *lVis_block,
+											*lVis_offset, *lVis_i,
+											tid, targetblock,
+											offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+									   *lVis_block != targetblock)
+		ereport(WARNING,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			errmsg("index uniqueness may be violated for index \"%s\": "
+					"Index tid=(%u,%u) doesn't have visible heap tids and key "
+					"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+					"Cross-page unique constraint violation can be missed. "
+					"Vacuum the table and repeat the check.",
+					RelationGetRelationName(state->rel),
+					targetblock, offset,
+					*lVis_block, *lVis_offset, psprintf(" posting %u", *lVis_i),
+					ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+					ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+}
+
 /*
  * Function performs the following checks on target page, or pages ancillary to
  * target page:
@@ -1026,6 +1208,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint check that only one of table entries for
+ *   equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1047,6 +1232,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber offset;
 	OffsetNumber max;
 	BTPageOpaque topaque;
+	/* last visible entry info for checking indexes with unique constraint */
+	int			 lVis_i = -1; /* the position of last visible item for posting
+							   * tuple. for non-posting tuple (-1)
+							   */
+	ItemPointer	 lVis_tid = NULL;
+	BlockNumber	 lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
 
 	topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
@@ -1446,6 +1638,39 @@ bt_target_page_check(BtreeCheckState *state)
 										(uint32) state->targetlsn)));
 		}
 
+		/*
+		 * If the index is unique, verify entries uniqueness by checking
+		 * heap tuples visibility.
+		 */
+		if (state->indexinfo->ii_Unique && P_ISLEAF(topaque))
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+					&lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+				 OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+			/* Invalidate scankey tid to make _bt_compare compare only keys
+			 * in the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple).
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+						OffsetNumberNext(offset)) != 0)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid; /* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1463,12 +1688,14 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1503,6 +1730,43 @@ bt_target_page_check(BtreeCheckState *state)
 											(uint32) (state->targetlsn >> 32),
 											(uint32) state->targetlsn)));
 			}
+
+			/*
+			 * If index has unique constraint check that not more than one found
+			 * equal items is visible.
+			 */
+			if (state->indexinfo->ii_Unique && rightkey && P_ISLEAF(topaque))
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* First key on next page is same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  state->targetblock + 1);
+					topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+							break;
+
+					itemid = PageGetItemIdCareful(state, state->targetblock + 1,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, state->targetblock + 1, rightfirstoffset,
+									&lVis_i, &lVis_tid, &lVis_offset,
+									&lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1548,9 +1812,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1713,6 +1979,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
-- 
2.28.0

From 34ed10d11a479cbab8afed224f1718de099d0f35 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Mon, 8 Feb 2021 12:26:08 +0400
Subject: [PATCH v4] Make amcheck checking UNIQUE constraint for btree index.
 On index with unique constraint ake check that only one table entry for the
 equal keys (including all posting list entries) is visible. Report error if
 not and show all index entries violating the constraint under warning level.

Authors: Anastasia Lubennikova <a.lubennikova@postgrespro.ru>, Pavel Borisov <pashkin.elfe@gmail.com>
---
 contrib/amcheck/Makefile                   |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql      |  28 ++
 contrib/amcheck/amcheck.control            |   2 +-
 contrib/amcheck/expected/check_btree.out   | 146 +++++++++
 contrib/amcheck/expected/check_btree_1.out | 333 +++++++++++++++++++++
 contrib/amcheck/sql/check_btree.sql        |  27 ++
 contrib/amcheck/verify_nbtree.c            | 311 ++++++++++++++++++-
 7 files changed, 832 insertions(+), 17 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/expected/check_btree_1.out

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50b..88271687a3e 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 00000000000..f76e6941ffe
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,28 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded version of the 1.3 function signature
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f754..e67ace01c99 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 5a3f1ef737c..b191a4df862 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -177,11 +177,157 @@ SELECT bt_index_check('toasty', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+CREATE TABLE bttest_unique(a varchar(50), b varchar(1500), c bytea, d varchar(50));
+CREATE UNIQUE INDEX bttest_unique_idx ON bttest_unique(a,b);
+UPDATE pg_catalog.pg_index SET indisunique = false
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+INSERT INTO bttest_unique
+	SELECT 	i::text::varchar,
+			array_to_string(array(
+				SELECT substr('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', ((random()*(36-1)+1)::integer), 1)
+			FROM generate_series(1,1300)),'')::varchar,
+	i::text::bytea, i::text::varchar
+	FROM generate_series(0,1) AS i, generate_series(0,30) AS x;
+UPDATE pg_catalog.pg_index SET indisunique = true
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+DELETE FROM bttest_unique WHERE ctid::text='(0,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,3)';
+DELETE FROM bttest_unique WHERE ctid::text='(9,3)';
+-- Check unique index with no uniqueness check. Should not complain.
+SELECT bt_index_check('bttest_unique_idx', true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+-- Check unique indes with uniquensee check. Should detect constraint violation cases.
+SELECT bt_index_check('bttest_unique_idx', true, true);
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 0 and posting 2 (point to heap tid=(0,1) and tid=(0,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 2 and posting 3 (point to heap tid=(0,3) and tid=(0,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 3 and posting 4 (point to heap tid=(0,4) and tid=(0,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 4 and tid=(1,3) posting 0 (point to heap tid=(0,5) and tid=(0,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 0 and posting 1 (point to heap tid=(0,6) and tid=(1,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 1 and posting 2 (point to heap tid=(1,1) and tid=(1,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 2 and posting 3 (point to heap tid=(1,2) and tid=(1,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 3 and posting 4 (point to heap tid=(1,3) and tid=(1,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 4 and tid=(1,4) posting 0 (point to heap tid=(1,4) and tid=(1,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 0 and posting 1 (point to heap tid=(1,5) and tid=(1,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 1 and posting 2 (point to heap tid=(1,6) and tid=(2,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 2 and posting 3 (point to heap tid=(2,1) and tid=(2,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 3 and posting 4 (point to heap tid=(2,2) and tid=(2,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 4 and tid=(1,5) posting 0 (point to heap tid=(2,3) and tid=(2,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 0 and posting 1 (point to heap tid=(2,4) and tid=(2,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 1 and posting 2 (point to heap tid=(2,5) and tid=(2,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 2 and posting 3 (point to heap tid=(2,6) and tid=(3,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 3 and posting 4 (point to heap tid=(3,1) and tid=(3,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 4 and tid=(1,6) posting 0 (point to heap tid=(3,2) and tid=(3,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 0 and posting 1 (point to heap tid=(3,3) and tid=(3,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 1 and posting 2 (point to heap tid=(3,4) and tid=(3,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 2 and posting 3 (point to heap tid=(3,5) and tid=(3,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 3 and posting 4 (point to heap tid=(3,6) and tid=(4,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 4 and tid=(2,2) posting 2 (point to heap tid=(4,1) and tid=(4,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 2 and posting 3 (point to heap tid=(4,4) and tid=(4,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 3 and posting 4 (point to heap tid=(4,5) and tid=(4,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 4 and tid=(2,3) (point to heap tid=(4,6) and tid=(5,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 0 and posting 1 (point to heap tid=(5,2) and tid=(5,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 1 and posting 2 (point to heap tid=(5,3) and tid=(5,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 2 and posting 3 (point to heap tid=(5,4) and tid=(5,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 3 and posting 4 (point to heap tid=(5,5) and tid=(5,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 4 and tid=(4,3) posting 0 (point to heap tid=(5,6) and tid=(6,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 0 and posting 1 (point to heap tid=(6,1) and tid=(6,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 1 and posting 2 (point to heap tid=(6,2) and tid=(6,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 2 and posting 3 (point to heap tid=(6,3) and tid=(6,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 3 and posting 4 (point to heap tid=(6,4) and tid=(6,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 4 and tid=(4,4) posting 0 (point to heap tid=(6,5) and tid=(6,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 0 and posting 1 (point to heap tid=(6,6) and tid=(7,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 1 and posting 2 (point to heap tid=(7,1) and tid=(7,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 2 and posting 3 (point to heap tid=(7,2) and tid=(7,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 3 and posting 4 (point to heap tid=(7,3) and tid=(7,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 4 and tid=(4,5) posting 0 (point to heap tid=(7,4) and tid=(7,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 0 and posting 1 (point to heap tid=(7,5) and tid=(7,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 1 and posting 2 (point to heap tid=(7,6) and tid=(8,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 2 and posting 3 (point to heap tid=(8,1) and tid=(8,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 3 and posting 4 (point to heap tid=(8,2) and tid=(8,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 4 and tid=(4,6) posting 0 (point to heap tid=(8,3) and tid=(8,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 0 and posting 1 (point to heap tid=(8,4) and tid=(8,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 1 and posting 2 (point to heap tid=(8,5) and tid=(8,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 2 and posting 3 (point to heap tid=(8,6) and tid=(9,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 3 and posting 4 (point to heap tid=(9,1) and tid=(9,2)).
+WARNING:  index uniqueness may be violated for index "bttest_unique_idx": Index tid=(5,1) doesn't have visible heap tids and key is equal to the tid=(4,6) posting 4 (points to heap tid=(9,2)). Cross-page unique constraint violation can be missed. Vacuum the table and repeat the check.
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,2) and tid=(5,3) (point to heap tid=(9,4) and tid=(9,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,3) and tid=(5,4) (point to heap tid=(9,5) and tid=(9,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,4) and tid=(5,5) (point to heap tid=(9,6) and tid=(10,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,5) and tid=(5,6) (point to heap tid=(10,1) and tid=(10,2)).
+ERROR:  index "bttest_unique_idx" is corrupted. There are tuples violating UNIQUE constraint
+DETAIL:  Details are in the previous log messages under WARNING priority
+VACUUM bttest_unique;
+SELECT bt_index_check('bttest_unique_idx', true, true);
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 0 and posting 1 (point to heap tid=(0,1) and tid=(0,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 1 and posting 2 (point to heap tid=(0,3) and tid=(0,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 2 and posting 3 (point to heap tid=(0,4) and tid=(0,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 3 and tid=(1,3) posting 0 (point to heap tid=(0,5) and tid=(0,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 0 and posting 1 (point to heap tid=(0,6) and tid=(1,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 1 and posting 2 (point to heap tid=(1,1) and tid=(1,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 2 and posting 3 (point to heap tid=(1,2) and tid=(1,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 3 and posting 4 (point to heap tid=(1,3) and tid=(1,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 4 and tid=(1,4) posting 0 (point to heap tid=(1,4) and tid=(1,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 0 and posting 1 (point to heap tid=(1,5) and tid=(1,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 1 and posting 2 (point to heap tid=(1,6) and tid=(2,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 2 and posting 3 (point to heap tid=(2,1) and tid=(2,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 3 and posting 4 (point to heap tid=(2,2) and tid=(2,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 4 and tid=(1,5) posting 0 (point to heap tid=(2,3) and tid=(2,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 0 and posting 1 (point to heap tid=(2,4) and tid=(2,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 1 and posting 2 (point to heap tid=(2,5) and tid=(2,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 2 and posting 3 (point to heap tid=(2,6) and tid=(3,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 3 and posting 4 (point to heap tid=(3,1) and tid=(3,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 4 and tid=(1,6) posting 0 (point to heap tid=(3,2) and tid=(3,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 0 and posting 1 (point to heap tid=(3,3) and tid=(3,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 1 and posting 2 (point to heap tid=(3,4) and tid=(3,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 2 and posting 3 (point to heap tid=(3,5) and tid=(3,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 3 and posting 4 (point to heap tid=(3,6) and tid=(4,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 4 and tid=(2,2) posting 0 (point to heap tid=(4,1) and tid=(4,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 0 and posting 1 (point to heap tid=(4,4) and tid=(4,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 1 and posting 2 (point to heap tid=(4,5) and tid=(4,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(2,2) posting 2 and tid=(2,3) (point to heap tid=(4,6) and tid=(5,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 0 and posting 1 (point to heap tid=(5,2) and tid=(5,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 1 and posting 2 (point to heap tid=(5,3) and tid=(5,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 2 and posting 3 (point to heap tid=(5,4) and tid=(5,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 3 and posting 4 (point to heap tid=(5,5) and tid=(5,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 4 and tid=(4,3) posting 0 (point to heap tid=(5,6) and tid=(6,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 0 and posting 1 (point to heap tid=(6,1) and tid=(6,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 1 and posting 2 (point to heap tid=(6,2) and tid=(6,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 2 and posting 3 (point to heap tid=(6,3) and tid=(6,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 3 and posting 4 (point to heap tid=(6,4) and tid=(6,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 4 and tid=(4,4) posting 0 (point to heap tid=(6,5) and tid=(6,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 0 and posting 1 (point to heap tid=(6,6) and tid=(7,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 1 and posting 2 (point to heap tid=(7,1) and tid=(7,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 2 and posting 3 (point to heap tid=(7,2) and tid=(7,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 3 and posting 4 (point to heap tid=(7,3) and tid=(7,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 4 and tid=(4,5) posting 0 (point to heap tid=(7,4) and tid=(7,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 0 and posting 1 (point to heap tid=(7,5) and tid=(7,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 1 and posting 2 (point to heap tid=(7,6) and tid=(8,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 2 and posting 3 (point to heap tid=(8,1) and tid=(8,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 3 and posting 4 (point to heap tid=(8,2) and tid=(8,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 4 and tid=(4,6) posting 0 (point to heap tid=(8,3) and tid=(8,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 0 and posting 1 (point to heap tid=(8,4) and tid=(8,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 1 and posting 2 (point to heap tid=(8,5) and tid=(8,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 2 and posting 3 (point to heap tid=(8,6) and tid=(9,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 3 and posting 4 (point to heap tid=(9,1) and tid=(9,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,6) posting 4 and tid=(5,1) (point to heap tid=(9,2) and tid=(9,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,1) and tid=(5,2) (point to heap tid=(9,4) and tid=(9,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,2) and tid=(5,3) (point to heap tid=(9,5) and tid=(9,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,3) and tid=(5,4) (point to heap tid=(9,6) and tid=(10,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(5,4) and tid=(5,5) (point to heap tid=(10,1) and tid=(10,2)).
+ERROR:  index "bttest_unique_idx" is corrupted. There are tuples violating UNIQUE constraint
+DETAIL:  Details are in the previous log messages under WARNING priority
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/expected/check_btree_1.out b/contrib/amcheck/expected/check_btree_1.out
new file mode 100644
index 00000000000..6aa66e42118
--- /dev/null
+++ b/contrib/amcheck/expected/check_btree_1.out
@@ -0,0 +1,333 @@
+CREATE TABLE bttest_a(id int8);
+CREATE TABLE bttest_b(id int8);
+CREATE TABLE bttest_multi(id int8, data int8);
+CREATE TABLE delete_test_table (a bigint, b bigint, c bigint, d bigint);
+-- Stabalize tests
+ALTER TABLE bttest_a SET (autovacuum_enabled = false);
+ALTER TABLE bttest_b SET (autovacuum_enabled = false);
+ALTER TABLE bttest_multi SET (autovacuum_enabled = false);
+ALTER TABLE delete_test_table SET (autovacuum_enabled = false);
+INSERT INTO bttest_a SELECT * FROM generate_series(1, 100000);
+INSERT INTO bttest_b SELECT * FROM generate_series(100000, 1, -1);
+INSERT INTO bttest_multi SELECT i, i%2  FROM generate_series(1, 100000) as i;
+CREATE INDEX bttest_a_idx ON bttest_a USING btree (id) WITH (deduplicate_items = ON);
+CREATE INDEX bttest_b_idx ON bttest_b USING btree (id);
+CREATE UNIQUE INDEX bttest_multi_idx ON bttest_multi
+USING btree (id) INCLUDE (data);
+CREATE ROLE regress_bttest_role;
+-- verify permissions are checked (error due to function not callable)
+SET ROLE regress_bttest_role;
+SELECT bt_index_check('bttest_a_idx'::regclass);
+ERROR:  permission denied for function bt_index_check
+SELECT bt_index_parent_check('bttest_a_idx'::regclass);
+ERROR:  permission denied for function bt_index_parent_check
+RESET ROLE;
+-- we, intentionally, don't check relation permissions - it's useful
+-- to run this cluster-wide with a restricted account, and as tested
+-- above explicit permission has to be granted for that.
+GRANT EXECUTE ON FUNCTION bt_index_check(regclass) TO regress_bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_parent_check(regclass) TO regress_bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_check(regclass, boolean) TO regress_bttest_role;
+GRANT EXECUTE ON FUNCTION bt_index_parent_check(regclass, boolean) TO regress_bttest_role;
+SET ROLE regress_bttest_role;
+SELECT bt_index_check('bttest_a_idx');
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx');
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+RESET ROLE;
+-- verify plain tables are rejected (error)
+SELECT bt_index_check('bttest_a');
+ERROR:  "bttest_a" is not an index
+SELECT bt_index_parent_check('bttest_a');
+ERROR:  "bttest_a" is not an index
+-- verify non-existing indexes are rejected (error)
+SELECT bt_index_check(17);
+ERROR:  could not open relation with OID 17
+SELECT bt_index_parent_check(17);
+ERROR:  could not open relation with OID 17
+-- verify wrong index types are rejected (error)
+BEGIN;
+CREATE INDEX bttest_a_brin_idx ON bttest_a USING brin(id);
+SELECT bt_index_parent_check('bttest_a_brin_idx');
+ERROR:  only B-Tree indexes are supported as targets for verification
+DETAIL:  Relation "bttest_a_brin_idx" is not a B-Tree index.
+ROLLBACK;
+-- normal check outside of xact
+SELECT bt_index_check('bttest_a_idx');
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+-- more expansive tests
+SELECT bt_index_check('bttest_a_idx', true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+BEGIN;
+SELECT bt_index_check('bttest_a_idx');
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx');
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- make sure we don't have any leftover locks
+SELECT * FROM pg_locks
+WHERE relation = ANY(ARRAY['bttest_a', 'bttest_a_idx', 'bttest_b', 'bttest_b_idx']::regclass[])
+    AND pid = pg_backend_pid();
+ locktype | database | relation | page | tuple | virtualxid | transactionid | classid | objid | objsubid | virtualtransaction | pid | mode | granted | fastpath | waitstart 
+----------+----------+----------+------+-------+------------+---------------+---------+-------+----------+--------------------+-----+------+---------+----------+-----------
+(0 rows)
+
+COMMIT;
+-- Deduplication
+TRUNCATE bttest_a;
+INSERT INTO bttest_a SELECT 42 FROM generate_series(1, 2000);
+SELECT bt_index_check('bttest_a_idx', true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+-- normal check outside of xact for index with included columns
+SELECT bt_index_check('bttest_multi_idx');
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+-- more expansive tests for index with included columns
+SELECT bt_index_parent_check('bttest_multi_idx', true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- repeat expansive tests for index built using insertions
+TRUNCATE bttest_multi;
+INSERT INTO bttest_multi SELECT i, i%2  FROM generate_series(1, 100000) as i;
+SELECT bt_index_parent_check('bttest_multi_idx', true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+--
+-- Test for multilevel page deletion/downlink present checks, and rootdescend
+-- checks
+--
+INSERT INTO delete_test_table SELECT i, 1, 2, 3 FROM generate_series(1,80000) i;
+ALTER TABLE delete_test_table ADD PRIMARY KEY (a,b,c,d);
+-- Delete most entries, and vacuum, deleting internal pages and creating "fast
+-- root"
+DELETE FROM delete_test_table WHERE a < 79990;
+VACUUM delete_test_table;
+SELECT bt_index_parent_check('delete_test_table_pkey', true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+--
+-- BUG #15597: must not assume consistent input toasting state when forming
+-- tuple.  Bloom filter must fingerprint normalized index tuple representation.
+--
+CREATE TABLE toast_bug(buggy text);
+ALTER TABLE toast_bug ALTER COLUMN buggy SET STORAGE extended;
+CREATE INDEX toasty ON toast_bug(buggy);
+-- pg_attribute entry for toasty.buggy (the index) will have plain storage:
+UPDATE pg_attribute SET attstorage = 'p'
+WHERE attrelid = 'toasty'::regclass AND attname = 'buggy';
+-- Whereas pg_attribute entry for toast_bug.buggy (the table) still has extended storage:
+SELECT attstorage FROM pg_attribute
+WHERE attrelid = 'toast_bug'::regclass AND attname = 'buggy';
+ attstorage 
+------------
+ x
+(1 row)
+
+-- Insert compressible heap tuple (comfortably exceeds TOAST_TUPLE_THRESHOLD):
+INSERT INTO toast_bug SELECT repeat('a', 2200);
+-- Should not get false positive report of corruption:
+SELECT bt_index_check('toasty', true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+-- UNIQUE constraint check
+CREATE TABLE bttest_unique(a varchar(50), b varchar(1500), c bytea, d varchar(50));
+CREATE UNIQUE INDEX bttest_unique_idx ON bttest_unique(a,b);
+UPDATE pg_catalog.pg_index SET indisunique = false
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+INSERT INTO bttest_unique
+	SELECT 	i::text::varchar,
+			array_to_string(array(
+				SELECT substr('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', ((random()*(36-1)+1)::integer), 1)
+			FROM generate_series(1,1300)),'')::varchar,
+	i::text::bytea, i::text::varchar
+	FROM generate_series(0,1) AS i, generate_series(0,30) AS x;
+UPDATE pg_catalog.pg_index SET indisunique = true
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+DELETE FROM bttest_unique WHERE ctid::text='(0,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,3)';
+DELETE FROM bttest_unique WHERE ctid::text='(9,3)';
+-- Check unique index with no uniqueness check. Should not complain.
+SELECT bt_index_check('bttest_unique_idx', true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+-- Check unique indes with uniquensee check. Should detect constraint violation cases.
+SELECT bt_index_check('bttest_unique_idx', true, true);
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 0 and posting 2 (point to heap tid=(0,1) and tid=(0,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 2 and posting 3 (point to heap tid=(0,3) and tid=(0,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 3 and posting 4 (point to heap tid=(0,4) and tid=(0,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 4 and posting 5 (point to heap tid=(0,5) and tid=(0,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 5 and tid=(1,3) posting 0 (point to heap tid=(0,6) and tid=(1,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 0 and posting 1 (point to heap tid=(1,1) and tid=(1,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 1 and posting 2 (point to heap tid=(1,2) and tid=(1,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 2 and posting 3 (point to heap tid=(1,3) and tid=(1,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 3 and posting 4 (point to heap tid=(1,4) and tid=(1,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 4 and posting 5 (point to heap tid=(1,5) and tid=(1,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 5 and tid=(1,4) posting 0 (point to heap tid=(1,6) and tid=(2,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 0 and posting 1 (point to heap tid=(2,1) and tid=(2,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 1 and posting 2 (point to heap tid=(2,2) and tid=(2,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 2 and posting 3 (point to heap tid=(2,3) and tid=(2,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 3 and posting 4 (point to heap tid=(2,4) and tid=(2,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 4 and posting 5 (point to heap tid=(2,5) and tid=(2,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 5 and tid=(1,5) posting 0 (point to heap tid=(2,6) and tid=(3,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 0 and posting 1 (point to heap tid=(3,1) and tid=(3,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 1 and posting 2 (point to heap tid=(3,2) and tid=(3,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 2 and posting 3 (point to heap tid=(3,3) and tid=(3,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 3 and posting 4 (point to heap tid=(3,4) and tid=(3,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 4 and posting 5 (point to heap tid=(3,5) and tid=(3,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 5 and tid=(1,6) posting 0 (point to heap tid=(3,6) and tid=(4,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 0 and posting 3 (point to heap tid=(4,1) and tid=(4,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 3 and posting 4 (point to heap tid=(4,4) and tid=(4,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 4 and posting 5 (point to heap tid=(4,5) and tid=(4,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 5 and tid=(2,2) (point to heap tid=(4,6) and tid=(5,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,1) posting 0 and posting 1 (point to heap tid=(5,2) and tid=(5,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,1) posting 1 and posting 2 (point to heap tid=(5,3) and tid=(5,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,1) posting 2 and posting 3 (point to heap tid=(5,4) and tid=(5,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,1) posting 3 and posting 4 (point to heap tid=(5,5) and tid=(5,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,1) posting 4 and posting 5 (point to heap tid=(5,6) and tid=(6,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,1) posting 5 and tid=(4,2) posting 0 (point to heap tid=(6,1) and tid=(6,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 0 and posting 1 (point to heap tid=(6,2) and tid=(6,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 1 and posting 2 (point to heap tid=(6,3) and tid=(6,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 2 and posting 3 (point to heap tid=(6,4) and tid=(6,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 3 and posting 4 (point to heap tid=(6,5) and tid=(6,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 4 and posting 5 (point to heap tid=(6,6) and tid=(7,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 5 and tid=(4,3) posting 0 (point to heap tid=(7,1) and tid=(7,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 0 and posting 1 (point to heap tid=(7,2) and tid=(7,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 1 and posting 2 (point to heap tid=(7,3) and tid=(7,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 2 and posting 3 (point to heap tid=(7,4) and tid=(7,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 3 and posting 4 (point to heap tid=(7,5) and tid=(7,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 4 and posting 5 (point to heap tid=(7,6) and tid=(8,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 5 and tid=(4,4) posting 0 (point to heap tid=(8,1) and tid=(8,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 0 and posting 1 (point to heap tid=(8,2) and tid=(8,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 1 and posting 2 (point to heap tid=(8,3) and tid=(8,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 2 and posting 3 (point to heap tid=(8,4) and tid=(8,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 3 and posting 4 (point to heap tid=(8,5) and tid=(8,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 4 and posting 5 (point to heap tid=(8,6) and tid=(9,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 5 and tid=(4,5) posting 0 (point to heap tid=(9,1) and tid=(9,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 0 and posting 2 (point to heap tid=(9,2) and tid=(9,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 2 and posting 3 (point to heap tid=(9,4) and tid=(9,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 3 and posting 4 (point to heap tid=(9,5) and tid=(9,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 4 and posting 5 (point to heap tid=(9,6) and tid=(10,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 5 and tid=(4,6) (point to heap tid=(10,1) and tid=(10,2)).
+ERROR:  index "bttest_unique_idx" is corrupted. There are tuples violating UNIQUE constraint
+DETAIL:  Details are in the previous log messages under WARNING priority
+VACUUM bttest_unique;
+SELECT bt_index_check('bttest_unique_idx', true, true);
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 0 and posting 1 (point to heap tid=(0,1) and tid=(0,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 1 and posting 2 (point to heap tid=(0,3) and tid=(0,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 2 and posting 3 (point to heap tid=(0,4) and tid=(0,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 3 and posting 4 (point to heap tid=(0,5) and tid=(0,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,2) posting 4 and tid=(1,3) posting 0 (point to heap tid=(0,6) and tid=(1,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 0 and posting 1 (point to heap tid=(1,1) and tid=(1,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 1 and posting 2 (point to heap tid=(1,2) and tid=(1,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 2 and posting 3 (point to heap tid=(1,3) and tid=(1,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 3 and posting 4 (point to heap tid=(1,4) and tid=(1,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 4 and posting 5 (point to heap tid=(1,5) and tid=(1,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,3) posting 5 and tid=(1,4) posting 0 (point to heap tid=(1,6) and tid=(2,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 0 and posting 1 (point to heap tid=(2,1) and tid=(2,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 1 and posting 2 (point to heap tid=(2,2) and tid=(2,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 2 and posting 3 (point to heap tid=(2,3) and tid=(2,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 3 and posting 4 (point to heap tid=(2,4) and tid=(2,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 4 and posting 5 (point to heap tid=(2,5) and tid=(2,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,4) posting 5 and tid=(1,5) posting 0 (point to heap tid=(2,6) and tid=(3,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 0 and posting 1 (point to heap tid=(3,1) and tid=(3,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 1 and posting 2 (point to heap tid=(3,2) and tid=(3,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 2 and posting 3 (point to heap tid=(3,3) and tid=(3,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 3 and posting 4 (point to heap tid=(3,4) and tid=(3,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 4 and posting 5 (point to heap tid=(3,5) and tid=(3,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,5) posting 5 and tid=(1,6) posting 0 (point to heap tid=(3,6) and tid=(4,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 0 and posting 1 (point to heap tid=(4,1) and tid=(4,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 1 and posting 2 (point to heap tid=(4,4) and tid=(4,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 2 and posting 3 (point to heap tid=(4,5) and tid=(4,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(1,6) posting 3 and tid=(2,2) (point to heap tid=(4,6) and tid=(5,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,1) posting 0 and posting 1 (point to heap tid=(5,2) and tid=(5,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,1) posting 1 and posting 2 (point to heap tid=(5,3) and tid=(5,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,1) posting 2 and posting 3 (point to heap tid=(5,4) and tid=(5,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,1) posting 3 and posting 4 (point to heap tid=(5,5) and tid=(5,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,1) posting 4 and posting 5 (point to heap tid=(5,6) and tid=(6,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,1) posting 5 and tid=(4,2) posting 0 (point to heap tid=(6,1) and tid=(6,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 0 and posting 1 (point to heap tid=(6,2) and tid=(6,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 1 and posting 2 (point to heap tid=(6,3) and tid=(6,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 2 and posting 3 (point to heap tid=(6,4) and tid=(6,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 3 and posting 4 (point to heap tid=(6,5) and tid=(6,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 4 and posting 5 (point to heap tid=(6,6) and tid=(7,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,2) posting 5 and tid=(4,3) posting 0 (point to heap tid=(7,1) and tid=(7,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 0 and posting 1 (point to heap tid=(7,2) and tid=(7,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 1 and posting 2 (point to heap tid=(7,3) and tid=(7,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 2 and posting 3 (point to heap tid=(7,4) and tid=(7,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 3 and posting 4 (point to heap tid=(7,5) and tid=(7,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 4 and posting 5 (point to heap tid=(7,6) and tid=(8,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,3) posting 5 and tid=(4,4) posting 0 (point to heap tid=(8,1) and tid=(8,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 0 and posting 1 (point to heap tid=(8,2) and tid=(8,3)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 1 and posting 2 (point to heap tid=(8,3) and tid=(8,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 2 and posting 3 (point to heap tid=(8,4) and tid=(8,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 3 and posting 4 (point to heap tid=(8,5) and tid=(8,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 4 and posting 5 (point to heap tid=(8,6) and tid=(9,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,4) posting 5 and tid=(4,5) posting 0 (point to heap tid=(9,1) and tid=(9,2)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 0 and posting 1 (point to heap tid=(9,2) and tid=(9,4)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 1 and posting 2 (point to heap tid=(9,4) and tid=(9,5)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 2 and posting 3 (point to heap tid=(9,5) and tid=(9,6)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 3 and posting 4 (point to heap tid=(9,6) and tid=(10,1)).
+WARNING:  index uniqueness is violated for index "bttest_unique_idx": Index tid=(4,5) posting 4 and tid=(4,6) (point to heap tid=(10,1) and tid=(10,2)).
+ERROR:  index "bttest_unique_idx" is corrupted. There are tuples violating UNIQUE constraint
+DETAIL:  Details are in the previous log messages under WARNING priority
+-- cleanup
+DROP TABLE bttest_a;
+DROP TABLE bttest_b;
+DROP TABLE bttest_multi;
+DROP TABLE delete_test_table;
+DROP TABLE toast_bug;
+DROP TABLE bttest_unique;
+DROP OWNED BY regress_bttest_role; -- permissions
+DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 97a3e1a20d5..dbe8b48afc7 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -115,11 +115,38 @@ INSERT INTO toast_bug SELECT repeat('a', 2200);
 -- Should not get false positive report of corruption:
 SELECT bt_index_check('toasty', true);
 
+-- UNIQUE constraint check
+CREATE TABLE bttest_unique(a varchar(50), b varchar(1500), c bytea, d varchar(50));
+CREATE UNIQUE INDEX bttest_unique_idx ON bttest_unique(a,b);
+UPDATE pg_catalog.pg_index SET indisunique = false
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+INSERT INTO bttest_unique
+	SELECT 	i::text::varchar,
+			array_to_string(array(
+				SELECT substr('ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789', ((random()*(36-1)+1)::integer), 1)
+			FROM generate_series(1,1300)),'')::varchar,
+	i::text::bytea, i::text::varchar
+	FROM generate_series(0,1) AS i, generate_series(0,30) AS x;
+UPDATE pg_catalog.pg_index SET indisunique = true
+WHERE indrelid = (SELECT oid FROM pg_catalog.pg_class WHERE relname = 'bttest_unique');
+
+DELETE FROM bttest_unique WHERE ctid::text='(0,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,2)';
+DELETE FROM bttest_unique WHERE ctid::text='(4,3)';
+DELETE FROM bttest_unique WHERE ctid::text='(9,3)';
+-- Check unique index with no uniqueness check. Should not complain.
+SELECT bt_index_check('bttest_unique_idx', true);
+-- Check unique indes with uniquensee check. Should detect constraint violation cases.
+SELECT bt_index_check('bttest_unique_idx', true, true);
+VACUUM bttest_unique;
+SELECT bt_index_check('bttest_unique_idx', true, true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index c4ca6339182..68644fa3285 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -78,11 +78,20 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool 		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking.
+	 * Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -137,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -187,9 +210,10 @@ static ItemId PageGetItemIdCareful(BtreeCheckState *state, BlockNumber block,
 static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 													  IndexTuple itup, bool nonpivot);
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
+static bool errflag; /* Output ERROR at the end of amcheck */
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -202,17 +226,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool        checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -226,13 +253,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -242,7 +272,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -323,7 +353,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/*
@@ -417,7 +447,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -449,6 +480,18 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
+	/*
+	 * We need a snapshot it to check uniqueness of the index
+	 * For better performance, take it once per index check.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+			state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+	}
 
 	if (state->heapallindexed)
 	{
@@ -632,7 +675,16 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
+
+	if (errflag)
+		ereport(ERROR,
+				(errcode(ERRCODE_INDEX_CORRUPTED),
+				errmsg("index \"%s\" is corrupted. There are tuples violating UNIQUE constraint",
+						RelationGetRelationName(state->rel)),
+				errdetail_internal("Details are in the previous log messages under WARNING priority")));
 }
 
 /*
@@ -1006,6 +1058,149 @@ bt_recheck_sibling_links(BtreeCheckState *state,
 								btpo_prev_from_target)));
 }
 
+/* Check visibility of the table entry referenced from nbtree index */
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+							  tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print error message for unique constrain violation in the btree
+ * index under WARNING level and set flag to report ERROR at the end of check
+ */
+static void bt_report_duplicate(BtreeCheckState *state,
+				 ItemPointer tid, BlockNumber block, OffsetNumber offset,
+				 int posting,
+				 ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+				 int nposting)
+{
+	char	   	*htid,
+				*nhtid,
+				*itid,
+				*nitid = "",
+				*pposting = "",
+				*pnposting = "";
+
+	errflag = true;
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(nexttid),
+					ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+		ereport(WARNING,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s).",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid)));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+		BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+		OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool has_visible_entry = false;
+
+	/*
+	 * Current tuple has posting list. If TID of any posting list entry is
+	 * visible, and lVis_tid is already valid report duplicate.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid (*lVis_tid))
+				{
+					bt_report_duplicate(state,
+											*lVis_tid, *lVis_block,
+											*lVis_offset, *lVis_i,
+											tid, targetblock,
+											offset, i);
+				}
+				/*
+				 * Prevent double reporting unique violation between the posting
+				 * list entries of a first tuple on the page after cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid (*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list.
+	 * If TID is visible, save info about it for next comparisons in the loop in
+	 * bt_page_check(). If also lVis_tid is already valid, report duplicate.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid (*lVis_tid))
+			{
+				bt_report_duplicate(state,
+											*lVis_tid, *lVis_block,
+											*lVis_offset, *lVis_i,
+											tid, targetblock,
+											offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+									   *lVis_block != targetblock)
+		ereport(WARNING,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			errmsg("index uniqueness may be violated for index \"%s\": "
+					"Index tid=(%u,%u) doesn't have visible heap tids and key "
+					"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+					"Cross-page unique constraint violation can be missed. "
+					"Vacuum the table and repeat the check.",
+					RelationGetRelationName(state->rel),
+					targetblock, offset,
+					*lVis_block, *lVis_offset, psprintf(" posting %u", *lVis_i),
+					ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+					ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+}
+
 /*
  * Function performs the following checks on target page, or pages ancillary to
  * target page:
@@ -1026,6 +1221,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint check that only one of table entries for
+ *   equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1047,6 +1245,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber offset;
 	OffsetNumber max;
 	BTPageOpaque topaque;
+	/* last visible entry info for checking indexes with unique constraint */
+	int			 lVis_i = -1; /* the position of last visible item for posting
+							   * tuple. for non-posting tuple (-1)
+							   */
+	ItemPointer	 lVis_tid = NULL;
+	BlockNumber	 lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
 
 	topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
@@ -1438,6 +1643,39 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique, verify entries uniqueness by checking
+		 * heap tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque))
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+					&lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+				 OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+			/* Invalidate scankey tid to make _bt_compare compare only keys
+			 * in the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple).
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+						OffsetNumberNext(offset)) != 0)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid; /* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1455,12 +1693,14 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1494,6 +1734,44 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint check that not more than one found
+			 * equal items is visible.
+			 */
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+					rightkey && P_ISLEAF(topaque))
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* First key on next page is same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  state->targetblock + 1);
+					topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+							break;
+
+					itemid = PageGetItemIdCareful(state, state->targetblock + 1,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, state->targetblock + 1, rightfirstoffset,
+									&lVis_i, &lVis_tid, &lVis_offset,
+									&lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1539,9 +1817,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1704,6 +1984,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
-- 
2.28.0

From b9bdb2dde7d97e09e9a95daf43c8c843ff3876d8 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Mon, 20 Dec 2021 16:57:03 +0400
Subject: [PATCH v5] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

With 'checkunique' option bt_index_check() and bt_index_parent_check()
for btree indexes that has unique constraint will check it i.e.
will check that only one heap entry for all equal keys in the index
(including posting list entries) is visible. Report error if not.

pg_amcheck called with --checkunique option will do the same for
all indexes it checks

Authors:
Anastasia Lubennikova <a.lubennikova@postgrespro.ru>
Pavel Borisov <pashkin.elfe@gmail.com>
Maxim Orlov <m.orlov@postgrespro.ru>
---
 contrib/amcheck/Makefile                   |   2 +-
 contrib/amcheck/amcheck.control            |   2 +-
 contrib/amcheck/expected/check_btree.out   |  27 ++
 contrib/amcheck/sql/check_btree.sql        |   7 +
 contrib/amcheck/verify_nbtree.c            | 327 ++++++++++++++++++++-
 doc/src/sgml/amcheck.sgml                  |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml           |  11 +
 src/bin/pg_amcheck/pg_amcheck.c            |  17 +-
 src/bin/pg_amcheck/t/005_opclass_damage.pl |  75 ++++-
 9 files changed, 457 insertions(+), 25 deletions(-)

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50b..88271687a3e 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f754..e67ace01c99 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 5a3f1ef737c..47250ec2f0f 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -177,11 +177,38 @@ SELECT bt_index_check('toasty', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', false, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique;
+ERROR:  table "bttest_unique" does not exist
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 97a3e1a20d5..1acee3a2439 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -115,11 +115,18 @@ INSERT INTO toast_bug SELECT repeat('a', 2200);
 -- Should not get false positive report of corruption:
 SELECT bt_index_check('toasty', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+SELECT bt_index_check('bttest_b_idx', false, true);
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index d3b29d3d890..80ddf3c819b 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -79,11 +79,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -138,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -190,7 +212,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -203,17 +225,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -227,13 +252,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -243,7 +271,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -323,7 +351,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/*
@@ -418,7 +446,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -450,6 +479,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -507,6 +538,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot it to check uniqueness of the index For better
+	 * performance, take it once per index check. If snapshot already taken,
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -633,6 +681,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -873,6 +923,162 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced from nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print error message for unique constrain violation in the btree
+ * index under WARNING level and set flag to report ERROR at the end of check
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. If TID of any posting list entry is
+	 * visible, and lVis_tid is already valid report duplicate.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique violation between the
+				 * posting list entries of a first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible, save info about
+	 * it for next comparisons in the loop in bt_page_check(). If also
+	 * lVis_tid is already valid, report duplicate.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1027,6 +1233,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint check that only one of table entries for
+ *   equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1049,6 +1258,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1439,6 +1655,41 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique, verify entries uniqueness by checking heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque))
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple).
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1456,12 +1707,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1495,6 +1750,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint check that not more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* First key on next page is same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1540,9 +1834,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1709,6 +2005,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 11d1eb5af23..0f23bbd575b 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c04655..61dacf1ee44 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index d4a53c8e636..7089ed6459f 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,7 +133,8 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
-	.no_btree_expansion = false
+	.no_btree_expansion = false,
+	.checkunique = false
 };
 
 static const char *progname = NULL;
@@ -267,6 +269,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -449,6 +452,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				fprintf(stderr,
 						_("Try \"%s --help\" for more information.\n"),
@@ -871,7 +877,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s, "
+						  "checkunique := %s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -880,11 +887,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (opts.checkunique ? "true" : "false"),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s,"
+						  "checkunique := %s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -892,6 +901,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (opts.checkunique ? "true" : "false"),
 						  rel->reloid);
 }
 
@@ -1187,6 +1197,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index 2f86f4f2a40..78ecb7c6321 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -8,7 +8,7 @@ use strict;
 use warnings;
 use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
-use Test::More tests => 5;
+use Test::More tests => 10;
 
 my $node = PostgreSQL::Test::Cluster->new('test');
 $node->init;
@@ -22,6 +22,17 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
@@ -30,6 +41,21 @@ $node->safe_psql(
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+
+	CREATE OPERATOR CLASS int4_custom_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
+
+	CREATE TABLE bttest_unique (i int4);
+	INSERT INTO bttest_unique
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON bttest_unique
+						USING btree (i int4_custom_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -57,3 +83,50 @@ $node->command_checks_all(
 	[],
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
+
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
-- 
2.24.3 (Apple Git-128)

From cf2d11a2e359d173bb92f6345a7f060630c03d64 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Wed, 22 Dec 2021 11:39:15 +0400
Subject: [PATCH v6] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

With 'checkunique' option bt_index_check() and bt_index_parent_check()
for btree indexes that has unique constraint will check it i.e.
will check that only one heap entry for all equal keys in the index
(including posting list entries) is visible. Report error if not.

pg_amcheck called with --checkunique option will do the same for
all indexes it checks

Authors:
Anastasia Lubennikova <lubennikovaav@gmail.com>
Pavel Borisov <pashkin.elfe@gmail.com>
Maxim Orlov <m.orlov@postgrespro.ru>
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  25 ++
 contrib/amcheck/sql/check_btree.sql           |   6 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 234 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 327 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  56 ++-
 src/bin/pg_amcheck/t/003_check.pl             |  48 ++-
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  75 +++-
 12 files changed, 798 insertions(+), 31 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50b..88271687a3e 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 00000000000..1caba148aa4
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f754..e67ace01c99 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 5a3f1ef737c..0144767b36e 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -177,6 +177,31 @@ SELECT bt_index_check('toasty', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', false, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 97a3e1a20d5..4eb5ffb21d3 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -115,6 +115,12 @@ INSERT INTO toast_bug SELECT repeat('a', 2200);
 -- Should not get false positive report of corruption:
 SELECT bt_index_check('toasty', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+SELECT bt_index_check('bttest_b_idx', false, true);
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 00000000000..a99e474f1f2
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,234 @@
+
+# Copyright (c) 2021, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More tests => 6;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 looks equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck get uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck get uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication possible only for same values, but
+# with different visibility.
+$node->safe_psql('postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index d3b29d3d890..80ddf3c819b 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -79,11 +79,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -138,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -190,7 +212,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -203,17 +225,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -227,13 +252,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -243,7 +271,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -323,7 +351,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/*
@@ -418,7 +446,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -450,6 +479,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -507,6 +538,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot it to check uniqueness of the index For better
+	 * performance, take it once per index check. If snapshot already taken,
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -633,6 +681,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -873,6 +923,162 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced from nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print error message for unique constrain violation in the btree
+ * index under WARNING level and set flag to report ERROR at the end of check
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. If TID of any posting list entry is
+	 * visible, and lVis_tid is already valid report duplicate.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique violation between the
+				 * posting list entries of a first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible, save info about
+	 * it for next comparisons in the loop in bt_page_check(). If also
+	 * lVis_tid is already valid, report duplicate.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1027,6 +1233,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint check that only one of table entries for
+ *   equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1049,6 +1258,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1439,6 +1655,41 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique, verify entries uniqueness by checking heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque))
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple).
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1456,12 +1707,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1495,6 +1750,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint check that not more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* First key on next page is same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1540,9 +1834,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1709,6 +2005,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 11d1eb5af23..0f23bbd575b 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c04655..61dacf1ee44 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index d4a53c8e636..30914db55b2 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,7 +133,8 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
-	.no_btree_expansion = false
+	.no_btree_expansion = false,
+	.checkunique = false
 };
 
 static const char *progname = NULL;
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -449,6 +453,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				fprintf(stderr,
 						_("Try \"%s --help\" for more information.\n"),
@@ -614,6 +621,34 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		if (opts.checkunique == true)
+		{
+			dat->is_checkunique = true;
+
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = pstrdup(PQgetvalue(result, 0, 1));
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -871,7 +906,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -880,11 +916,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -892,6 +930,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1100,17 +1139,17 @@ verify_btree_slot_handler(PGresult *res, PGconn *conn, void *context)
 
 	if (PQresultStatus(res) == PGRES_TUPLES_OK)
 	{
-		int                     ntups = PQntuples(res);
+		int			ntups = PQntuples(res);
 
 		if (ntups > 1)
 		{
 			/*
 			 * We expect the btree checking functions to return one void row
 			 * each, or zero rows if the check was skipped due to the object
-			 * being in the wrong state to be checked, so we should output some
-			 * sort of warning if we get anything more, not because it
-			 * indicates corruption, but because it suggests a mismatch between
-			 * amcheck and pg_amcheck versions.
+			 * being in the wrong state to be checked, so we should output
+			 * some sort of warning if we get anything more, not because it
+			 * indicates corruption, but because it suggests a mismatch
+			 * between amcheck and pg_amcheck versions.
 			 *
 			 * In conjunction with --progress, anything written to stderr at
 			 * this time would present strangely to the user without an extra
@@ -1187,6 +1226,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index 5913fcc5305..d3f0259bcbd 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -8,7 +8,7 @@ use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
 
 use Fcntl qw(:seek);
-use Test::More tests => 63;
+use Test::More tests => 75;
 
 my ($node, $port, %corrupt_page, %remove_relation);
 
@@ -258,6 +258,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -517,3 +520,46 @@ $node->command_checks_all(
 	[ @cmd, '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique',  'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend');
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
+	0, [$no_output_re], [$no_output_re],
+	'pg_amcheck excluding all corrupt schemas');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', 'db3' ],
+		0,
+		[$no_output_re],
+		[qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
\ No newline at end of file
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index 2f86f4f2a40..78ecb7c6321 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -8,7 +8,7 @@ use strict;
 use warnings;
 use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
-use Test::More tests => 5;
+use Test::More tests => 10;
 
 my $node = PostgreSQL::Test::Cluster->new('test');
 $node->init;
@@ -22,6 +22,17 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
@@ -30,6 +41,21 @@ $node->safe_psql(
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+
+	CREATE OPERATOR CLASS int4_custom_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
+
+	CREATE TABLE bttest_unique (i int4);
+	INSERT INTO bttest_unique
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON bttest_unique
+						USING btree (i int4_custom_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -57,3 +83,50 @@ $node->command_checks_all(
 	[],
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
+
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
-- 
2.24.3 (Apple Git-128)

From 10c94688e3e0aae2f3ac94026c04161f7b66b04a Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Mon, 8 Feb 2021 12:26:08 +0400
Subject: [PATCH v7] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

With 'checkunique' option bt_index_check() and bt_index_parent_check()
for btree indexes that has unique constraint will check it i.e.
will check that only one heap entry for all equal keys in the index
(including posting list entries) is visible. Report error if not.

pg_amcheck called with --checkunique option will do the same for
all indexes it checks

Authors:
Anastasia Lubennikova <lubennikovaav@gmail.com>
Pavel Borisov <pashkin.elfe@gmail.com>
Maxim Orlov <orlovmg@gmail.com>
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  25 ++
 contrib/amcheck/sql/check_btree.sql           |   6 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 234 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 327 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  56 ++-
 src/bin/pg_amcheck/t/003_check.pl             |  48 ++-
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  68 +++-
 12 files changed, 791 insertions(+), 31 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50b..88271687a3e 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 00000000000..1caba148aa4
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f754..e67ace01c99 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 5a3f1ef737c..0144767b36e 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -177,6 +177,31 @@ SELECT bt_index_check('toasty', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', false, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 97a3e1a20d5..4eb5ffb21d3 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -115,6 +115,12 @@ INSERT INTO toast_bug SELECT repeat('a', 2200);
 -- Should not get false positive report of corruption:
 SELECT bt_index_check('toasty', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+SELECT bt_index_check('bttest_b_idx', false, true);
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 00000000000..a99e474f1f2
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,234 @@
+
+# Copyright (c) 2021, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More tests => 6;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 looks equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck get uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck get uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication possible only for same values, but
+# with different visibility.
+$node->safe_psql('postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index d3b29d3d890..80ddf3c819b 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -79,11 +79,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -138,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -190,7 +212,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -203,17 +225,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -227,13 +252,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -243,7 +271,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -323,7 +351,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/*
@@ -418,7 +446,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -450,6 +479,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -507,6 +538,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot it to check uniqueness of the index For better
+	 * performance, take it once per index check. If snapshot already taken,
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -633,6 +681,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -873,6 +923,162 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced from nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print error message for unique constrain violation in the btree
+ * index under WARNING level and set flag to report ERROR at the end of check
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. If TID of any posting list entry is
+	 * visible, and lVis_tid is already valid report duplicate.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique violation between the
+				 * posting list entries of a first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible, save info about
+	 * it for next comparisons in the loop in bt_page_check(). If also
+	 * lVis_tid is already valid, report duplicate.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1027,6 +1233,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint check that only one of table entries for
+ *   equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1049,6 +1258,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1439,6 +1655,41 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique, verify entries uniqueness by checking heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque))
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple).
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1456,12 +1707,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1495,6 +1750,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint check that not more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* First key on next page is same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1540,9 +1834,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1709,6 +2005,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 11d1eb5af23..0f23bbd575b 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c04655..61dacf1ee44 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index d4a53c8e636..c4711d4a956 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,7 +133,8 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
-	.no_btree_expansion = false
+	.no_btree_expansion = false,
+	.checkunique = false
 };
 
 static const char *progname = NULL;
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -449,6 +453,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				fprintf(stderr,
 						_("Try \"%s --help\" for more information.\n"),
@@ -614,6 +621,34 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		if (opts.checkunique == true)
+		{
+			dat->is_checkunique = true;
+
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = PQgetvalue(result, 0, 1);
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -871,7 +906,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -880,11 +916,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -892,6 +930,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1100,17 +1139,17 @@ verify_btree_slot_handler(PGresult *res, PGconn *conn, void *context)
 
 	if (PQresultStatus(res) == PGRES_TUPLES_OK)
 	{
-		int                     ntups = PQntuples(res);
+		int			ntups = PQntuples(res);
 
 		if (ntups > 1)
 		{
 			/*
 			 * We expect the btree checking functions to return one void row
 			 * each, or zero rows if the check was skipped due to the object
-			 * being in the wrong state to be checked, so we should output some
-			 * sort of warning if we get anything more, not because it
-			 * indicates corruption, but because it suggests a mismatch between
-			 * amcheck and pg_amcheck versions.
+			 * being in the wrong state to be checked, so we should output
+			 * some sort of warning if we get anything more, not because it
+			 * indicates corruption, but because it suggests a mismatch
+			 * between amcheck and pg_amcheck versions.
 			 *
 			 * In conjunction with --progress, anything written to stderr at
 			 * this time would present strangely to the user without an extra
@@ -1187,6 +1226,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index 5913fcc5305..de125ed3c0c 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -8,7 +8,7 @@ use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
 
 use Fcntl qw(:seek);
-use Test::More tests => 63;
+use Test::More tests => 75;
 
 my ($node, $port, %corrupt_page, %remove_relation);
 
@@ -258,6 +258,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -517,3 +520,46 @@ $node->command_checks_all(
 	[ @cmd, '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique',  'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
+	0, [$no_output_re], [$no_output_re],
+	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', 'db3' ],
+		0,
+		[$no_output_re],
+		[qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index 2f86f4f2a40..5219c6ec0f5 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -8,7 +8,7 @@ use strict;
 use warnings;
 use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
-use Test::More tests => 5;
+use Test::More tests => 10;
 
 my $node = PostgreSQL::Test::Cluster->new('test');
 $node->init;
@@ -22,14 +22,33 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
 	    OPERATOR 5 > (int4, int4), FUNCTION 1 int4_asc_cmp(int4, int4);
 
+	CREATE OPERATOR CLASS int4_unique_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON int4tbl
+						USING btree (i int4_unique_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -57,3 +76,50 @@ $node->command_checks_all(
 	[],
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
+
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
-- 
2.32.0 (Apple Git-132)

From 5f4a18a7bdd1e5a2e7d63304241fa29b541cfee3 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Wed, 12 Jan 2022 11:46:24 +0400
Subject: [PATCH v8] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

With 'checkunique' option bt_index_check() and bt_index_parent_check()
for btree indexes that has unique constraint will check it i.e.
will check that only one heap entry for all equal keys in the index
(including posting list entries) is visible. Report error if not.

pg_amcheck called with --checkunique option will do the same for
all indexes it checks

Authors:
Anastasia Lubennikova <lubennikovaav@gmail.com>
Pavel Borisov <pashkin.elfe@gmail.com>
Maxim Orlov <orlovmg@gmail.com>
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  25 ++
 contrib/amcheck/sql/check_btree.sql           |   6 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 234 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 327 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  60 +++-
 src/bin/pg_amcheck/t/003_check.pl             |  48 ++-
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  68 +++-
 12 files changed, 795 insertions(+), 31 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50b..88271687a3e 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 00000000000..1caba148aa4
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f754..e67ace01c99 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 5a3f1ef737c..0144767b36e 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -177,6 +177,31 @@ SELECT bt_index_check('toasty', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', false, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 97a3e1a20d5..4eb5ffb21d3 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -115,6 +115,12 @@ INSERT INTO toast_bug SELECT repeat('a', 2200);
 -- Should not get false positive report of corruption:
 SELECT bt_index_check('toasty', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+SELECT bt_index_check('bttest_b_idx', false, true);
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 00000000000..a99e474f1f2
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,234 @@
+
+# Copyright (c) 2021, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More tests => 6;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 looks equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck get uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck get uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication possible only for same values, but
+# with different visibility.
+$node->safe_psql('postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index d2510ee6480..4e074b56bc5 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -79,11 +79,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -138,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -190,7 +212,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -203,17 +225,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -227,13 +252,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -243,7 +271,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -323,7 +351,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/*
@@ -418,7 +446,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -450,6 +479,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -507,6 +538,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot it to check uniqueness of the index For better
+	 * performance, take it once per index check. If snapshot already taken,
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -633,6 +681,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -873,6 +923,162 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced from nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print error message for unique constrain violation in the btree
+ * index under WARNING level and set flag to report ERROR at the end of check
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. If TID of any posting list entry is
+	 * visible, and lVis_tid is already valid report duplicate.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique violation between the
+				 * posting list entries of a first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible, save info about
+	 * it for next comparisons in the loop in bt_page_check(). If also
+	 * lVis_tid is already valid, report duplicate.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1027,6 +1233,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint check that only one of table entries for
+ *   equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1049,6 +1258,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1439,6 +1655,41 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique, verify entries uniqueness by checking heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque))
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple).
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1456,12 +1707,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1495,6 +1750,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint check that not more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* First key on next page is same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1540,9 +1834,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1709,6 +2005,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 11d1eb5af23..0f23bbd575b 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c04655..61dacf1ee44 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index 6607f729382..b3d393c500b 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,7 +133,8 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
-	.no_btree_expansion = false
+	.no_btree_expansion = false,
+	.checkunique = false
 };
 
 static const char *progname = NULL;
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -449,6 +453,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				fprintf(stderr,
 						_("Try \"%s --help\" for more information.\n"),
@@ -614,6 +621,38 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		/*
+		 * Check version of amcheck extension. Skip requested unique constraint
+		 * check with warning if it is not yet supported by amcheck.
+		 */
+		if (opts.checkunique == true)
+		{
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = PQgetvalue(result, 0, 1);
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+			else
+				dat->is_checkunique = true;
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -871,7 +910,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -880,11 +920,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -892,6 +934,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1100,17 +1143,17 @@ verify_btree_slot_handler(PGresult *res, PGconn *conn, void *context)
 
 	if (PQresultStatus(res) == PGRES_TUPLES_OK)
 	{
-		int                     ntups = PQntuples(res);
+		int			ntups = PQntuples(res);
 
 		if (ntups > 1)
 		{
 			/*
 			 * We expect the btree checking functions to return one void row
 			 * each, or zero rows if the check was skipped due to the object
-			 * being in the wrong state to be checked, so we should output some
-			 * sort of warning if we get anything more, not because it
-			 * indicates corruption, but because it suggests a mismatch between
-			 * amcheck and pg_amcheck versions.
+			 * being in the wrong state to be checked, so we should output
+			 * some sort of warning if we get anything more, not because it
+			 * indicates corruption, but because it suggests a mismatch
+			 * between amcheck and pg_amcheck versions.
 			 *
 			 * In conjunction with --progress, anything written to stderr at
 			 * this time would present strangely to the user without an extra
@@ -1187,6 +1230,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index 9df027b37f0..d8d03dabd6e 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -8,7 +8,7 @@ use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
 
 use Fcntl qw(:seek);
-use Test::More tests => 63;
+use Test::More tests => 75;
 
 my ($node, $port, %corrupt_page, %remove_relation);
 
@@ -258,6 +258,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -517,3 +520,46 @@ $node->command_checks_all(
 	[ @cmd, '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique',  'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
+	0, [$no_output_re], [$no_output_re],
+	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', 'db3' ],
+		0,
+		[$no_output_re],
+		[qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index d81c9583de2..905d3902c2d 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -8,7 +8,7 @@ use strict;
 use warnings;
 use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
-use Test::More tests => 5;
+use Test::More tests => 10;
 
 my $node = PostgreSQL::Test::Cluster->new('test');
 $node->init;
@@ -22,14 +22,33 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
 	    OPERATOR 5 > (int4, int4), FUNCTION 1 int4_asc_cmp(int4, int4);
 
+	CREATE OPERATOR CLASS int4_unique_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON int4tbl
+						USING btree (i int4_unique_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -57,3 +76,50 @@ $node->command_checks_all(
 	[],
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
+
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
-- 
2.24.3 (Apple Git-128)

From b3793c95f07f5104abfb1542d27d6ba2a67587bf Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Thu, 13 Jan 2022 11:56:38 +0400
Subject: [PATCH v9] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

With 'checkunique' option bt_index_check() and bt_index_parent_check()
for btree indexes that has unique constraint will check it i.e.
will check that only one heap entry for all equal keys in the index
(including posting list entries) is visible. Report error if not.

pg_amcheck called with --checkunique option will do the same for
all indexes it checks

Authors:
Anastasia Lubennikova <lubennikovaav@gmail.com>
Pavel Borisov <pashkin.elfe@gmail.com>
Maxim Orlov <orlovmg@gmail.com>
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  42 +++
 contrib/amcheck/sql/check_btree.sql           |  14 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 234 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 329 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  60 +++-
 src/bin/pg_amcheck/t/003_check.pl             |  48 ++-
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  68 +++-
 12 files changed, 822 insertions(+), 31 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50b..88271687a3e 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 00000000000..1caba148aa4
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f754..e67ace01c99 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 5a3f1ef737c..d6d578e9995 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -177,11 +177,53 @@ SELECT bt_index_check('toasty', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', false, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- Check null values in unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 97a3e1a20d5..8e09f43c373 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -115,11 +115,25 @@ INSERT INTO toast_bug SELECT repeat('a', 2200);
 -- Should not get false positive report of corruption:
 SELECT bt_index_check('toasty', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+SELECT bt_index_check('bttest_b_idx', false, true);
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+
+-- Check null values in unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', true, true);
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', true, true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 00000000000..a99e474f1f2
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,234 @@
+
+# Copyright (c) 2021, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More tests => 6;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 looks equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck get uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck get uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication possible only for same values, but
+# with different visibility.
+$node->safe_psql('postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index d2510ee6480..4b947558225 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -79,11 +79,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -138,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -190,7 +212,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -203,17 +225,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -227,13 +252,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -243,7 +271,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -323,7 +351,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/*
@@ -418,7 +446,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -450,6 +479,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -507,6 +538,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot it to check uniqueness of the index For better
+	 * performance, take it once per index check. If snapshot already taken,
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -633,6 +681,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -873,6 +923,162 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced from nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print error message for unique constrain violation in the btree
+ * index under WARNING level and set flag to report ERROR at the end of check
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. If TID of any posting list entry is
+	 * visible, and lVis_tid is already valid report duplicate.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique violation between the
+				 * posting list entries of a first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible, save info about
+	 * it for next comparisons in the loop in bt_page_check(). If also
+	 * lVis_tid is already valid, report duplicate.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1027,6 +1233,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint check that only one of table entries for
+ *   equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1049,6 +1258,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1439,6 +1655,43 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique, verify entries uniqueness by checking heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) && !skey->anynullkeys)
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple). Key
+			 * containing null value does not violate unique constraint and
+			 * treated as different to any other key.
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1456,12 +1709,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1495,6 +1752,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint check that not more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* First key on next page is same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1540,9 +1836,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1709,6 +2007,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 11d1eb5af23..0f23bbd575b 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c04655..61dacf1ee44 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index 6607f729382..b3d393c500b 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,7 +133,8 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
-	.no_btree_expansion = false
+	.no_btree_expansion = false,
+	.checkunique = false
 };
 
 static const char *progname = NULL;
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -449,6 +453,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				fprintf(stderr,
 						_("Try \"%s --help\" for more information.\n"),
@@ -614,6 +621,38 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		/*
+		 * Check version of amcheck extension. Skip requested unique constraint
+		 * check with warning if it is not yet supported by amcheck.
+		 */
+		if (opts.checkunique == true)
+		{
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = PQgetvalue(result, 0, 1);
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+			else
+				dat->is_checkunique = true;
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -871,7 +910,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -880,11 +920,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -892,6 +934,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1100,17 +1143,17 @@ verify_btree_slot_handler(PGresult *res, PGconn *conn, void *context)
 
 	if (PQresultStatus(res) == PGRES_TUPLES_OK)
 	{
-		int                     ntups = PQntuples(res);
+		int			ntups = PQntuples(res);
 
 		if (ntups > 1)
 		{
 			/*
 			 * We expect the btree checking functions to return one void row
 			 * each, or zero rows if the check was skipped due to the object
-			 * being in the wrong state to be checked, so we should output some
-			 * sort of warning if we get anything more, not because it
-			 * indicates corruption, but because it suggests a mismatch between
-			 * amcheck and pg_amcheck versions.
+			 * being in the wrong state to be checked, so we should output
+			 * some sort of warning if we get anything more, not because it
+			 * indicates corruption, but because it suggests a mismatch
+			 * between amcheck and pg_amcheck versions.
 			 *
 			 * In conjunction with --progress, anything written to stderr at
 			 * this time would present strangely to the user without an extra
@@ -1187,6 +1230,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index 9df027b37f0..d8d03dabd6e 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -8,7 +8,7 @@ use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
 
 use Fcntl qw(:seek);
-use Test::More tests => 63;
+use Test::More tests => 75;
 
 my ($node, $port, %corrupt_page, %remove_relation);
 
@@ -258,6 +258,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -517,3 +520,46 @@ $node->command_checks_all(
 	[ @cmd, '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique',  'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
+	0, [$no_output_re], [$no_output_re],
+	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', 'db3' ],
+		0,
+		[$no_output_re],
+		[qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index d81c9583de2..905d3902c2d 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -8,7 +8,7 @@ use strict;
 use warnings;
 use PostgreSQL::Test::Cluster;
 use PostgreSQL::Test::Utils;
-use Test::More tests => 5;
+use Test::More tests => 10;
 
 my $node = PostgreSQL::Test::Cluster->new('test');
 $node->init;
@@ -22,14 +22,33 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
 	    OPERATOR 5 > (int4, int4), FUNCTION 1 int4_asc_cmp(int4, int4);
 
+	CREATE OPERATOR CLASS int4_unique_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON int4tbl
+						USING btree (i int4_unique_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -57,3 +76,50 @@ $node->command_checks_all(
 	[],
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
+
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
-- 
2.24.3 (Apple Git-128)

From d23722b8f083ecfec69307e24c4748c9ddb09ff0 Mon Sep 17 00:00:00 2001
From: Maxim Orlov <m.orlov@postgrespro.ru>
Date: Mon, 21 Feb 2022 17:03:59 +0300
Subject: [PATCH v10] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

With 'checkunique' option bt_index_check() and bt_index_parent_check()
for btree indexes that has unique constraint will check it i.e.
will check that only one heap entry for all equal keys in the index
(including posting list entries) is visible. Report error if not.

pg_amcheck called with --checkunique option will do the same for
all indexes it checks

Authors:
Anastasia Lubennikova <lubennikovaav@gmail.com>
Pavel Borisov <pashkin.elfe@gmail.com>
Maxim Orlov <orlovmg@gmail.com>
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  42 +++
 contrib/amcheck/sql/check_btree.sql           |  14 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 234 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 329 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  60 +++-
 src/bin/pg_amcheck/t/003_check.pl             |  45 +++
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  65 ++++
 12 files changed, 818 insertions(+), 29 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50b..88271687a3e 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 00000000000..1caba148aa4
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f754..e67ace01c99 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 5a3f1ef737c..d6d578e9995 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -177,11 +177,53 @@ SELECT bt_index_check('toasty', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', false, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- Check null values in unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 97a3e1a20d5..8e09f43c373 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -115,11 +115,25 @@ INSERT INTO toast_bug SELECT repeat('a', 2200);
 -- Should not get false positive report of corruption:
 SELECT bt_index_check('toasty', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+SELECT bt_index_check('bttest_b_idx', false, true);
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+
+-- Check null values in unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', true, true);
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', true, true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 00000000000..a99e474f1f2
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,234 @@
+
+# Copyright (c) 2021, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More tests => 6;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 looks equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck get uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck get uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication possible only for same values, but
+# with different visibility.
+$node->safe_psql('postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index d2510ee6480..4b947558225 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -79,11 +79,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -138,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -190,7 +212,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -203,17 +225,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -227,13 +252,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -243,7 +271,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -323,7 +351,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/*
@@ -418,7 +446,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -450,6 +479,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -507,6 +538,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot it to check uniqueness of the index For better
+	 * performance, take it once per index check. If snapshot already taken,
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -633,6 +681,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -873,6 +923,162 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced from nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print error message for unique constrain violation in the btree
+ * index under WARNING level and set flag to report ERROR at the end of check
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. If TID of any posting list entry is
+	 * visible, and lVis_tid is already valid report duplicate.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique violation between the
+				 * posting list entries of a first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible, save info about
+	 * it for next comparisons in the loop in bt_page_check(). If also
+	 * lVis_tid is already valid, report duplicate.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1027,6 +1233,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint check that only one of table entries for
+ *   equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1049,6 +1258,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1439,6 +1655,43 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique, verify entries uniqueness by checking heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) && !skey->anynullkeys)
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple). Key
+			 * containing null value does not violate unique constraint and
+			 * treated as different to any other key.
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1456,12 +1709,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1495,6 +1752,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint check that not more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* First key on next page is same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1540,9 +1836,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1709,6 +2007,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 11d1eb5af23..0f23bbd575b 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c04655..61dacf1ee44 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index 6607f729382..b3d393c500b 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,7 +133,8 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
-	.no_btree_expansion = false
+	.no_btree_expansion = false,
+	.checkunique = false
 };
 
 static const char *progname = NULL;
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -449,6 +453,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				fprintf(stderr,
 						_("Try \"%s --help\" for more information.\n"),
@@ -614,6 +621,38 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		/*
+		 * Check version of amcheck extension. Skip requested unique constraint
+		 * check with warning if it is not yet supported by amcheck.
+		 */
+		if (opts.checkunique == true)
+		{
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = PQgetvalue(result, 0, 1);
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+			else
+				dat->is_checkunique = true;
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -871,7 +910,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -880,11 +920,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -892,6 +934,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1100,17 +1143,17 @@ verify_btree_slot_handler(PGresult *res, PGconn *conn, void *context)
 
 	if (PQresultStatus(res) == PGRES_TUPLES_OK)
 	{
-		int                     ntups = PQntuples(res);
+		int			ntups = PQntuples(res);
 
 		if (ntups > 1)
 		{
 			/*
 			 * We expect the btree checking functions to return one void row
 			 * each, or zero rows if the check was skipped due to the object
-			 * being in the wrong state to be checked, so we should output some
-			 * sort of warning if we get anything more, not because it
-			 * indicates corruption, but because it suggests a mismatch between
-			 * amcheck and pg_amcheck versions.
+			 * being in the wrong state to be checked, so we should output
+			 * some sort of warning if we get anything more, not because it
+			 * indicates corruption, but because it suggests a mismatch
+			 * between amcheck and pg_amcheck versions.
 			 *
 			 * In conjunction with --progress, anything written to stderr at
 			 * this time would present strangely to the user without an extra
@@ -1187,6 +1230,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index d984eacb24f..eb701cb85e3 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -258,6 +258,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -518,4 +521,46 @@ $node->command_checks_all(
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
 
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique',  'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
+	0, [$no_output_re], [$no_output_re],
+	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', 'db3' ],
+		0,
+		[$no_output_re],
+		[qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
 done_testing();
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index a5e82082700..dcaa333133a 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -22,14 +22,33 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
 	    OPERATOR 5 > (int4, int4), FUNCTION 1 int4_asc_cmp(int4, int4);
 
+	CREATE OPERATOR CLASS int4_unique_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON int4tbl
+						USING btree (i int4_unique_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -58,4 +77,50 @@ $node->command_checks_all(
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
 
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
 done_testing();
-- 
2.25.1

From b2bae648a344bf42b874c41a5d633c949e7609f5 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Mon, 4 Apr 2022 12:27:02 +0400
Subject: [PATCH v11] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

With 'checkunique' option bt_index_check() and bt_index_parent_check()
for btree indexes that has unique constraint will check it i.e.
will check that only one heap entry for all equal keys in the index
(including posting list entries) is visible. Report error if not.

pg_amcheck called with --checkunique option will do the same for
all indexes it checks

Authors:
Anastasia Lubennikova <lubennikovaav@gmail.com>
Pavel Borisov <pashkin.elfe@gmail.com>
Maxim Orlov <orlovmg@gmail.com>
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  42 +++
 contrib/amcheck/sql/check_btree.sql           |  14 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 234 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 329 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  60 +++-
 src/bin/pg_amcheck/t/003_check.pl             |  45 +++
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  65 ++++
 12 files changed, 818 insertions(+), 29 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50b..88271687a3e 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 00000000000..1caba148aa4
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f754..e67ace01c99 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 5a3f1ef737c..d6d578e9995 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -177,11 +177,53 @@ SELECT bt_index_check('toasty', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', false, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- Check null values in unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 97a3e1a20d5..8e09f43c373 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -115,11 +115,25 @@ INSERT INTO toast_bug SELECT repeat('a', 2200);
 -- Should not get false positive report of corruption:
 SELECT bt_index_check('toasty', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+SELECT bt_index_check('bttest_b_idx', false, true);
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+
+-- Check null values in unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', true, true);
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', true, true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
 DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 00000000000..a99e474f1f2
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,234 @@
+
+# Copyright (c) 2021, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More tests => 6;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 looks equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck get uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck get uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication possible only for same values, but
+# with different visibility.
+$node->safe_psql('postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 70278c4f932..ddd9c6783e4 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -79,11 +79,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -138,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -190,7 +212,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -203,17 +225,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -227,13 +252,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -243,7 +271,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -323,7 +351,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/*
@@ -418,7 +446,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -450,6 +479,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -507,6 +538,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot it to check uniqueness of the index For better
+	 * performance, take it once per index check. If snapshot already taken,
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -633,6 +681,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -873,6 +923,162 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced from nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print error message for unique constrain violation in the btree
+ * index under WARNING level and set flag to report ERROR at the end of check
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. If TID of any posting list entry is
+	 * visible, and lVis_tid is already valid report duplicate.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique violation between the
+				 * posting list entries of a first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible, save info about
+	 * it for next comparisons in the loop in bt_page_check(). If also
+	 * lVis_tid is already valid, report duplicate.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1027,6 +1233,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint check that only one of table entries for
+ *   equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1049,6 +1258,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = BTPageGetOpaque(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1439,6 +1655,43 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique, verify entries uniqueness by checking heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) && !skey->anynullkeys)
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple). Key
+			 * containing null value does not violate unique constraint and
+			 * treated as different to any other key.
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1456,12 +1709,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1495,6 +1752,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint check that not more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* First key on next page is same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = BTPageGetOpaque(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1540,9 +1836,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1709,6 +2007,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 5d61a33936f..a8a83e7cc26 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c04655..61dacf1ee44 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index 6607f729382..b3d393c500b 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,7 +133,8 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
-	.no_btree_expansion = false
+	.no_btree_expansion = false,
+	.checkunique = false
 };
 
 static const char *progname = NULL;
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -449,6 +453,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				fprintf(stderr,
 						_("Try \"%s --help\" for more information.\n"),
@@ -614,6 +621,38 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		/*
+		 * Check version of amcheck extension. Skip requested unique constraint
+		 * check with warning if it is not yet supported by amcheck.
+		 */
+		if (opts.checkunique == true)
+		{
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = PQgetvalue(result, 0, 1);
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+			else
+				dat->is_checkunique = true;
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -871,7 +910,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -880,11 +920,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -892,6 +934,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1100,17 +1143,17 @@ verify_btree_slot_handler(PGresult *res, PGconn *conn, void *context)
 
 	if (PQresultStatus(res) == PGRES_TUPLES_OK)
 	{
-		int                     ntups = PQntuples(res);
+		int			ntups = PQntuples(res);
 
 		if (ntups > 1)
 		{
 			/*
 			 * We expect the btree checking functions to return one void row
 			 * each, or zero rows if the check was skipped due to the object
-			 * being in the wrong state to be checked, so we should output some
-			 * sort of warning if we get anything more, not because it
-			 * indicates corruption, but because it suggests a mismatch between
-			 * amcheck and pg_amcheck versions.
+			 * being in the wrong state to be checked, so we should output
+			 * some sort of warning if we get anything more, not because it
+			 * indicates corruption, but because it suggests a mismatch
+			 * between amcheck and pg_amcheck versions.
 			 *
 			 * In conjunction with --progress, anything written to stderr at
 			 * this time would present strangely to the user without an extra
@@ -1187,6 +1230,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index 0cf67065d6b..19a269c1b83 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -257,6 +257,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -517,4 +520,46 @@ $node->command_checks_all(
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
 
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique',  'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
+	0, [$no_output_re], [$no_output_re],
+	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', 'db3' ],
+		0,
+		[$no_output_re],
+		[qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
 done_testing();
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index a5e82082700..dcaa333133a 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -22,14 +22,33 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
 	    OPERATOR 5 > (int4, int4), FUNCTION 1 int4_asc_cmp(int4, int4);
 
+	CREATE OPERATOR CLASS int4_unique_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON int4tbl
+						USING btree (i int4_unique_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -58,4 +77,50 @@ $node->command_checks_all(
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
 
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
 done_testing();
-- 
2.24.3 (Apple Git-128)

From 097dbe8593d2b7d4229247c9e65e2136561c9fe8 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Wed, 11 May 2022 15:54:13 +0400
Subject: [PATCH v12] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

With 'checkunique' option bt_index_check() and bt_index_parent_check()
for btree indexes that has unique constraint will check it i.e.
will check that only one heap entry for all equal keys in the index
(including posting list entries) is visible. Report error if not.

pg_amcheck called with --checkunique option will do the same for
all indexes it checks

Authors:
Anastasia Lubennikova <lubennikovaav@gmail.com>
Pavel Borisov <pashkin.elfe@gmail.com>
Maxim Orlov <orlovmg@gmail.com>
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  42 +++
 contrib/amcheck/sql/check_btree.sql           |  14 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 234 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 329 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  60 +++-
 src/bin/pg_amcheck/t/003_check.pl             |  45 +++
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  65 ++++
 12 files changed, 818 insertions(+), 29 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50b..88271687a3e 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 00000000000..1caba148aa4
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f754..e67ace01c99 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 38791bbc1f4..9e257ac3bb2 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -199,6 +199,47 @@ SELECT bt_index_check('bttest_a_expr_idx', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', false, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- Check null values in unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -206,5 +247,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 033c04b4d05..5afe7f369d7 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -135,6 +135,19 @@ CREATE INDEX bttest_a_expr_idx ON bttest_a ((ifun(id) + ifun(0)))
 
 SELECT bt_index_check('bttest_a_expr_idx', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+SELECT bt_index_check('bttest_b_idx', false, true);
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+
+-- Check null values in unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', true, true);
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', true, true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -142,5 +155,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 00000000000..a99e474f1f2
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,234 @@
+
+# Copyright (c) 2021, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More tests => 6;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 looks equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck get uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck get uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication possible only for same values, but
+# with different visibility.
+$node->safe_psql('postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index a8791000f87..470db0698c7 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -79,11 +79,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -138,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -190,7 +212,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -203,17 +225,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -227,13 +252,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -243,7 +271,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -344,7 +372,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/* Roll back any GUC changes executed by index functions */
@@ -445,7 +473,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -477,6 +506,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -534,6 +565,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot it to check uniqueness of the index For better
+	 * performance, take it once per index check. If snapshot already taken,
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -660,6 +708,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -900,6 +950,162 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced from nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print error message for unique constrain violation in the btree
+ * index under WARNING level and set flag to report ERROR at the end of check
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. If TID of any posting list entry is
+	 * visible, and lVis_tid is already valid report duplicate.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique violation between the
+				 * posting list entries of a first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible, save info about
+	 * it for next comparisons in the loop in bt_page_check(). If also
+	 * lVis_tid is already valid, report duplicate.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1054,6 +1260,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint check that only one of table entries for
+ *   equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1076,6 +1285,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = BTPageGetOpaque(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1466,6 +1682,43 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique, verify entries uniqueness by checking heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) && !skey->anynullkeys)
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple). Key
+			 * containing null value does not violate unique constraint and
+			 * treated as different to any other key.
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1483,12 +1736,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1522,6 +1779,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint check that not more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* First key on next page is same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = BTPageGetOpaque(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1567,9 +1863,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1736,6 +2034,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 5d61a33936f..a8a83e7cc26 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c04655..61dacf1ee44 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index 48cee8c1c4e..3dfd7e918df 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,7 +133,8 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
-	.no_btree_expansion = false
+	.no_btree_expansion = false,
+	.checkunique = false
 };
 
 static const char *progname = NULL;
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -434,6 +438,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				/* getopt_long already emitted a complaint */
 				pg_log_error_hint("Try \"%s --help\" for more information.", progname);
@@ -589,6 +596,38 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		/*
+		 * Check version of amcheck extension. Skip requested unique constraint
+		 * check with warning if it is not yet supported by amcheck.
+		 */
+		if (opts.checkunique == true)
+		{
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = PQgetvalue(result, 0, 1);
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+			else
+				dat->is_checkunique = true;
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -845,7 +884,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -854,11 +894,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -866,6 +908,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1074,17 +1117,17 @@ verify_btree_slot_handler(PGresult *res, PGconn *conn, void *context)
 
 	if (PQresultStatus(res) == PGRES_TUPLES_OK)
 	{
-		int                     ntups = PQntuples(res);
+		int			ntups = PQntuples(res);
 
 		if (ntups > 1)
 		{
 			/*
 			 * We expect the btree checking functions to return one void row
 			 * each, or zero rows if the check was skipped due to the object
-			 * being in the wrong state to be checked, so we should output some
-			 * sort of warning if we get anything more, not because it
-			 * indicates corruption, but because it suggests a mismatch between
-			 * amcheck and pg_amcheck versions.
+			 * being in the wrong state to be checked, so we should output
+			 * some sort of warning if we get anything more, not because it
+			 * indicates corruption, but because it suggests a mismatch
+			 * between amcheck and pg_amcheck versions.
 			 *
 			 * In conjunction with --progress, anything written to stderr at
 			 * this time would present strangely to the user without an extra
@@ -1161,6 +1204,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index 0cf67065d6b..19a269c1b83 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -257,6 +257,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -517,4 +520,46 @@ $node->command_checks_all(
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
 
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique',  'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
+	0, [$no_output_re], [$no_output_re],
+	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', 'db3' ],
+		0,
+		[$no_output_re],
+		[qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
 done_testing();
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index a5e82082700..dcaa333133a 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -22,14 +22,33 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
 	    OPERATOR 5 > (int4, int4), FUNCTION 1 int4_asc_cmp(int4, int4);
 
+	CREATE OPERATOR CLASS int4_unique_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON int4tbl
+						USING btree (i int4_unique_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -58,4 +77,50 @@ $node->command_checks_all(
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
 
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
 done_testing();
-- 
2.24.3 (Apple Git-128)

From 21fe45c0ae8479e0733bd8caeb5d2a19d715e0d9 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Wed, 11 May 2022 15:54:13 +0400
Subject: [PATCH v13] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

With 'checkunique' option bt_index_check() and bt_index_parent_check()
for btree indexes that has unique constraint will check it i.e.
will check that only one heap entry for all equal keys in the index
(including posting list entries) is visible. Report error if not.

pg_amcheck called with --checkunique option will do the same for
all indexes it checks

Authors:
Anastasia Lubennikova <lubennikovaav@gmail.com>
Pavel Borisov <pashkin.elfe@gmail.com>
Maxim Orlov <orlovmg@gmail.com>
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  42 +++
 contrib/amcheck/sql/check_btree.sql           |  14 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 234 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 329 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  50 ++-
 src/bin/pg_amcheck/t/003_check.pl             |  45 +++
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  65 ++++
 12 files changed, 813 insertions(+), 24 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50b..88271687a3e 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 00000000000..1caba148aa4
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- Don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f754..e67ace01c99 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 38791bbc1f4..9e257ac3bb2 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -199,6 +199,47 @@ SELECT bt_index_check('bttest_a_expr_idx', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', false, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- Check null values in unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', true, true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -206,5 +247,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 033c04b4d05..5afe7f369d7 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -135,6 +135,19 @@ CREATE INDEX bttest_a_expr_idx ON bttest_a ((ifun(id) + ifun(0)))
 
 SELECT bt_index_check('bttest_a_expr_idx', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', true, true);
+SELECT bt_index_check('bttest_b_idx', false, true);
+SELECT bt_index_parent_check('bttest_a_idx', true, true, true);
+SELECT bt_index_parent_check('bttest_b_idx', true, false, true);
+
+-- Check null values in unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', true, true);
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', true, true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -142,5 +155,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 00000000000..a99e474f1f2
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,234 @@
+
+# Copyright (c) 2021, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More tests => 6;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 looks equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck get uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck get uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication possible only for same values, but
+# with different visibility.
+$node->safe_psql('postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index a8791000f87..470db0698c7 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -79,11 +79,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -138,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -190,7 +212,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -203,17 +225,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -227,13 +252,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -243,7 +271,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -344,7 +372,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/* Roll back any GUC changes executed by index functions */
@@ -445,7 +473,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -477,6 +506,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -534,6 +565,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot it to check uniqueness of the index For better
+	 * performance, take it once per index check. If snapshot already taken,
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -660,6 +708,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -900,6 +950,162 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced from nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print error message for unique constrain violation in the btree
+ * index under WARNING level and set flag to report ERROR at the end of check
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. If TID of any posting list entry is
+	 * visible, and lVis_tid is already valid report duplicate.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique violation between the
+				 * posting list entries of a first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible, save info about
+	 * it for next comparisons in the loop in bt_page_check(). If also
+	 * lVis_tid is already valid, report duplicate.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1054,6 +1260,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint check that only one of table entries for
+ *   equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1076,6 +1285,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = BTPageGetOpaque(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1466,6 +1682,43 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique, verify entries uniqueness by checking heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) && !skey->anynullkeys)
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple). Key
+			 * containing null value does not violate unique constraint and
+			 * treated as different to any other key.
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1483,12 +1736,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1522,6 +1779,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint check that not more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* First key on next page is same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = BTPageGetOpaque(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1567,9 +1863,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1736,6 +2034,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 5d61a33936f..a8a83e7cc26 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c04655..61dacf1ee44 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index f0b818e987a..3dfd7e918df 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,7 +133,8 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
-	.no_btree_expansion = false
+	.no_btree_expansion = false,
+	.checkunique = false
 };
 
 static const char *progname = NULL;
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -434,6 +438,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				/* getopt_long already emitted a complaint */
 				pg_log_error_hint("Try \"%s --help\" for more information.", progname);
@@ -589,6 +596,38 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		/*
+		 * Check version of amcheck extension. Skip requested unique constraint
+		 * check with warning if it is not yet supported by amcheck.
+		 */
+		if (opts.checkunique == true)
+		{
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = PQgetvalue(result, 0, 1);
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+			else
+				dat->is_checkunique = true;
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -845,7 +884,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -854,11 +894,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -866,6 +908,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1161,6 +1204,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index 0cf67065d6b..19a269c1b83 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -257,6 +257,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -517,4 +520,46 @@ $node->command_checks_all(
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
 
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique',  'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
+	0, [$no_output_re], [$no_output_re],
+	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', 'db3' ],
+		0,
+		[$no_output_re],
+		[qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
 done_testing();
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index ce376f239cf..81d392a34e6 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -22,14 +22,33 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
 	    OPERATOR 5 > (int4, int4), FUNCTION 1 int4_asc_cmp(int4, int4);
 
+	CREATE OPERATOR CLASS int4_unique_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON int4tbl
+						USING btree (i int4_unique_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -57,4 +76,50 @@ $node->command_checks_all(
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
 
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
 done_testing();
-- 
2.24.3 (Apple Git-128)

From 37553352e796e877329e111b8744844ec5fb64ee Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Wed, 11 May 2022 15:54:13 +0400
Subject: [PATCH v14] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

Add 'checkunique' argument to bt_index_check() and bt_index_parent_check().
When the flag is specified the procedures will check the unique constraint
violation for unique indexes. Only one heap entry for all equal keys in
the index should be visible (including posting list entries). Report an error
otherwise.

pg_amcheck called with --checkunique option will do the same check for all
the indexes it checks.

Author: Anastasia Lubennikova <lubennikovaav@gmail.com>
Author: Pavel Borisov <pashkin.elfe@gmail.com>
Author: Maxim Orlov <orlovmg@gmail.com>
Reviewed-by: Mark Dilger <mark.dilger@enterprisedb.com>
Reviewed-by: Zhihong Yu <zyu@yugabyte.com>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://postgr.es/m/CALT9ZEHRn5xAM5boga0qnrCmPV52bScEK2QnQ1HmUZDD301JEg%40mail.gmail.com
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  42 +++
 contrib/amcheck/sql/check_btree.sql           |  14 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 234 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 330 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  50 ++-
 src/bin/pg_amcheck/t/003_check.pl             |  45 +++
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  65 ++++
 12 files changed, 814 insertions(+), 24 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50..88271687a3 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 0000000000..75574eaa64
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- We don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f75..e67ace01c9 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 38791bbc1f..86b38d93f4 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -199,6 +199,47 @@ SELECT bt_index_check('bttest_a_expr_idx', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', heapallindexed => false, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, checkunique => true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, checkunique => true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- Check that null values in an unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -206,5 +247,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 033c04b4d0..aa461f7fb9 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -135,6 +135,19 @@ CREATE INDEX bttest_a_expr_idx ON bttest_a ((ifun(id) + ifun(0)))
 
 SELECT bt_index_check('bttest_a_expr_idx', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', heapallindexed => true, checkunique => true);
+SELECT bt_index_check('bttest_b_idx', heapallindexed => false, checkunique => true);
+SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, checkunique => true);
+SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, checkunique => true);
+
+-- Check that null values in an unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, checkunique => true);
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, checkunique => true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -142,5 +155,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 0000000000..1df106a232
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,234 @@
+
+# Copyright (c) 2022, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More tests => 6;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 look equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck finds the uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck finds the uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication is possible only for same values, but
+# with different visibility.
+$node->safe_psql('postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 2beeebb163..c14bf62ad4 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -79,11 +79,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -138,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -190,7 +212,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -203,17 +225,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -227,13 +252,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -243,7 +271,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -344,7 +372,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/* Roll back any GUC changes executed by index functions */
@@ -445,7 +473,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -477,6 +506,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -534,6 +565,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot to check the uniqueness of the index. For better
+	 * performance take it once per index check. If snapshot already taken
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -660,6 +708,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -900,6 +950,163 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced by nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare and print an error message for unique constrain violation in
+ * a btree index under WARNING level. Also set a flag to report ERROR
+ * at the end of the check.
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. Report duplicate if TID of any posting
+	 * list entry is visible and lVis_tid is valid.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique constraint violation between
+				 * the posting list entries of the first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible save info about
+	 * it for the next comparisons in the loop in bt_page_check(). Report
+	 * duplicate if lVis_tid is already valid.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1054,6 +1261,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint make sure that only one of table entries
+ *   for equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1076,6 +1286,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = BTPageGetOpaque(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1466,6 +1683,43 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique verify entries uniqueness by checking the heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) && !skey->anynullkeys)
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple). Key
+			 * containing null value does not violate unique constraint and
+			 * treated as different to any other key.
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1483,12 +1737,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1522,6 +1780,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint make sure that no more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* The first key on the next page is the same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = BTPageGetOpaque(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1567,9 +1864,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1736,6 +2035,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 5d61a33936..b6f3adc612 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_parent_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c0465..61dacf1ee4 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index 3cff319f02..3ad5927319 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,7 +133,8 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
-	.no_btree_expansion = false
+	.no_btree_expansion = false,
+	.checkunique = false
 };
 
 static const char *progname = NULL;
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -434,6 +438,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				/* getopt_long already emitted a complaint */
 				pg_log_error_hint("Try \"%s --help\" for more information.", progname);
@@ -589,6 +596,38 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		/*
+		 * Check the version of amcheck extension. Skip requested unique
+		 * constraint check with warning if it is not yet supported by amcheck.
+		 */
+		if (opts.checkunique == true)
+		{
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = PQgetvalue(result, 0, 1);
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+			else
+				dat->is_checkunique = true;
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -845,7 +884,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -854,11 +894,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -866,6 +908,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1163,6 +1206,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index 0cf67065d6..19a269c1b8 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -257,6 +257,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -517,4 +520,46 @@ $node->command_checks_all(
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
 
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique',  'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
+	0, [$no_output_re], [$no_output_re],
+	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', 'db3' ],
+		0,
+		[$no_output_re],
+		[qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
 done_testing();
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index ce376f239c..81d392a34e 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -22,14 +22,33 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
 	    OPERATOR 5 > (int4, int4), FUNCTION 1 int4_asc_cmp(int4, int4);
 
+	CREATE OPERATOR CLASS int4_unique_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON int4tbl
+						USING btree (i int4_unique_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -57,4 +76,50 @@ $node->command_checks_all(
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
 
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
 done_testing();
-- 
2.36.1

From 93a10abd0afb14b264e4cf59f7e92f619dd9b11a Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Wed, 11 May 2022 15:54:13 +0400
Subject: [PATCH v15] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

Add 'checkunique' argument to bt_index_check() and bt_index_parent_check().
When the flag is specified the procedures will check the unique constraint
violation for unique indexes. Only one heap entry for all equal keys in
the index should be visible (including posting list entries). Report an error
otherwise.

pg_amcheck called with --checkunique option will do the same check for all
the indexes it checks.

Author: Anastasia Lubennikova <lubennikovaav@gmail.com>
Author: Pavel Borisov <pashkin.elfe@gmail.com>
Author: Maxim Orlov <orlovmg@gmail.com>
Reviewed-by: Mark Dilger <mark.dilger@enterprisedb.com>
Reviewed-by: Zhihong Yu <zyu@yugabyte.com>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://postgr.es/m/CALT9ZEHRn5xAM5boga0qnrCmPV52bScEK2QnQ1HmUZDD301JEg%40mail.gmail.com
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  42 +++
 contrib/amcheck/sql/check_btree.sql           |  14 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 235 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 329 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  50 ++-
 src/bin/pg_amcheck/t/003_check.pl             |  45 +++
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  65 ++++
 12 files changed, 814 insertions(+), 24 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50..88271687a3 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 0000000000..75574eaa64
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- We don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f75..e67ace01c9 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 38791bbc1f..86b38d93f4 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -199,6 +199,47 @@ SELECT bt_index_check('bttest_a_expr_idx', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', heapallindexed => false, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, checkunique => true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, checkunique => true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- Check that null values in an unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -206,5 +247,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 033c04b4d0..aa461f7fb9 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -135,6 +135,19 @@ CREATE INDEX bttest_a_expr_idx ON bttest_a ((ifun(id) + ifun(0)))
 
 SELECT bt_index_check('bttest_a_expr_idx', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', heapallindexed => true, checkunique => true);
+SELECT bt_index_check('bttest_b_idx', heapallindexed => false, checkunique => true);
+SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, checkunique => true);
+SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, checkunique => true);
+
+-- Check that null values in an unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, checkunique => true);
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, checkunique => true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -142,5 +155,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 0000000000..83572959bd
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,235 @@
+
+# Copyright (c) 2022, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 look equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck finds the uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck finds the uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication is possible only for same values, but
+# with different visibility.
+$node->safe_psql('postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
+done_testing();
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 2beeebb163..169a16b82c 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -79,11 +79,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -138,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -190,7 +212,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -203,17 +225,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -227,13 +252,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -243,7 +271,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -344,7 +372,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/* Roll back any GUC changes executed by index functions */
@@ -445,7 +473,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -477,6 +506,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -534,6 +565,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot to check the uniqueness of the index. For better
+	 * performance take it once per index check. If snapshot already taken
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -660,6 +708,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -900,6 +950,162 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced by nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare an error message for unique constrain violation in
+ * a btree index and report ERROR.
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. Report duplicate if TID of any posting
+	 * list entry is visible and lVis_tid is valid.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique constraint violation between
+				 * the posting list entries of the first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible save info about
+	 * it for the next comparisons in the loop in bt_page_check(). Report
+	 * duplicate if lVis_tid is already valid.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1054,6 +1260,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint make sure that only one of table entries
+ *   for equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1076,6 +1285,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = BTPageGetOpaque(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1466,6 +1682,43 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique verify entries uniqueness by checking the heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) && !skey->anynullkeys)
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple). Key
+			 * containing null value does not violate unique constraint and
+			 * treated as different to any other key.
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1483,12 +1736,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1522,6 +1779,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint make sure that no more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* The first key on the next page is the same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = BTPageGetOpaque(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1567,9 +1863,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1736,6 +2034,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 5d61a33936..b6f3adc612 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_parent_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c0465..61dacf1ee4 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index fea35e4b14..956fb6f565 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,7 +133,8 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
-	.no_btree_expansion = false
+	.no_btree_expansion = false,
+	.checkunique = false
 };
 
 static const char *progname = NULL;
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -434,6 +438,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				/* getopt_long already emitted a complaint */
 				pg_log_error_hint("Try \"%s --help\" for more information.", progname);
@@ -589,6 +596,38 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		/*
+		 * Check the version of amcheck extension. Skip requested unique
+		 * constraint check with warning if it is not yet supported by amcheck.
+		 */
+		if (opts.checkunique == true)
+		{
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = PQgetvalue(result, 0, 1);
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+			else
+				dat->is_checkunique = true;
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -845,7 +884,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -854,11 +894,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -866,6 +908,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1163,6 +1206,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index 0cf67065d6..19a269c1b8 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -257,6 +257,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -517,4 +520,46 @@ $node->command_checks_all(
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
 
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique',  'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
+	0, [$no_output_re], [$no_output_re],
+	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', 'db3' ],
+		0,
+		[$no_output_re],
+		[qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
 done_testing();
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index ce376f239c..81d392a34e 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -22,14 +22,33 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
 	    OPERATOR 5 > (int4, int4), FUNCTION 1 int4_asc_cmp(int4, int4);
 
+	CREATE OPERATOR CLASS int4_unique_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON int4tbl
+						USING btree (i int4_unique_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -57,4 +76,50 @@ $node->command_checks_all(
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
 
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
 done_testing();
-- 
2.31.0.windows.1

From 56fe2b608b46c6c97900bbb63b2169e2997bc8cc Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Wed, 11 May 2022 15:54:13 +0400
Subject: [PATCH v16] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

Add 'checkunique' argument to bt_index_check() and bt_index_parent_check().
When the flag is specified the procedures will check the unique constraint
violation for unique indexes. Only one heap entry for all equal keys in
the index should be visible (including posting list entries). Report an error
otherwise.

pg_amcheck called with --checkunique option will do the same check for all
the indexes it checks.

Author: Anastasia Lubennikova <lubennikovaav@gmail.com>
Author: Pavel Borisov <pashkin.elfe@gmail.com>
Author: Maxim Orlov <orlovmg@gmail.com>
Reviewed-by: Mark Dilger <mark.dilger@enterprisedb.com>
Reviewed-by: Zhihong Yu <zyu@yugabyte.com>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://postgr.es/m/CALT9ZEHRn5xAM5boga0qnrCmPV52bScEK2QnQ1HmUZDD301JEg%40mail.gmail.com
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  42 +++
 contrib/amcheck/sql/check_btree.sql           |  14 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 235 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 329 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  48 ++-
 src/bin/pg_amcheck/t/003_check.pl             |  45 +++
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  65 ++++
 12 files changed, 813 insertions(+), 23 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50..88271687a3 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 0000000000..75574eaa64
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- We don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f75..e67ace01c9 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 38791bbc1f..86b38d93f4 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -199,6 +199,47 @@ SELECT bt_index_check('bttest_a_expr_idx', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', heapallindexed => false, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, checkunique => true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, checkunique => true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- Check that null values in an unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -206,5 +247,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 033c04b4d0..aa461f7fb9 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -135,6 +135,19 @@ CREATE INDEX bttest_a_expr_idx ON bttest_a ((ifun(id) + ifun(0)))
 
 SELECT bt_index_check('bttest_a_expr_idx', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', heapallindexed => true, checkunique => true);
+SELECT bt_index_check('bttest_b_idx', heapallindexed => false, checkunique => true);
+SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, checkunique => true);
+SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, checkunique => true);
+
+-- Check that null values in an unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, checkunique => true);
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, checkunique => true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -142,5 +155,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 0000000000..83572959bd
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,235 @@
+
+# Copyright (c) 2022, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 look equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck finds the uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck finds the uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication is possible only for same values, but
+# with different visibility.
+$node->safe_psql('postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
+done_testing();
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 2beeebb163..169a16b82c 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -79,11 +79,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -138,19 +146,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -190,7 +212,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -203,17 +225,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -227,13 +252,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -243,7 +271,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -344,7 +372,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/* Roll back any GUC changes executed by index functions */
@@ -445,7 +473,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -477,6 +506,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -534,6 +565,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot to check the uniqueness of the index. For better
+	 * performance take it once per index check. If snapshot already taken
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -660,6 +708,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -900,6 +950,162 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced by nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare an error message for unique constrain violation in
+ * a btree index and report ERROR.
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. Report duplicate if TID of any posting
+	 * list entry is visible and lVis_tid is valid.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique constraint violation between
+				 * the posting list entries of the first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible save info about
+	 * it for the next comparisons in the loop in bt_page_check(). Report
+	 * duplicate if lVis_tid is already valid.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1054,6 +1260,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint make sure that only one of table entries
+ *   for equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1076,6 +1285,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = BTPageGetOpaque(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1466,6 +1682,43 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique verify entries uniqueness by checking the heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) && !skey->anynullkeys)
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple). Key
+			 * containing null value does not violate unique constraint and
+			 * treated as different to any other key.
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1483,12 +1736,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1522,6 +1779,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint make sure that no more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* The first key on the next page is the same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = BTPageGetOpaque(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1567,9 +1863,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1736,6 +2034,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 5d61a33936..b6f3adc612 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_parent_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c0465..61dacf1ee4 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index fea35e4b14..ac9ce2a341 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,6 +133,7 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
+	.checkunique = false,
 	.no_btree_expansion = false
 };
 
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -434,6 +438,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				/* getopt_long already emitted a complaint */
 				pg_log_error_hint("Try \"%s --help\" for more information.", progname);
@@ -589,6 +596,38 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		/*
+		 * Check the version of amcheck extension. Skip requested unique
+		 * constraint check with warning if it is not yet supported by amcheck.
+		 */
+		if (opts.checkunique == true)
+		{
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = PQgetvalue(result, 0, 1);
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+			else
+				dat->is_checkunique = true;
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -845,7 +884,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -854,11 +894,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -866,6 +908,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1163,6 +1206,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index 0cf67065d6..19a269c1b8 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -257,6 +257,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -517,4 +520,46 @@ $node->command_checks_all(
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
 
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique',  'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
+	0, [$no_output_re], [$no_output_re],
+	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', 'db3' ],
+		0,
+		[$no_output_re],
+		[qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
 done_testing();
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index ce376f239c..81d392a34e 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -22,14 +22,33 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
 	    OPERATOR 5 > (int4, int4), FUNCTION 1 int4_asc_cmp(int4, int4);
 
+	CREATE OPERATOR CLASS int4_unique_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON int4tbl
+						USING btree (i int4_unique_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -57,4 +76,50 @@ $node->command_checks_all(
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
 
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
 done_testing();
-- 
2.25.1

From b7989f959429087328f7b1e521072567154d1167 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Wed, 11 May 2022 15:54:13 +0400
Subject: [PATCH v17] Add option for amcheck and pg_amcheck to check unique
 constraint for btree indexes.

Add 'checkunique' argument to bt_index_check() and bt_index_parent_check().
When the flag is specified the procedures will check the unique constraint
violation for unique indexes. Only one heap entry for all equal keys in
the index should be visible (including posting list entries). Report an error
otherwise.

pg_amcheck called with --checkunique option will do the same check for all
the indexes it checks.

Author: Anastasia Lubennikova <lubennikovaav@gmail.com>
Author: Pavel Borisov <pashkin.elfe@gmail.com>
Author: Maxim Orlov <orlovmg@gmail.com>
Reviewed-by: Mark Dilger <mark.dilger@enterprisedb.com>
Reviewed-by: Zhihong Yu <zyu@yugabyte.com>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://postgr.es/m/CALT9ZEHRn5xAM5boga0qnrCmPV52bScEK2QnQ1HmUZDD301JEg%40mail.gmail.com
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  42 +++
 contrib/amcheck/meson.build                   |   2 +
 contrib/amcheck/sql/check_btree.sql           |  14 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 235 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 329 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  48 ++-
 src/bin/pg_amcheck/t/003_check.pl             |  45 +++
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  65 ++++
 13 files changed, 815 insertions(+), 23 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50..88271687a3 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 0000000000..75574eaa64
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- We don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f75..e67ace01c9 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 38791bbc1f..86b38d93f4 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -199,6 +199,47 @@ SELECT bt_index_check('bttest_a_expr_idx', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', heapallindexed => false, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, checkunique => true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, checkunique => true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- Check that null values in an unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -206,5 +247,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/meson.build b/contrib/amcheck/meson.build
index 1db3d20349..1a550ff762 100644
--- a/contrib/amcheck/meson.build
+++ b/contrib/amcheck/meson.build
@@ -12,6 +12,7 @@ install_data(
   'amcheck--1.0--1.1.sql',
   'amcheck--1.1--1.2.sql',
   'amcheck--1.2--1.3.sql',
+  'amcheck--1.3--1.4.sql',
   kwargs: contrib_data_args,
 )
 
@@ -31,6 +32,7 @@ tests += {
       't/001_verify_heapam.pl',
       't/002_cic.pl',
       't/003_cic_2pc.pl',
+      't/004_verify_nbtree_unique.pl',
     ],
   },
 }
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 033c04b4d0..aa461f7fb9 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -135,6 +135,19 @@ CREATE INDEX bttest_a_expr_idx ON bttest_a ((ifun(id) + ifun(0)))
 
 SELECT bt_index_check('bttest_a_expr_idx', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', heapallindexed => true, checkunique => true);
+SELECT bt_index_check('bttest_b_idx', heapallindexed => false, checkunique => true);
+SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, checkunique => true);
+SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, checkunique => true);
+
+-- Check that null values in an unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, checkunique => true);
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, checkunique => true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -142,5 +155,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 0000000000..83572959bd
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,235 @@
+
+# Copyright (c) 2022, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 look equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck finds the uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck finds the uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication is possible only for same values, but
+# with different visibility.
+$node->safe_psql('postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql('postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql('postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok($stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
+done_testing();
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 9021d156eb..cc75697798 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -80,11 +80,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -139,19 +147,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -191,7 +213,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -204,17 +226,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -228,13 +253,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -244,7 +272,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -345,7 +373,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/* Roll back any GUC changes executed by index functions */
@@ -446,7 +474,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -478,6 +507,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -535,6 +566,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot to check the uniqueness of the index. For better
+	 * performance take it once per index check. If snapshot already taken
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -661,6 +709,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -901,6 +951,162 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced by nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare an error message for unique constrain violation in
+ * a btree index and report ERROR.
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\": "
+					"Index %s%s and%s%s "
+					"(point to heap %s and %s) page lsn=%X/%X.",
+					RelationGetRelationName(state->rel),
+					itid, pposting, nitid, pnposting, htid, nhtid,
+					LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i, ItemPointer *lVis_tid,
+					  OffsetNumber *lVis_offset, BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. Report duplicate if TID of any posting
+	 * list entry is visible and lVis_tid is valid.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique constraint violation between
+				 * the posting list entries of the first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible save info about
+	 * it for the next comparisons in the loop in bt_page_check(). Report
+	 * duplicate if lVis_tid is already valid.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) "
+						"in index \"%s\". It doesn't have visible heap tids and key "
+						"is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)). "
+						"Vacuum the table and repeat the check.",
+						targetblock, offset,
+						RelationGetRelationName(state->rel),
+						*lVis_block, *lVis_offset, posting,
+						ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						ItemPointerGetOffsetNumberNoCheck(*lVis_tid))));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1055,6 +1261,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint make sure that only one of table entries
+ *   for equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1077,6 +1286,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = BTPageGetOpaque(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1467,6 +1683,43 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique verify entries uniqueness by checking the heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) && !skey->anynullkeys)
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset, &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique && P_ISLEAF(topaque) &&
+			OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple). Key
+			 * containing null value does not violate unique constraint and
+			 * treated as different to any other key.
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1484,12 +1737,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1523,6 +1780,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint make sure that no more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later
+				 */
+				rightkey->scantid = NULL;
+
+				/* The first key on the next page is the same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = BTPageGetOpaque(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1568,9 +1864,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1737,6 +2035,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 5d61a33936..b6f3adc612 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_parent_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index cfef6c0465..61dacf1ee4 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index 9ce4b11f1e..6c27f857d1 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,6 +133,7 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
+	.checkunique = false,
 	.no_btree_expansion = false
 };
 
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -434,6 +438,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				/* getopt_long already emitted a complaint */
 				pg_log_error_hint("Try \"%s --help\" for more information.", progname);
@@ -589,6 +596,38 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		/*
+		 * Check the version of amcheck extension. Skip requested unique
+		 * constraint check with warning if it is not yet supported by amcheck.
+		 */
+		if (opts.checkunique == true)
+		{
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = PQgetvalue(result, 0, 1);
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+			else
+				dat->is_checkunique = true;
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -845,7 +884,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -854,11 +894,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -866,6 +908,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1163,6 +1206,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index 0cf67065d6..19a269c1b8 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -257,6 +257,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -517,4 +520,46 @@ $node->command_checks_all(
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
 
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique',  'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', '-d', 'db1', '-d', 'db2', '-d', 'db3', '-S', 's*' ],
+	0, [$no_output_re], [$no_output_re],
+	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', 'db3' ],
+		0,
+		[$no_output_re],
+		[qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
 done_testing();
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index ce376f239c..81d392a34e 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -22,14 +22,33 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
 	    OPERATOR 5 > (int4, int4), FUNCTION 1 int4_asc_cmp(int4, int4);
 
+	CREATE OPERATOR CLASS int4_unique_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON int4tbl
+						USING btree (i int4_unique_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -57,4 +76,50 @@ $node->command_checks_all(
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
 
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
 done_testing();
-- 
2.37.0 (Apple Git-136)

From a405bdcbca38a9f44da70a20312a9bc81e50c76c Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Tue, 24 Oct 2023 22:07:30 +0300
Subject: [PATCH] Teach contrib/amcheck to check the unique constraint
 violation

Add the 'checkunique' argument to bt_index_check() and bt_index_parent_check().
When the flag is specified the procedures will check the unique constraint
violation for unique indexes.  Only one heap entry for all equal keys in
the index should be visible (including posting list entries).  Report an error
otherwise.

pg_amcheck called with the --checkunique option will do the same check for all
the indexes it checks.

Author: Anastasia Lubennikova <lubennikovaav@gmail.com>
Author: Pavel Borisov <pashkin.elfe@gmail.com>
Author: Maxim Orlov <orlovmg@gmail.com>
Reviewed-by: Mark Dilger <mark.dilger@enterprisedb.com>
Reviewed-by: Zhihong Yu <zyu@yugabyte.com>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: Aleksander Alekseev <aleksander@timescale.com>
Discussion: https://postgr.es/m/CALT9ZEHRn5xAM5boga0qnrCmPV52bScEK2QnQ1HmUZDD301JEg%40mail.gmail.com
---
 contrib/amcheck/Makefile                      |   2 +-
 contrib/amcheck/amcheck--1.3--1.4.sql         |  29 ++
 contrib/amcheck/amcheck.control               |   2 +-
 contrib/amcheck/expected/check_btree.out      |  42 +++
 contrib/amcheck/meson.build                   |   2 +
 contrib/amcheck/sql/check_btree.sql           |  14 +
 contrib/amcheck/t/004_verify_nbtree_unique.pl | 244 +++++++++++++
 contrib/amcheck/verify_nbtree.c               | 330 +++++++++++++++++-
 doc/src/sgml/amcheck.sgml                     |  14 +-
 doc/src/sgml/ref/pg_amcheck.sgml              |  11 +
 src/bin/pg_amcheck/pg_amcheck.c               |  48 ++-
 src/bin/pg_amcheck/t/003_check.pl             |  50 +++
 src/bin/pg_amcheck/t/005_opclass_damage.pl    |  65 ++++
 13 files changed, 830 insertions(+), 23 deletions(-)
 create mode 100644 contrib/amcheck/amcheck--1.3--1.4.sql
 create mode 100644 contrib/amcheck/t/004_verify_nbtree_unique.pl

diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
index b82f221e50b..88271687a3e 100644
--- a/contrib/amcheck/Makefile
+++ b/contrib/amcheck/Makefile
@@ -7,7 +7,7 @@ OBJS = \
 	verify_nbtree.o
 
 EXTENSION = amcheck
-DATA = amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
+DATA = amcheck--1.3--1.4.sql amcheck--1.2--1.3.sql amcheck--1.1--1.2.sql amcheck--1.0--1.1.sql amcheck--1.0.sql
 PGFILEDESC = "amcheck - function for verifying relation integrity"
 
 REGRESS = check check_btree check_heap
diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
new file mode 100644
index 00000000000..75574eaa64b
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -0,0 +1,29 @@
+/* contrib/amcheck/amcheck--1.3--1.4.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "ALTER EXTENSION amcheck UPDATE TO '1.4'" to load this file. \quit
+
+-- In order to avoid issues with dependencies when updating amcheck to 1.4,
+-- create new, overloaded versions of the 1.2 bt_index_parent_check signature,
+-- and 1.1 bt_index_check signature.
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass,
+    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass,
+    heapallindexed boolean, checkunique boolean)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT PARALLEL RESTRICTED;
+
+-- We don't want this to be available to public
+REVOKE ALL ON FUNCTION bt_index_parent_check(regclass, boolean, boolean, boolean) FROM PUBLIC;
+REVOKE ALL ON FUNCTION bt_index_check(regclass, boolean, boolean) FROM PUBLIC;
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
index ab50931f754..e67ace01c99 100644
--- a/contrib/amcheck/amcheck.control
+++ b/contrib/amcheck/amcheck.control
@@ -1,5 +1,5 @@
 # amcheck extension
 comment = 'functions for verifying relation integrity'
-default_version = '1.3'
+default_version = '1.4'
 module_pathname = '$libdir/amcheck'
 relocatable = true
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index 38791bbc1f4..86b38d93f41 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -199,6 +199,47 @@ SELECT bt_index_check('bttest_a_expr_idx', true);
  
 (1 row)
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_check('bttest_b_idx', heapallindexed => false, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, checkunique => true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, checkunique => true);
+ bt_index_parent_check 
+-----------------------
+ 
+(1 row)
+
+-- Check that null values in an unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, checkunique => true);
+ bt_index_check 
+----------------
+ 
+(1 row)
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -206,5 +247,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/meson.build b/contrib/amcheck/meson.build
index 5b55cf343a9..4c8e2e2f13c 100644
--- a/contrib/amcheck/meson.build
+++ b/contrib/amcheck/meson.build
@@ -23,6 +23,7 @@ install_data(
   'amcheck--1.0--1.1.sql',
   'amcheck--1.1--1.2.sql',
   'amcheck--1.2--1.3.sql',
+  'amcheck--1.3--1.4.sql',
   kwargs: contrib_data_args,
 )
 
@@ -42,6 +43,7 @@ tests += {
       't/001_verify_heapam.pl',
       't/002_cic.pl',
       't/003_cic_2pc.pl',
+      't/004_verify_nbtree_unique.pl',
     ],
   },
 }
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 033c04b4d05..aa461f7fb97 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -135,6 +135,19 @@ CREATE INDEX bttest_a_expr_idx ON bttest_a ((ifun(id) + ifun(0)))
 
 SELECT bt_index_check('bttest_a_expr_idx', true);
 
+-- UNIQUE constraint check
+SELECT bt_index_check('bttest_a_idx', heapallindexed => true, checkunique => true);
+SELECT bt_index_check('bttest_b_idx', heapallindexed => false, checkunique => true);
+SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, checkunique => true);
+SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, checkunique => true);
+
+-- Check that null values in an unique index are not treated as equal
+CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
+INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
+SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, checkunique => true);
+CREATE INDEX on bttest_unique_nulls (b,c);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, checkunique => true);
+
 -- cleanup
 DROP TABLE bttest_a;
 DROP TABLE bttest_b;
@@ -142,5 +155,6 @@ DROP TABLE bttest_multi;
 DROP TABLE delete_test_table;
 DROP TABLE toast_bug;
 DROP FUNCTION ifun(int8);
+DROP TABLE bttest_unique_nulls;
 DROP OWNED BY regress_bttest_role; -- permissions
 DROP ROLE regress_bttest_role;
diff --git a/contrib/amcheck/t/004_verify_nbtree_unique.pl b/contrib/amcheck/t/004_verify_nbtree_unique.pl
new file mode 100644
index 00000000000..b999ab9c176
--- /dev/null
+++ b/contrib/amcheck/t/004_verify_nbtree_unique.pl
@@ -0,0 +1,244 @@
+
+# Copyright (c) 2023, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings;
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create a custom operator class and an index which uses it.
+$node->safe_psql(
+	'postgres', q(
+	CREATE EXTENSION amcheck;
+
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	---
+	--- Check 1: uniqueness violation.
+	---
+	CREATE FUNCTION ok_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	---
+	--- Make values 768 and 769 look equal.
+	---
+	CREATE FUNCTION bad_cmp1 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 ELSE ok_cmp($1, $2)
+			END;
+	$$;
+
+	---
+	--- Check 2: uniqueness violation without deduplication.
+	---
+	CREATE FUNCTION ok_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp2 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 = $2 AND $1 = 400 THEN -1
+			ELSE ok_cmp($1, $2)
+		END;
+	$$;
+
+	---
+	--- Check 3: uniqueness violation with deduplication.
+	---
+	CREATE FUNCTION ok_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT ok_cmp($1, $2);
+	$$;
+
+	CREATE FUNCTION bad_cmp3 (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT bad_cmp2($1, $2);
+	$$;
+
+	---
+	--- Create data.
+	---
+	CREATE TABLE bttest_unique1 (i int4);
+	INSERT INTO bttest_unique1
+		(SELECT * FROM generate_series(1, 1024) gs);
+
+	CREATE TABLE bttest_unique2 (i int4);
+	INSERT INTO bttest_unique2(i)
+		(SELECT * FROM generate_series(1, 400) gs);
+	INSERT INTO bttest_unique2
+		(SELECT * FROM generate_series(400, 1024) gs);
+
+	CREATE TABLE bttest_unique3 (i int4);
+	INSERT INTO bttest_unique3
+		SELECT * FROM bttest_unique2;
+
+	CREATE OPERATOR CLASS int4_custom_ops1 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp1(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops2 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp2(int4, int4);
+	CREATE OPERATOR CLASS int4_custom_ops3 FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 bad_cmp3(int4, int4);
+
+	CREATE UNIQUE INDEX bttest_unique_idx1
+						ON bttest_unique1
+						USING btree (i int4_custom_ops1)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx2
+						ON bttest_unique2
+						USING btree (i int4_custom_ops2)
+						WITH (deduplicate_items = off);
+	CREATE UNIQUE INDEX bttest_unique_idx3
+						ON bttest_unique3
+						USING btree (i int4_custom_ops3)
+						WITH (deduplicate_items = on);
+));
+
+my ($result, $stdout, $stderr);
+
+#
+# Test 1.
+#  - insert seq values
+#  - create unique index
+#  - break cmp function
+#  - amcheck finds the uniqueness violation
+#
+
+# We have not yet broken the index, so we should get no corruption
+$result = $node->safe_psql(
+	'postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+is($result, '', 'run amcheck on non-broken bttest_unique_idx1');
+
+# Change the operator class to use a function which considers certain different
+# values to be equal.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'bad_cmp1'::regproc
+	WHERE amproc = 'ok_cmp1'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql(
+	'postgres', q(
+	SELECT bt_index_check('bttest_unique_idx1', true, true);
+));
+ok( $stderr =~ /index uniqueness is violated for index "bttest_unique_idx1"/,
+	'detected uniqueness violation for index "bttest_unique_idx1"');
+
+#
+# Test 2.
+#  - break cmp function
+#  - insert seq values with duplicates
+#  - create unique index
+#  - make cmp function correct
+#  - amcheck finds the uniqueness violation
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql(
+	'postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok( $stderr =~ /item order invariant violated for index "bttest_unique_idx2"/,
+	'detected item order invariant violation for index "bttest_unique_idx2"');
+
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp2'::regproc
+	WHERE amproc = 'bad_cmp2'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql(
+	'postgres', q(
+	SELECT bt_index_check('bttest_unique_idx2', true, true);
+));
+ok( $stderr =~ /index uniqueness is violated for index "bttest_unique_idx2"/,
+	'detected uniqueness violation for index "bttest_unique_idx2"');
+
+#
+# Test 3.
+#  - same as Test 2, but with index deduplication
+#
+# Then uniqueness violation is detected between different posting list
+# entries inside one index entry.
+#
+
+# Due to bad cmp function we expect amcheck to detect item order violation,
+# but no uniqueness violation.
+($result, $stdout, $stderr) = $node->psql(
+	'postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok( $stderr =~ /item order invariant violated for index "bttest_unique_idx3"/,
+	'detected item order invariant violation for index "bttest_unique_idx3"');
+
+# For unique index deduplication is possible only for same values, but
+# with different visibility.
+$node->safe_psql(
+	'postgres', q(
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+	DELETE FROM bttest_unique3 WHERE 380 <= i AND i <= 420;
+	INSERT INTO bttest_unique3 (SELECT * FROM generate_series(380, 420));
+	INSERT INTO bttest_unique3 VALUES (400);
+));
+
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc SET
+		   amproc = 'ok_cmp3'::regproc
+	WHERE amproc = 'bad_cmp3'::regproc;
+));
+
+($result, $stdout, $stderr) = $node->psql(
+	'postgres', q(
+	SELECT bt_index_check('bttest_unique_idx3', true, true);
+));
+ok( $stderr =~ /index uniqueness is violated for index "bttest_unique_idx3"/,
+	'detected uniqueness violation for index "bttest_unique_idx3"');
+
+$node->stop;
+done_testing();
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 3e07a3e35f4..7282cf7fc80 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -81,11 +81,19 @@ typedef struct BtreeCheckState
 	bool		heapallindexed;
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
+	/* Also check uniqueness constraint if index is unique */
+	bool		checkunique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
 	BufferAccessStrategy checkstrategy;
 
+	/*
+	 * Info for uniqueness checking. Fill these fields once per index check.
+	 */
+	IndexInfo  *indexinfo;
+	Snapshot	snapshot;
+
 	/*
 	 * Mutable state, for verification of particular page:
 	 */
@@ -140,19 +148,33 @@ PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
-									bool heapallindexed, bool rootdescend);
+									bool heapallindexed, bool rootdescend,
+									bool checkunique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend);
+								 bool rootdescend, bool checkunique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
+static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
+static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
+								BlockNumber block, OffsetNumber offset,
+								int posting, ItemPointer nexttid,
+								BlockNumber nblock, OffsetNumber noffset,
+								int nposting);
+static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+								  BlockNumber targetblock,
+								  OffsetNumber offset, int *lVis_i,
+								  ItemPointer *lVis_tid,
+								  OffsetNumber *lVis_offset,
+								  BlockNumber *lVis_block);
 static void bt_target_page_check(BtreeCheckState *state);
-static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state);
+static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
+												OffsetNumber *rightfirstoffset);
 static void bt_child_check(BtreeCheckState *state, BTScanInsert targetkey,
 						   OffsetNumber downlinkoffnum);
 static void bt_child_highkey_check(BtreeCheckState *state,
@@ -192,7 +214,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -205,17 +227,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
+	bool		checkunique = false;
 
-	if (PG_NARGS() == 2)
+	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
+	if (PG_NARGS() == 3)
+		checkunique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -229,13 +254,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
+	bool		checkunique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
-	if (PG_NARGS() == 3)
+	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
+	if (PG_NARGS() == 4)
+		checkunique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
 
 	PG_RETURN_VOID();
 }
@@ -245,7 +273,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend)
+						bool rootdescend, bool checkunique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -356,7 +384,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend);
+							 heapallindexed, rootdescend, checkunique);
 	}
 
 	/* Roll back any GUC changes executed by index functions */
@@ -457,7 +485,8 @@ btree_index_mainfork_expected(Relation rel)
  */
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
-					 bool readonly, bool heapallindexed, bool rootdescend)
+					 bool readonly, bool heapallindexed, bool rootdescend,
+					 bool checkunique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -489,6 +518,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
+	state->checkunique = checkunique;
+	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
 	{
@@ -546,6 +577,23 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 		}
 	}
 
+	/*
+	 * We need a snapshot to check the uniqueness of the index. For better
+	 * performance take it once per index check. If snapshot already taken
+	 * reuse it.
+	 */
+	if (state->checkunique)
+	{
+		state->indexinfo = BuildIndexInfo(state->rel);
+		if (state->indexinfo->ii_Unique)
+		{
+			if (snapshot != SnapshotAny)
+				state->snapshot = snapshot;
+			else
+				state->snapshot = RegisterSnapshot(GetTransactionSnapshot());
+		}
+	}
+
 	Assert(!state->rootdescend || state->readonly);
 	if (state->rootdescend && !state->heapkeyspace)
 		ereport(ERROR,
@@ -672,6 +720,8 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	}
 
 	/* Be tidy: */
+	if (snapshot == SnapshotAny && state->snapshot != InvalidSnapshot)
+		UnregisterSnapshot(state->snapshot);
 	MemoryContextDelete(state->targetcontext);
 }
 
@@ -912,6 +962,161 @@ nextpage:
 	return nextleveldown;
 }
 
+/* Check visibility of the table entry referenced by nbtree index */
+static bool
+heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
+{
+	bool		tid_visible;
+
+	TupleTableSlot *slot = table_slot_create(state->heaprel, NULL);
+
+	tid_visible = table_tuple_fetch_row_version(state->heaprel,
+												tid, state->snapshot, slot);
+	if (slot != NULL)
+		ExecDropSingleTupleTableSlot(slot);
+
+	return tid_visible;
+}
+
+/*
+ * Prepare an error message for unique constrain violation in
+ * a btree index and report ERROR.
+ */
+static void
+bt_report_duplicate(BtreeCheckState *state,
+					ItemPointer tid, BlockNumber block, OffsetNumber offset,
+					int posting,
+					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
+					int nposting)
+{
+	char	   *htid,
+			   *nhtid,
+			   *itid,
+			   *nitid = "",
+			   *pposting = "",
+			   *pnposting = "";
+
+	htid = psprintf("tid=(%u,%u)",
+					ItemPointerGetBlockNumberNoCheck(tid),
+					ItemPointerGetOffsetNumberNoCheck(tid));
+	nhtid = psprintf("tid=(%u,%u)",
+					 ItemPointerGetBlockNumberNoCheck(nexttid),
+					 ItemPointerGetOffsetNumberNoCheck(nexttid));
+	itid = psprintf("tid=(%u,%u)", block, offset);
+
+	if (nblock != block || noffset != offset)
+		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
+
+	if (posting >= 0)
+		pposting = psprintf(" posting %u", posting);
+
+	if (nposting >= 0)
+		pnposting = psprintf(" posting %u", nposting);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_INDEX_CORRUPTED),
+			 errmsg("index uniqueness is violated for index \"%s\"",
+					RelationGetRelationName(state->rel)),
+			 errdetail("Index %s%s and%s%s (point to heap %s and %s) page lsn=%X/%X.",
+					   itid, pposting, nitid, pnposting, htid, nhtid,
+					   LSN_FORMAT_ARGS(state->targetlsn))));
+}
+
+/* Check if current nbtree leaf entry complies with UNIQUE constraint */
+static void
+bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
+					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i,
+					  ItemPointer *lVis_tid, OffsetNumber *lVis_offset,
+					  BlockNumber *lVis_block)
+{
+	ItemPointer tid;
+	bool		has_visible_entry = false;
+
+	Assert(targetblock != P_NONE);
+
+	/*
+	 * Current tuple has posting list. Report duplicate if TID of any posting
+	 * list entry is visible and lVis_tid is valid.
+	 */
+	if (BTreeTupleIsPosting(itup))
+	{
+		for (int i = 0; i < BTreeTupleGetNPosting(itup); i++)
+		{
+			tid = BTreeTupleGetPostingN(itup, i);
+			if (heap_entry_is_visible(state, tid))
+			{
+				has_visible_entry = true;
+				if (ItemPointerIsValid(*lVis_tid))
+				{
+					bt_report_duplicate(state,
+										*lVis_tid, *lVis_block,
+										*lVis_offset, *lVis_i,
+										tid, targetblock,
+										offset, i);
+				}
+
+				/*
+				 * Prevent double reporting unique constraint violation between
+				 * the posting list entries of the first tuple on the page after
+				 * cross-page check.
+				 */
+				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+					return;
+
+				*lVis_i = i;
+				*lVis_tid = tid;
+				*lVis_offset = offset;
+				*lVis_block = targetblock;
+			}
+		}
+	}
+
+	/*
+	 * Current tuple has no posting list. If TID is visible save info about
+	 * it for the next comparisons in the loop in bt_page_check(). Report
+	 * duplicate if lVis_tid is already valid.
+	 */
+	else
+	{
+		tid = BTreeTupleGetHeapTID(itup);
+		if (heap_entry_is_visible(state, tid))
+		{
+			has_visible_entry = true;
+			if (ItemPointerIsValid(*lVis_tid))
+			{
+				bt_report_duplicate(state,
+									*lVis_tid, *lVis_block,
+									*lVis_offset, *lVis_i,
+									tid, targetblock,
+									offset, -1);
+			}
+			*lVis_i = -1;
+			*lVis_tid = tid;
+			*lVis_offset = offset;
+			*lVis_block = targetblock;
+		}
+	}
+
+	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
+		*lVis_block != targetblock)
+	{
+		char	   *posting = "";
+
+		if (*lVis_i >= 0)
+			posting = psprintf(" posting %u", *lVis_i);
+		ereport(DEBUG1,
+				(errcode(ERRCODE_NO_DATA),
+				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) in index \"%s\"",
+						targetblock, offset,
+						RelationGetRelationName(state->rel)),
+				 errdetail("It doesn't have visible heap tids and key is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)).",
+						   *lVis_block, *lVis_offset, posting,
+						   ItemPointerGetBlockNumberNoCheck(*lVis_tid),
+						   ItemPointerGetOffsetNumberNoCheck(*lVis_tid)),
+				 errhint("VACUUM the table and repeat the check.")));
+	}
+}
+
 /*
  * Raise an error when target page's left link does not point back to the
  * previous target page, called leftcurrent here.  The leftcurrent page's
@@ -1066,6 +1271,9 @@ bt_recheck_sibling_links(BtreeCheckState *state,
  * - Various checks on the structure of tuples themselves.  For example, check
  *	 that non-pivot tuples have no truncated attributes.
  *
+ * - For index with unique constraint make sure that only one of table entries
+ *   for equal keys is visible.
+ *
  * Furthermore, when state passed shows ShareLock held, function also checks:
  *
  * - That all child pages respect strict lower bound from parent's pivot
@@ -1088,6 +1296,13 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
+	/* last visible entry info for checking indexes with unique constraint */
+	int			lVis_i = -1;	/* the position of last visible item for
+								 * posting tuple. for non-posting tuple (-1) */
+	ItemPointer lVis_tid = NULL;
+	BlockNumber lVis_block = InvalidBlockNumber;
+	OffsetNumber lVis_offset = InvalidOffsetNumber;
+
 	topaque = BTPageGetOpaque(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
 
@@ -1478,6 +1693,45 @@ bt_target_page_check(BtreeCheckState *state)
 										LSN_FORMAT_ARGS(state->targetlsn))));
 		}
 
+		/*
+		 * If the index is unique verify entries uniqueness by checking the heap
+		 * tuples visibility.
+		 */
+		if (state->checkunique && state->indexinfo->ii_Unique &&
+			P_ISLEAF(topaque) && !skey->anynullkeys)
+			bt_entry_unique_check(state, itup, state->targetblock, offset,
+								  &lVis_i, &lVis_tid, &lVis_offset,
+								  &lVis_block);
+
+		if (state->checkunique && state->indexinfo->ii_Unique &&
+			P_ISLEAF(topaque) && OffsetNumberNext(offset) <= max)
+		{
+			/* Save current scankey tid */
+			scantid = skey->scantid;
+
+			/*
+			 * Invalidate scankey tid to make _bt_compare compare only keys in
+			 * the item to report equality even if heap TIDs are different
+			 */
+			skey->scantid = NULL;
+
+			/*
+			 * If next key tuple is different, invalidate last visible entry
+			 * data (whole index tuple or last posting in index tuple). Key
+			 * containing null value does not violate unique constraint and
+			 * treated as different to any other key.
+			 */
+			if (_bt_compare(state->rel, skey, state->target,
+							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
+			{
+				lVis_i = -1;
+				lVis_tid = NULL;
+				lVis_block = InvalidBlockNumber;
+				lVis_offset = InvalidOffsetNumber;
+			}
+			skey->scantid = scantid;	/* Restore saved scan key state */
+		}
+
 		/*
 		 * * Last item check *
 		 *
@@ -1495,12 +1749,16 @@ bt_target_page_check(BtreeCheckState *state)
 		 * available from sibling for various reasons, though (e.g., target is
 		 * the rightmost page on level).
 		 */
-		else if (offset == max)
+		if (offset == max)
 		{
 			BTScanInsert rightkey;
+			BlockNumber rightblock_number;
+
+			/* first offset on a right index page (log only) */
+			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
 
 			/* Get item in next/right page */
-			rightkey = bt_right_page_check_scankey(state);
+			rightkey = bt_right_page_check_scankey(state, &rightfirstoffset);
 
 			if (rightkey &&
 				!invariant_g_offset(state, rightkey, max))
@@ -1534,6 +1792,45 @@ bt_target_page_check(BtreeCheckState *state)
 											state->targetblock, offset,
 											LSN_FORMAT_ARGS(state->targetlsn))));
 			}
+
+			/*
+			 * If index has unique constraint make sure that no more than one
+			 * found equal items is visible.
+			 */
+			rightblock_number = topaque->btpo_next;
+			if (state->checkunique && state->indexinfo->ii_Unique &&
+				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+			{
+				elog(DEBUG2, "check cross page unique condition");
+
+				/*
+				 * Make _bt_compare compare only index keys without heap TIDs.
+				 * rightkey->scantid is modified destructively but it is ok
+				 * for it is not used later.
+				 */
+				rightkey->scantid = NULL;
+
+				/* The first key on the next page is the same */
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
+				{
+					elog(DEBUG2, "cross page equal keys");
+					state->target = palloc_btree_page(state,
+													  rightblock_number);
+					topaque = BTPageGetOpaque(state->target);
+
+					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
+						break;
+
+					itemid = PageGetItemIdCareful(state, rightblock_number,
+												  state->target,
+												  rightfirstoffset);
+					itup = (IndexTuple) PageGetItem(state->target, itemid);
+
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
+										  &lVis_i, &lVis_tid, &lVis_offset,
+										  &lVis_block);
+				}
+			}
 		}
 
 		/*
@@ -1579,9 +1876,11 @@ bt_target_page_check(BtreeCheckState *state)
  *
  * Note that !readonly callers must reverify that target page has not
  * been concurrently deleted.
+ *
+ * Save rightfirstdataoffset for detailed error message.
  */
 static BTScanInsert
-bt_right_page_check_scankey(BtreeCheckState *state)
+bt_right_page_check_scankey(BtreeCheckState *state, OffsetNumber *rightfirstoffset)
 {
 	BTPageOpaque opaque;
 	ItemId		rightitem;
@@ -1748,6 +2047,7 @@ bt_right_page_check_scankey(BtreeCheckState *state)
 		/* Return first data item (if any) */
 		rightitem = PageGetItemIdCareful(state, targetnext, rightpage,
 										 P_FIRSTDATAKEY(opaque));
+		*rightfirstoffset = P_FIRSTDATAKEY(opaque);
 	}
 	else if (!P_ISLEAF(opaque) &&
 			 nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 2b9c1a9205f..780fd05a73b 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -58,7 +58,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -115,7 +115,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When a routine, lightweight test for
+      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When a routine, lightweight test for
       corruption is required in a live production environment, using
       <function>bt_index_check</function> often provides the best
       trade-off between thoroughness of verification and limiting the
@@ -126,7 +129,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -139,7 +142,10 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When the optional <parameter>rootdescend</parameter>
+      index.  When <parameter>checkunique</parameter>
+      is <literal>true</literal> <function>bt_index_parent_check</function> will
+      check that no more than one among duplicate entries in unique
+      index is visible.  When the optional <parameter>rootdescend</parameter>
       argument is <literal>true</literal>, verification re-finds
       tuples on the leaf level by performing a new search from the
       root page for each tuple.  The checks that can be performed by
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index 20c2897accb..067c806b46d 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -432,6 +432,17 @@ PostgreSQL documentation
       </para>
      </listitem>
     </varlistentry>
+
+    <varlistentry>
+     <term><option>--checkunique</option></term>
+     <listitem>
+      <para>
+       For each index with unique constraint checked, verify that no more than
+       one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
+       <option>checkunique</option> option.
+      </para>
+     </listitem>
+    </varlistentry>
    </variablelist>
   </para>
 
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index 8ac7051ff4d..57c7c1917c4 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,6 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
+	bool		checkunique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -132,6 +133,7 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
+	.checkunique = false,
 	.no_btree_expansion = false
 };
 
@@ -148,6 +150,7 @@ typedef struct DatabaseInfo
 {
 	char	   *datname;
 	char	   *amcheck_schema; /* escaped, quoted literal */
+	bool		is_checkunique;
 } DatabaseInfo;
 
 typedef struct RelationInfo
@@ -267,6 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
+		{"checkunique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -434,6 +438,9 @@ main(int argc, char *argv[])
 				if (optarg)
 					opts.install_schema = pg_strdup(optarg);
 				break;
+			case 14:
+				opts.checkunique = true;
+				break;
 			default:
 				/* getopt_long already emitted a complaint */
 				pg_log_error_hint("Try \"%s --help\" for more information.", progname);
@@ -589,6 +596,38 @@ main(int argc, char *argv[])
 						PQdb(conn), PQgetvalue(result, 0, 1), amcheck_schema);
 		dat->amcheck_schema = PQescapeIdentifier(conn, amcheck_schema,
 												 strlen(amcheck_schema));
+
+		/*
+		 * Check the version of amcheck extension. Skip requested unique
+		 * constraint check with warning if it is not yet supported by amcheck.
+		 */
+		if (opts.checkunique == true)
+		{
+			/*
+			 * Now amcheck has only major and minor versions in the string but
+			 * we also support revision just in case. Now it is expected to be
+			 * zero.
+			 */
+			int			vmaj = 0,
+						vmin = 0,
+						vrev = 0;
+			const char *amcheck_version = PQgetvalue(result, 0, 1);
+
+			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
+
+			/*
+			 * checkunique option is supported in amcheck since version 1.4
+			 */
+			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
+			{
+				pg_log_warning("--checkunique option is not supported by amcheck "
+							   "version \"%s\"", amcheck_version);
+				dat->is_checkunique = false;
+			}
+			else
+				dat->is_checkunique = true;
+		}
+
 		PQclear(result);
 
 		compile_relation_list_one_db(conn, &relations, dat, &pagestotal);
@@ -845,7 +884,8 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 	if (opts.parent_check)
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_parent_check("
-						  "index := c.oid, heapallindexed := %s, rootdescend := %s)"
+						  "index := c.oid, heapallindexed := %s, rootdescend := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -854,11 +894,13 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
 						  "SELECT %s.bt_index_check("
-						  "index := c.oid, heapallindexed := %s)"
+						  "index := c.oid, heapallindexed := %s "
+						  "%s)"
 						  "\nFROM pg_catalog.pg_class c, pg_catalog.pg_index i "
 						  "WHERE c.oid = %u "
 						  "AND c.oid = i.indexrelid "
@@ -866,6 +908,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
+						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1163,6 +1206,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
+	printf(_("      --checkunique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index d577cffa30d..2b7ef198552 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -257,6 +257,9 @@ for my $dbname (qw(db1 db2 db3))
 
 			CREATE INDEX t1_spgist ON $schema.t1 USING SPGIST (ir);
 			CREATE INDEX t2_spgist ON $schema.t2 USING SPGIST (ir);
+
+			CREATE UNIQUE INDEX t1_btree_unique ON $schema.t1 USING BTREE (i);
+			CREATE UNIQUE INDEX t2_btree_unique ON $schema.t2 USING BTREE (i);
 		));
 	}
 }
@@ -517,4 +520,51 @@ $node->command_checks_all(
 	0, [$no_output_re], [$no_output_re],
 	'pg_amcheck excluding all corrupt schemas');
 
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
+		'--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --parent-check --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
+		'--rootdescend', '--checkunique', 'db1'
+	],
+	2,
+	[$index_missing_relation_fork_re],
+	[$no_output_re],
+	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+
+$node->command_checks_all(
+	[
+		@cmd, '--checkunique', '-d', 'db1', '-d', 'db2',
+		'-d', 'db3', '-S', 's*'
+	],
+	0,
+	[$no_output_re],
+	[$no_output_re],
+	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+
+#
+# Smoke test for checkunique option for not supported versions.
+#
+$node->safe_psql(
+	'db3', q(
+		DROP EXTENSION amcheck;
+		CREATE EXTENSION amcheck WITH SCHEMA amcheck_schema VERSION '1.3' ;
+));
+
+$node->command_checks_all(
+	[ @cmd, '--checkunique', 'db3' ],
+	0,
+	[$no_output_re],
+	[
+		qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+	],
+	'pg_amcheck smoke test --checkunique');
 done_testing();
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index fd476179f49..a5ef2c0f33d 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -22,14 +22,33 @@ $node->safe_psql(
 	CREATE FUNCTION int4_asc_cmp (a int4, b int4) RETURNS int LANGUAGE sql AS $$
 		SELECT CASE WHEN $1 = $2 THEN 0 WHEN $1 > $2 THEN 1 ELSE -1 END; $$;
 
+	CREATE FUNCTION ok_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
 	CREATE OPERATOR CLASS int4_fickle_ops FOR TYPE int4 USING btree AS
 	    OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
 	    OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
 	    OPERATOR 5 > (int4, int4), FUNCTION 1 int4_asc_cmp(int4, int4);
 
+	CREATE OPERATOR CLASS int4_unique_ops FOR TYPE int4 USING btree AS
+		OPERATOR 1 < (int4, int4), OPERATOR 2 <= (int4, int4),
+		OPERATOR 3 = (int4, int4), OPERATOR 4 >= (int4, int4),
+		OPERATOR 5 > (int4, int4), FUNCTION 1 ok_cmp(int4, int4);
+
 	CREATE TABLE int4tbl (i int4);
 	INSERT INTO int4tbl (SELECT * FROM generate_series(1,1000) gs);
 	CREATE INDEX fickleidx ON int4tbl USING btree (i int4_fickle_ops);
+	CREATE UNIQUE INDEX bttest_unique_idx
+						ON int4tbl
+						USING btree (i int4_unique_ops)
+						WITH (deduplicate_items = off);
 ));
 
 # We have not yet broken the index, so we should get no corruption
@@ -57,4 +76,50 @@ $node->command_checks_all(
 	'pg_amcheck all schemas, tables and indexes reports fickleidx corruption'
 );
 
+#
+# Check unique constraints
+#
+
+# Repair broken opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'int4_asc_cmp'::regproc
+		WHERE amproc = 'int4_desc_cmp'::regproc
+));
+
+# We should get no corruptions
+$node->command_like(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	qr/^$/,
+	'pg_amcheck all schemas, tables and indexes reports no corruption');
+
+# Break opclass for check unique tests.
+$node->safe_psql(
+	'postgres', q(
+	CREATE FUNCTION bad_cmp (int4, int4)
+	RETURNS int LANGUAGE sql AS
+	$$
+		SELECT
+			CASE WHEN ($1 = 768 AND $2 = 769) OR
+					  ($1 = 769 AND $2 = 768) THEN 0
+				 WHEN $1 < $2 THEN -1
+				 WHEN $1 > $2 THEN  1
+				 ELSE 0
+			END;
+	$$;
+
+	UPDATE pg_catalog.pg_amproc
+		SET amproc = 'bad_cmp'::regproc
+		WHERE amproc = 'ok_cmp'::regproc
+));
+
+# Unique index corruption should now be reported
+$node->command_checks_all(
+	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	2,
+	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
+	[],
+	'pg_amcheck all schemas, tables and indexes reports bttest_unique_idx corruption'
+);
 done_testing();
-- 
2.39.3 (Apple Git-145)

From 92a085925e84cd6c34c59861f2775e53a1048665 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Thu, 25 Apr 2024 14:07:45 +0400
Subject: [PATCH v1 2/5] Amcheck: code refactoring

Use structure to store and transfer info about last visible heap entry
among the equal index/posting list entries.

Reported-by: Alexander Korotkov
Discussion: https://www.postgresql.org/message-id/CAPpHfdsVbB9ToriaB1UHuOKwjKxiZmTFQcEF%3DjuzzC_nby31uA%40mail.gmail.com
---
 contrib/amcheck/verify_nbtree.c | 112 ++++++++++++++------------------
 1 file changed, 49 insertions(+), 63 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index ae8012f15b..66f2e619a8 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -145,6 +145,15 @@ typedef struct BtreeLevel
 	bool		istruerootlevel;
 } BtreeLevel;
 
+/* Info for last visible entry for checking unique constraint */
+typedef struct lVisInfo
+{
+	ItemPointer tid; 	/* Heap tid */
+	BlockNumber block;	/* Index block */
+	OffsetNumber offset;	/* Offset on index block */
+	int i;			/* Number in posting list. (-1 for non-deduplicated) */
+} lVisInfo;
+
 PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
@@ -165,17 +174,13 @@ static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
 static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
-static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
-								BlockNumber block, OffsetNumber offset,
-								int posting, ItemPointer nexttid,
+static void bt_report_duplicate(BtreeCheckState *state, lVisInfo *lVis,
+								ItemPointer nexttid,
 								BlockNumber nblock, OffsetNumber noffset,
 								int nposting);
 static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 								  BlockNumber targetblock,
-								  OffsetNumber offset, int *lVis_i,
-								  ItemPointer *lVis_tid,
-								  OffsetNumber *lVis_offset,
-								  BlockNumber *lVis_block);
+								  OffsetNumber offset, lVisInfo *lVis);
 static void bt_target_page_check(BtreeCheckState *state);
 static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
 												OffsetNumber *rightfirstoffset);
@@ -997,8 +1002,7 @@ heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
  */
 static void
 bt_report_duplicate(BtreeCheckState *state,
-					ItemPointer tid, BlockNumber block, OffsetNumber offset,
-					int posting,
+					lVisInfo *lVis,
 					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
 					int nposting)
 {
@@ -1010,18 +1014,18 @@ bt_report_duplicate(BtreeCheckState *state,
 			   *pnposting = "";
 
 	htid = psprintf("tid=(%u,%u)",
-					ItemPointerGetBlockNumberNoCheck(tid),
-					ItemPointerGetOffsetNumberNoCheck(tid));
+					ItemPointerGetBlockNumberNoCheck(lVis->tid),
+					ItemPointerGetOffsetNumberNoCheck(lVis->tid));
 	nhtid = psprintf("tid=(%u,%u)",
 					 ItemPointerGetBlockNumberNoCheck(nexttid),
 					 ItemPointerGetOffsetNumberNoCheck(nexttid));
-	itid = psprintf("tid=(%u,%u)", block, offset);
+	itid = psprintf("tid=(%u,%u)", lVis->block, lVis->offset);
 
-	if (nblock != block || noffset != offset)
+	if (nblock != lVis->block || noffset != lVis->offset)
 		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
 
-	if (posting >= 0)
-		pposting = psprintf(" posting %u", posting);
+	if (lVis->i >= 0)
+		pposting = psprintf(" posting %u", lVis->i);
 
 	if (nposting >= 0)
 		pnposting = psprintf(" posting %u", nposting);
@@ -1038,9 +1042,7 @@ bt_report_duplicate(BtreeCheckState *state,
 /* Check if current nbtree leaf entry complies with UNIQUE constraint */
 static void
 bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
-					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i,
-					  ItemPointer *lVis_tid, OffsetNumber *lVis_offset,
-					  BlockNumber *lVis_block)
+					  BlockNumber targetblock, OffsetNumber offset, lVisInfo *lVis)
 {
 	ItemPointer tid;
 	bool		has_visible_entry = false;
@@ -1049,7 +1051,7 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 
 	/*
 	 * Current tuple has posting list. Report duplicate if TID of any posting
-	 * list entry is visible and lVis_tid is valid.
+	 * list entry is visible and lVis->tid is valid.
 	 */
 	if (BTreeTupleIsPosting(itup))
 	{
@@ -1059,11 +1061,10 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 			if (heap_entry_is_visible(state, tid))
 			{
 				has_visible_entry = true;
-				if (ItemPointerIsValid(*lVis_tid))
+				if (ItemPointerIsValid(lVis->tid))
 				{
 					bt_report_duplicate(state,
-										*lVis_tid, *lVis_block,
-										*lVis_offset, *lVis_i,
+										lVis,
 										tid, targetblock,
 										offset, i);
 				}
@@ -1073,13 +1074,13 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 				 * between the posting list entries of the first tuple on the
 				 * page after cross-page check.
 				 */
-				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+				if (lVis->block != targetblock && ItemPointerIsValid(lVis->tid))
 					return;
 
-				*lVis_i = i;
-				*lVis_tid = tid;
-				*lVis_offset = offset;
-				*lVis_block = targetblock;
+				lVis->i = i;
+				lVis->tid = tid;
+				lVis->offset = offset;
+				lVis->block = targetblock;
 			}
 		}
 	}
@@ -1095,37 +1096,36 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 		if (heap_entry_is_visible(state, tid))
 		{
 			has_visible_entry = true;
-			if (ItemPointerIsValid(*lVis_tid))
+			if (ItemPointerIsValid(lVis->tid))
 			{
 				bt_report_duplicate(state,
-									*lVis_tid, *lVis_block,
-									*lVis_offset, *lVis_i,
+									lVis,
 									tid, targetblock,
 									offset, -1);
 			}
-			*lVis_i = -1;
-			*lVis_tid = tid;
-			*lVis_offset = offset;
-			*lVis_block = targetblock;
+			lVis->i = -1;
+			lVis->tid = tid;
+			lVis->offset = offset;
+			lVis->block = targetblock;
 		}
 	}
 
-	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
-		*lVis_block != targetblock)
+	if (!has_visible_entry && lVis->block != InvalidBlockNumber &&
+		lVis->block != targetblock)
 	{
 		char	   *posting = "";
 
-		if (*lVis_i >= 0)
-			posting = psprintf(" posting %u", *lVis_i);
+		if (lVis->i >= 0)
+			posting = psprintf(" posting %u", lVis->i);
 		ereport(DEBUG1,
 				(errcode(ERRCODE_NO_DATA),
 				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) in index \"%s\"",
 						targetblock, offset,
 						RelationGetRelationName(state->rel)),
 				 errdetail("It doesn't have visible heap tids and key is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)).",
-						   *lVis_block, *lVis_offset, posting,
-						   ItemPointerGetBlockNumberNoCheck(*lVis_tid),
-						   ItemPointerGetOffsetNumberNoCheck(*lVis_tid)),
+						   lVis->block, lVis->offset, posting,
+						   ItemPointerGetBlockNumberNoCheck(lVis->tid),
+						   ItemPointerGetOffsetNumberNoCheck(lVis->tid)),
 				 errhint("VACUUM the table and repeat the check.")));
 	}
 }
@@ -1373,11 +1373,7 @@ bt_target_page_check(BtreeCheckState *state)
 	BTPageOpaque topaque;
 
 	/* last visible entry info for checking indexes with unique constraint */
-	int			lVis_i = -1;	/* the position of last visible item for
-								 * posting tuple. for non-posting tuple (-1) */
-	ItemPointer lVis_tid = NULL;
-	BlockNumber lVis_block = InvalidBlockNumber;
-	OffsetNumber lVis_offset = InvalidOffsetNumber;
+	lVisInfo 	lVis = {NULL, InvalidBlockNumber, InvalidOffsetNumber, -1};
 
 	topaque = BTPageGetOpaque(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
@@ -1776,11 +1772,9 @@ bt_target_page_check(BtreeCheckState *state)
 		 */
 		if (state->checkunique && state->indexinfo->ii_Unique &&
 			P_ISLEAF(topaque) && !skey->anynullkeys &&
-			(BTreeTupleIsPosting(itup) || ItemPointerIsValid(lVis_tid)))
+			(BTreeTupleIsPosting(itup) || ItemPointerIsValid(lVis.tid)))
 		{
-			bt_entry_unique_check(state, itup, state->targetblock, offset,
-								  &lVis_i, &lVis_tid, &lVis_offset,
-								  &lVis_block);
+			bt_entry_unique_check(state, itup, state->targetblock, offset, &lVis);
 			unique_checked = true;
 		}
 
@@ -1805,17 +1799,13 @@ bt_target_page_check(BtreeCheckState *state)
 			if (_bt_compare(state->rel, skey, state->target,
 							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
 			{
-				lVis_i = -1;
-				lVis_tid = NULL;
-				lVis_block = InvalidBlockNumber;
-				lVis_offset = InvalidOffsetNumber;
+				lVis = (lVisInfo) {NULL, InvalidBlockNumber, InvalidOffsetNumber, -1};
 			}
 			else if (!unique_checked)
 			{
-				bt_entry_unique_check(state, itup, state->targetblock, offset,
-									  &lVis_i, &lVis_tid, &lVis_offset,
-									  &lVis_block);
+				bt_entry_unique_check(state, itup, state->targetblock, offset, &lVis);
 			}
+
 			skey->scantid = scantid;	/* Restore saved scan key state */
 		}
 
@@ -1901,9 +1891,7 @@ bt_target_page_check(BtreeCheckState *state)
 				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
 				{
 					if (!unique_checked)
-						bt_entry_unique_check(state, itup, state->targetblock, offset,
-											  &lVis_i, &lVis_tid, &lVis_offset,
-											  &lVis_block);
+						bt_entry_unique_check(state, itup, state->targetblock, offset, &lVis);
 
 					elog(DEBUG2, "cross page equal keys");
 					state->target = palloc_btree_page(state,
@@ -1918,9 +1906,7 @@ bt_target_page_check(BtreeCheckState *state)
 												  rightfirstoffset);
 					itup = (IndexTuple) PageGetItem(state->target, itemid);
 
-					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
-										  &lVis_i, &lVis_tid, &lVis_offset,
-										  &lVis_block);
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset, &lVis);
 				}
 			}
 		}
-- 
2.34.1

From 526409e76419b387d9d4a40e94881231f256fcc8 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Thu, 25 Apr 2024 14:20:45 +0400
Subject: [PATCH v1 3/5] Amcheck: Don't load rightpage into BtreeCheckState

For cross-page unique constraint check use a local variable in the
similar way as implemented in bt_right_page_check_scankey().

Reported-by: Peter Geoghegan
---
 contrib/amcheck/verify_nbtree.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 66f2e619a8..d6f70206db 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -1890,23 +1890,27 @@ bt_target_page_check(BtreeCheckState *state)
 				/* The first key on the next page is the same */
 				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
 				{
+					Page	rightpage;
+
 					if (!unique_checked)
 						bt_entry_unique_check(state, itup, state->targetblock, offset, &lVis);
 
 					elog(DEBUG2, "cross page equal keys");
-					state->target = palloc_btree_page(state,
+					rightpage = palloc_btree_page(state,
 													  rightblock_number);
-					topaque = BTPageGetOpaque(state->target);
+					topaque = BTPageGetOpaque(rightpage);
 
 					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
 						break;
 
 					itemid = PageGetItemIdCareful(state, rightblock_number,
-												  state->target,
+												  rightpage,
 												  rightfirstoffset);
-					itup = (IndexTuple) PageGetItem(state->target, itemid);
+					itup = (IndexTuple) PageGetItem(rightpage, itemid);
 
 					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset, &lVis);
+
+					pfree(rightpage);
 				}
 			}
 		}
-- 
2.34.1

From a2950357675198285dfdbc974346bce12dbd3c55 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Thu, 25 Apr 2024 15:06:36 +0400
Subject: [PATCH v1 4/5] Amcheck: Report error when next page to a leaf is not
 a leaf

This is a very unlikely condition during checking unique constraint,
meaning that index connectivity is violated badly and we shouldn't
continue checking to avoid neverending loops etc. So it's better
to  honestly throw an error.

Reported-by: Peter Geoghegan
---
 contrib/amcheck/verify_nbtree.c | 23 ++++++++++++++++-------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index d6f70206db..dfc9ed769f 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -1829,7 +1829,6 @@ bt_target_page_check(BtreeCheckState *state)
 		if (offset == max)
 		{
 			BTScanInsert rightkey;
-			BlockNumber rightblock_number;
 
 			/* first offset on a right index page (log only) */
 			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
@@ -1874,12 +1873,12 @@ bt_target_page_check(BtreeCheckState *state)
 			 * If index has unique constraint make sure that no more than one
 			 * found equal items is visible.
 			 */
-			rightblock_number = topaque->btpo_next;
 			if (state->checkunique && state->indexinfo->ii_Unique &&
-				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+				rightkey && P_ISLEAF(topaque) && !P_RIGHTMOST(topaque))
 			{
-				elog(DEBUG2, "check cross page unique condition");
+				BlockNumber rightblock_number = topaque->btpo_next;
 
+				elog(DEBUG2, "check cross page unique condition");
 				/*
 				 * Make _bt_compare compare only index keys without heap TIDs.
 				 * rightkey->scantid is modified destructively but it is ok
@@ -1900,9 +1899,19 @@ bt_target_page_check(BtreeCheckState *state)
 													  rightblock_number);
 					topaque = BTPageGetOpaque(rightpage);
 
-					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
-						break;
-
+					if (P_IGNORE(topaque))
+					{
+						if (unlikely(!P_ISLEAF(topaque)))
+							ereport(ERROR,
+								(errcode(ERRCODE_INDEX_CORRUPTED),
+								errmsg("right block of leaf block is non-leaf for index \"%s\"",
+								RelationGetRelationName(state->rel)),
+								errdetail_internal("Block=%u page lsn=%X/%X.",
+								state->targetblock,
+								LSN_FORMAT_ARGS(state->targetlsn))));
+						else
+							break;
+					}
 					itemid = PageGetItemIdCareful(state, rightblock_number,
 												  rightpage,
 												  rightfirstoffset);
-- 
2.34.1

From 2847e83ef91bfa3e3ebda6a8acd60bd835950716 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Thu, 25 Apr 2024 13:08:39 +0400
Subject: [PATCH v1 1/5] Amcheck: optimize speed of checking unique constraint

Check heap visibility only for non-equal non-deduplicated index
tuples. For deduplicated tuples we still need to check visibility
of all posting list entries because they represent equal
key values.

Reported-by: Noah Misch
---
 contrib/amcheck/verify_nbtree.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 20da4a46ba..ae8012f15b 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -1428,6 +1428,7 @@ bt_target_page_check(BtreeCheckState *state)
 		BTScanInsert skey;
 		bool		lowersizelimit;
 		ItemPointer scantid;
+		bool		unique_checked = false;
 
 		CHECK_FOR_INTERRUPTS();
 
@@ -1774,10 +1775,14 @@ bt_target_page_check(BtreeCheckState *state)
 		 * heap tuples visibility.
 		 */
 		if (state->checkunique && state->indexinfo->ii_Unique &&
-			P_ISLEAF(topaque) && !skey->anynullkeys)
+			P_ISLEAF(topaque) && !skey->anynullkeys &&
+			(BTreeTupleIsPosting(itup) || ItemPointerIsValid(lVis_tid)))
+		{
 			bt_entry_unique_check(state, itup, state->targetblock, offset,
 								  &lVis_i, &lVis_tid, &lVis_offset,
 								  &lVis_block);
+			unique_checked = true;
+		}
 
 		if (state->checkunique && state->indexinfo->ii_Unique &&
 			P_ISLEAF(topaque) && OffsetNumberNext(offset) <= max)
@@ -1805,6 +1810,12 @@ bt_target_page_check(BtreeCheckState *state)
 				lVis_block = InvalidBlockNumber;
 				lVis_offset = InvalidOffsetNumber;
 			}
+			else if (!unique_checked)
+			{
+				bt_entry_unique_check(state, itup, state->targetblock, offset,
+									  &lVis_i, &lVis_tid, &lVis_offset,
+									  &lVis_block);
+			}
 			skey->scantid = scantid;	/* Restore saved scan key state */
 		}
 
@@ -1889,6 +1900,11 @@ bt_target_page_check(BtreeCheckState *state)
 				/* The first key on the next page is the same */
 				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
 				{
+					if (!unique_checked)
+						bt_entry_unique_check(state, itup, state->targetblock, offset,
+											  &lVis_i, &lVis_tid, &lVis_offset,
+											  &lVis_block);
+
 					elog(DEBUG2, "cross page equal keys");
 					state->target = palloc_btree_page(state,
 													  rightblock_number);
-- 
2.34.1

From ff4d989173084431914220d1a66646a332fa9d71 Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Thu, 25 Apr 2024 15:36:38 +0400
Subject: [PATCH v1 5/5] Rename checkunique parameter for amcheck and
 pg_amcheck

Use more user friendly naming: --check-unique for pg_amcheck
command line, check_unique for amcheck sql functions

Reported-by: Peter Eisentraut
---
 contrib/amcheck/amcheck--1.3--1.4.sql      |  4 +--
 contrib/amcheck/expected/check_btree.out   | 12 +++----
 contrib/amcheck/sql/check_btree.sql        | 12 +++----
 contrib/amcheck/verify_nbtree.c            | 38 +++++++++++-----------
 doc/src/sgml/amcheck.sgml                  |  8 ++---
 doc/src/sgml/ref/pg_amcheck.sgml           |  4 +--
 src/bin/pg_amcheck/pg_amcheck.c            | 20 ++++++------
 src/bin/pg_amcheck/t/003_check.pl          | 20 ++++++------
 src/bin/pg_amcheck/t/005_opclass_damage.pl |  4 +--
 9 files changed, 61 insertions(+), 61 deletions(-)

diff --git a/contrib/amcheck/amcheck--1.3--1.4.sql b/contrib/amcheck/amcheck--1.3--1.4.sql
index 75574eaa64..e0d4f92085 100644
--- a/contrib/amcheck/amcheck--1.3--1.4.sql
+++ b/contrib/amcheck/amcheck--1.3--1.4.sql
@@ -11,7 +11,7 @@
 -- bt_index_parent_check()
 --
 CREATE FUNCTION bt_index_parent_check(index regclass,
-    heapallindexed boolean, rootdescend boolean, checkunique boolean)
+    heapallindexed boolean, rootdescend boolean, check_unique boolean)
 RETURNS VOID
 AS 'MODULE_PATHNAME', 'bt_index_parent_check'
 LANGUAGE C STRICT PARALLEL RESTRICTED;
@@ -19,7 +19,7 @@ LANGUAGE C STRICT PARALLEL RESTRICTED;
 -- bt_index_check()
 --
 CREATE FUNCTION bt_index_check(index regclass,
-    heapallindexed boolean, checkunique boolean)
+    heapallindexed boolean, check_unique boolean)
 RETURNS VOID
 AS 'MODULE_PATHNAME', 'bt_index_check'
 LANGUAGE C STRICT PARALLEL RESTRICTED;
diff --git a/contrib/amcheck/expected/check_btree.out b/contrib/amcheck/expected/check_btree.out
index e7fb5f5515..ebb91d5d78 100644
--- a/contrib/amcheck/expected/check_btree.out
+++ b/contrib/amcheck/expected/check_btree.out
@@ -200,25 +200,25 @@ SELECT bt_index_check('bttest_a_expr_idx', true);
 (1 row)
 
 -- UNIQUE constraint check
-SELECT bt_index_check('bttest_a_idx', heapallindexed => true, checkunique => true);
+SELECT bt_index_check('bttest_a_idx', heapallindexed => true, check_unique => true);
  bt_index_check 
 ----------------
  
 (1 row)
 
-SELECT bt_index_check('bttest_b_idx', heapallindexed => false, checkunique => true);
+SELECT bt_index_check('bttest_b_idx', heapallindexed => false, check_unique => true);
  bt_index_check 
 ----------------
  
 (1 row)
 
-SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, checkunique => true);
+SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, check_unique => true);
  bt_index_parent_check 
 -----------------------
  
 (1 row)
 
-SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, checkunique => true);
+SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, check_unique => true);
  bt_index_parent_check 
 -----------------------
  
@@ -227,14 +227,14 @@ SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend
 -- Check that null values in an unique index are not treated as equal
 CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
 INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
-SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, checkunique => true);
+SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, check_unique => true);
  bt_index_check 
 ----------------
  
 (1 row)
 
 CREATE INDEX on bttest_unique_nulls (b,c);
-SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, checkunique => true);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, check_unique => true);
  bt_index_check 
 ----------------
  
diff --git a/contrib/amcheck/sql/check_btree.sql b/contrib/amcheck/sql/check_btree.sql
index 0793dbfeeb..f9ae33a6a4 100644
--- a/contrib/amcheck/sql/check_btree.sql
+++ b/contrib/amcheck/sql/check_btree.sql
@@ -136,17 +136,17 @@ CREATE INDEX bttest_a_expr_idx ON bttest_a ((ifun(id) + ifun(0)))
 SELECT bt_index_check('bttest_a_expr_idx', true);
 
 -- UNIQUE constraint check
-SELECT bt_index_check('bttest_a_idx', heapallindexed => true, checkunique => true);
-SELECT bt_index_check('bttest_b_idx', heapallindexed => false, checkunique => true);
-SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, checkunique => true);
-SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, checkunique => true);
+SELECT bt_index_check('bttest_a_idx', heapallindexed => true, check_unique => true);
+SELECT bt_index_check('bttest_b_idx', heapallindexed => false, check_unique => true);
+SELECT bt_index_parent_check('bttest_a_idx', heapallindexed => true, rootdescend => true, check_unique => true);
+SELECT bt_index_parent_check('bttest_b_idx', heapallindexed => true, rootdescend => false, check_unique => true);
 
 -- Check that null values in an unique index are not treated as equal
 CREATE TABLE bttest_unique_nulls (a serial, b int, c int UNIQUE);
 INSERT INTO bttest_unique_nulls VALUES (generate_series(1, 10000), 2, default);
-SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, checkunique => true);
+SELECT bt_index_check('bttest_unique_nulls_c_key', heapallindexed => true, check_unique => true);
 CREATE INDEX on bttest_unique_nulls (b,c);
-SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, checkunique => true);
+SELECT bt_index_check('bttest_unique_nulls_b_c_idx', heapallindexed => true, check_unique => true);
 
 -- Check support of both 1B and 4B header sizes of short varlena datum
 CREATE TABLE varlena_bug (v text);
diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index dfc9ed769f..3b673bac95 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -83,7 +83,7 @@ typedef struct BtreeCheckState
 	/* Also making sure non-pivot tuples can be found by new search? */
 	bool		rootdescend;
 	/* Also check uniqueness constraint if index is unique */
-	bool		checkunique;
+	bool		check_unique;
 	/* Per-page context */
 	MemoryContext targetcontext;
 	/* Buffer access strategy */
@@ -159,12 +159,12 @@ PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
 static void bt_index_check_internal(Oid indrelid, bool parentcheck,
 									bool heapallindexed, bool rootdescend,
-									bool checkunique);
+									bool check_unique);
 static inline void btree_index_checkable(Relation rel);
 static inline bool btree_index_mainfork_expected(Relation rel);
 static void bt_check_every_level(Relation rel, Relation heaprel,
 								 bool heapkeyspace, bool readonly, bool heapallindexed,
-								 bool rootdescend, bool checkunique);
+								 bool rootdescend, bool check_unique);
 static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
 											   BtreeLevel level);
 static bool bt_leftmost_ignoring_half_dead(BtreeCheckState *state,
@@ -223,7 +223,7 @@ static inline ItemPointer BTreeTupleGetHeapTIDCareful(BtreeCheckState *state,
 static inline ItemPointer BTreeTupleGetPointsToTID(IndexTuple itup);
 
 /*
- * bt_index_check(index regclass, heapallindexed boolean, checkunique boolean)
+ * bt_index_check(index regclass, heapallindexed boolean, check_unique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -236,20 +236,20 @@ bt_index_check(PG_FUNCTION_ARGS)
 {
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
-	bool		checkunique = false;
+	bool		check_unique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
 	if (PG_NARGS() == 3)
-		checkunique = PG_GETARG_BOOL(2);
+		check_unique = PG_GETARG_BOOL(2);
 
-	bt_index_check_internal(indrelid, false, heapallindexed, false, checkunique);
+	bt_index_check_internal(indrelid, false, heapallindexed, false, check_unique);
 
 	PG_RETURN_VOID();
 }
 
 /*
- * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean)
+ * bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, check_unique boolean)
  *
  * Verify integrity of B-Tree index.
  *
@@ -263,16 +263,16 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
 	Oid			indrelid = PG_GETARG_OID(0);
 	bool		heapallindexed = false;
 	bool		rootdescend = false;
-	bool		checkunique = false;
+	bool		check_unique = false;
 
 	if (PG_NARGS() >= 2)
 		heapallindexed = PG_GETARG_BOOL(1);
 	if (PG_NARGS() >= 3)
 		rootdescend = PG_GETARG_BOOL(2);
 	if (PG_NARGS() == 4)
-		checkunique = PG_GETARG_BOOL(3);
+		check_unique = PG_GETARG_BOOL(3);
 
-	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, checkunique);
+	bt_index_check_internal(indrelid, true, heapallindexed, rootdescend, check_unique);
 
 	PG_RETURN_VOID();
 }
@@ -282,7 +282,7 @@ bt_index_parent_check(PG_FUNCTION_ARGS)
  */
 static void
 bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
-						bool rootdescend, bool checkunique)
+						bool rootdescend, bool check_unique)
 {
 	Oid			heapid;
 	Relation	indrel;
@@ -394,7 +394,7 @@ bt_index_check_internal(Oid indrelid, bool parentcheck, bool heapallindexed,
 
 		/* Check index, possibly against table it is an index on */
 		bt_check_every_level(indrel, heaprel, heapkeyspace, parentcheck,
-							 heapallindexed, rootdescend, checkunique);
+							 heapallindexed, rootdescend, check_unique);
 	}
 
 	/* Roll back any GUC changes executed by index functions */
@@ -496,7 +496,7 @@ btree_index_mainfork_expected(Relation rel)
 static void
 bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 					 bool readonly, bool heapallindexed, bool rootdescend,
-					 bool checkunique)
+					 bool check_unique)
 {
 	BtreeCheckState *state;
 	Page		metapage;
@@ -528,7 +528,7 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	state->readonly = readonly;
 	state->heapallindexed = heapallindexed;
 	state->rootdescend = rootdescend;
-	state->checkunique = checkunique;
+	state->check_unique = check_unique;
 	state->snapshot = InvalidSnapshot;
 
 	if (state->heapallindexed)
@@ -592,7 +592,7 @@ bt_check_every_level(Relation rel, Relation heaprel, bool heapkeyspace,
 	 * performance take it once per index check. If snapshot already taken
 	 * reuse it.
 	 */
-	if (state->checkunique)
+	if (state->check_unique)
 	{
 		state->indexinfo = BuildIndexInfo(state->rel);
 		if (state->indexinfo->ii_Unique)
@@ -1770,7 +1770,7 @@ bt_target_page_check(BtreeCheckState *state)
 		 * If the index is unique verify entries uniqueness by checking the
 		 * heap tuples visibility.
 		 */
-		if (state->checkunique && state->indexinfo->ii_Unique &&
+		if (state->check_unique && state->indexinfo->ii_Unique &&
 			P_ISLEAF(topaque) && !skey->anynullkeys &&
 			(BTreeTupleIsPosting(itup) || ItemPointerIsValid(lVis.tid)))
 		{
@@ -1778,7 +1778,7 @@ bt_target_page_check(BtreeCheckState *state)
 			unique_checked = true;
 		}
 
-		if (state->checkunique && state->indexinfo->ii_Unique &&
+		if (state->check_unique && state->indexinfo->ii_Unique &&
 			P_ISLEAF(topaque) && OffsetNumberNext(offset) <= max)
 		{
 			/* Save current scankey tid */
@@ -1873,7 +1873,7 @@ bt_target_page_check(BtreeCheckState *state)
 			 * If index has unique constraint make sure that no more than one
 			 * found equal items is visible.
 			 */
-			if (state->checkunique && state->indexinfo->ii_Unique &&
+			if (state->check_unique && state->indexinfo->ii_Unique &&
 				rightkey && P_ISLEAF(topaque) && !P_RIGHTMOST(topaque))
 			{
 				BlockNumber rightblock_number = topaque->btpo_next;
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
index 3af065615b..3464ab5e76 100644
--- a/doc/src/sgml/amcheck.sgml
+++ b/doc/src/sgml/amcheck.sgml
@@ -61,7 +61,7 @@
   <variablelist>
    <varlistentry>
     <term>
-     <function>bt_index_check(index regclass, heapallindexed boolean, checkunique boolean) returns void</function>
+     <function>bt_index_check(index regclass, heapallindexed boolean, check_unique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_check</primary>
      </indexterm>
@@ -118,7 +118,7 @@ ORDER BY c.relpages DESC LIMIT 10;
       that span child/parent relationships, but will verify the
       presence of all heap tuples as index tuples within the index
       when <parameter>heapallindexed</parameter> is
-      <literal>true</literal>.  When <parameter>checkunique</parameter>
+      <literal>true</literal>.  When <parameter>check_unique</parameter>
       is <literal>true</literal> <function>bt_index_check</function> will
       check that no more than one among duplicate entries in unique
       index is visible.  When a routine, lightweight test for
@@ -132,7 +132,7 @@ ORDER BY c.relpages DESC LIMIT 10;
 
    <varlistentry>
     <term>
-     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, checkunique boolean) returns void</function>
+     <function>bt_index_parent_check(index regclass, heapallindexed boolean, rootdescend boolean, check_unique boolean) returns void</function>
      <indexterm>
       <primary>bt_index_parent_check</primary>
      </indexterm>
@@ -145,7 +145,7 @@ ORDER BY c.relpages DESC LIMIT 10;
       Optionally, when the <parameter>heapallindexed</parameter>
       argument is <literal>true</literal>, the function verifies the
       presence of all heap tuples that should be found within the
-      index.  When <parameter>checkunique</parameter>
+      index.  When <parameter>check_unique</parameter>
       is <literal>true</literal> <function>bt_index_parent_check</function> will
       check that no more than one among duplicate entries in unique
       index is visible.  When the optional <parameter>rootdescend</parameter>
diff --git a/doc/src/sgml/ref/pg_amcheck.sgml b/doc/src/sgml/ref/pg_amcheck.sgml
index 067c806b46..0837045632 100644
--- a/doc/src/sgml/ref/pg_amcheck.sgml
+++ b/doc/src/sgml/ref/pg_amcheck.sgml
@@ -434,12 +434,12 @@ PostgreSQL documentation
     </varlistentry>
 
     <varlistentry>
-     <term><option>--checkunique</option></term>
+     <term><option>--check-unique</option></term>
      <listitem>
       <para>
        For each index with unique constraint checked, verify that no more than
        one among duplicate entries is visible in the index using <xref linkend="amcheck"/>'s
-       <option>checkunique</option> option.
+       <option>check-unique</option> option.
       </para>
      </listitem>
     </varlistentry>
diff --git a/src/bin/pg_amcheck/pg_amcheck.c b/src/bin/pg_amcheck/pg_amcheck.c
index 7e3101704d..3aaf69a3b4 100644
--- a/src/bin/pg_amcheck/pg_amcheck.c
+++ b/src/bin/pg_amcheck/pg_amcheck.c
@@ -102,7 +102,7 @@ typedef struct AmcheckOptions
 	bool		parent_check;
 	bool		rootdescend;
 	bool		heapallindexed;
-	bool		checkunique;
+	bool		check_unique;
 
 	/* heap and btree hybrid option */
 	bool		no_btree_expansion;
@@ -133,7 +133,7 @@ static AmcheckOptions opts = {
 	.parent_check = false,
 	.rootdescend = false,
 	.heapallindexed = false,
-	.checkunique = false,
+	.check_unique = false,
 	.no_btree_expansion = false
 };
 
@@ -270,7 +270,7 @@ main(int argc, char *argv[])
 		{"heapallindexed", no_argument, NULL, 11},
 		{"parent-check", no_argument, NULL, 12},
 		{"install-missing", optional_argument, NULL, 13},
-		{"checkunique", no_argument, NULL, 14},
+		{"check-unique", no_argument, NULL, 14},
 
 		{NULL, 0, NULL, 0}
 	};
@@ -439,7 +439,7 @@ main(int argc, char *argv[])
 					opts.install_schema = pg_strdup(optarg);
 				break;
 			case 14:
-				opts.checkunique = true;
+				opts.check_unique = true;
 				break;
 			default:
 				/* getopt_long already emitted a complaint */
@@ -602,7 +602,7 @@ main(int argc, char *argv[])
 		 * constraint check with warning if it is not yet supported by
 		 * amcheck.
 		 */
-		if (opts.checkunique == true)
+		if (opts.check_unique == true)
 		{
 			/*
 			 * Now amcheck has only major and minor versions in the string but
@@ -617,11 +617,11 @@ main(int argc, char *argv[])
 			sscanf(amcheck_version, "%d.%d.%d", &vmaj, &vmin, &vrev);
 
 			/*
-			 * checkunique option is supported in amcheck since version 1.4
+			 * check_unique option is supported in amcheck since version 1.4
 			 */
 			if ((vmaj == 1 && vmin < 4) || vmaj == 0)
 			{
-				pg_log_warning("--checkunique option is not supported by amcheck "
+				pg_log_warning("--check-unique option is not supported by amcheck "
 							   "version \"%s\"", amcheck_version);
 				dat->is_checkunique = false;
 			}
@@ -895,7 +895,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
 						  (opts.rootdescend ? "true" : "false"),
-						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
+						  (rel->datinfo->is_checkunique ? ", check_unique := true" : ""),
 						  rel->reloid);
 	else
 		appendPQExpBuffer(sql,
@@ -909,7 +909,7 @@ prepare_btree_command(PQExpBuffer sql, RelationInfo *rel, PGconn *conn)
 						  "AND i.indisready AND i.indisvalid AND i.indislive",
 						  rel->datinfo->amcheck_schema,
 						  (opts.heapallindexed ? "true" : "false"),
-						  (rel->datinfo->is_checkunique ? ", checkunique := true" : ""),
+						  (rel->datinfo->is_checkunique ? ", check_unique := true" : ""),
 						  rel->reloid);
 }
 
@@ -1208,7 +1208,7 @@ help(const char *progname)
 	printf(_("      --heapallindexed            check that all heap tuples are found within indexes\n"));
 	printf(_("      --parent-check              check index parent/child relationships\n"));
 	printf(_("      --rootdescend               search from root page to refind tuples\n"));
-	printf(_("      --checkunique               check unique constraint if index is unique\n"));
+	printf(_("      --check-unique               check unique constraint if index is unique\n"));
 	printf(_("\nConnection options:\n"));
 	printf(_("  -h, --host=HOSTNAME             database server host or socket directory\n"));
 	printf(_("  -p, --port=PORT                 database server port\n"));
diff --git a/src/bin/pg_amcheck/t/003_check.pl b/src/bin/pg_amcheck/t/003_check.pl
index 4b16bda6a4..0a8cf8bced 100644
--- a/src/bin/pg_amcheck/t/003_check.pl
+++ b/src/bin/pg_amcheck/t/003_check.pl
@@ -523,35 +523,35 @@ $node->command_checks_all(
 $node->command_checks_all(
 	[
 		@cmd, '-s', 's1', '-i', 't1_btree', '--parent-check',
-		'--checkunique', 'db1'
+		'--check-unique', 'db1'
 	],
 	2,
 	[$index_missing_relation_fork_re],
 	[$no_output_re],
-	'pg_amcheck smoke test --parent-check --checkunique');
+	'pg_amcheck smoke test --parent-check --check-unique');
 
 $node->command_checks_all(
 	[
 		@cmd, '-s', 's1', '-i', 't1_btree', '--heapallindexed',
-		'--rootdescend', '--checkunique', 'db1'
+		'--rootdescend', '--check-unique', 'db1'
 	],
 	2,
 	[$index_missing_relation_fork_re],
 	[$no_output_re],
-	'pg_amcheck smoke test --heapallindexed --rootdescend --checkunique');
+	'pg_amcheck smoke test --heapallindexed --rootdescend --check-unique');
 
 $node->command_checks_all(
 	[
-		@cmd, '--checkunique', '-d', 'db1', '-d', 'db2',
+		@cmd, '--check-unique', '-d', 'db1', '-d', 'db2',
 		'-d', 'db3', '-S', 's*'
 	],
 	0,
 	[$no_output_re],
 	[$no_output_re],
-	'pg_amcheck excluding all corrupt schemas with --checkunique option');
+	'pg_amcheck excluding all corrupt schemas with --check-unique option');
 
 #
-# Smoke test for checkunique option for not supported versions.
+# Smoke test for check-unique option for not supported versions.
 #
 $node->safe_psql(
 	'db3', q(
@@ -560,11 +560,11 @@ $node->safe_psql(
 ));
 
 $node->command_checks_all(
-	[ @cmd, '--checkunique', 'db3' ],
+	[ @cmd, '--check-unique', 'db3' ],
 	0,
 	[$no_output_re],
 	[
-		qr/pg_amcheck: warning: --checkunique option is not supported by amcheck version "1.3"/
+		qr/pg_amcheck: warning: --check-unique option is not supported by amcheck version "1.3"/
 	],
-	'pg_amcheck smoke test --checkunique');
+	'pg_amcheck smoke test --check-unique');
 done_testing();
diff --git a/src/bin/pg_amcheck/t/005_opclass_damage.pl b/src/bin/pg_amcheck/t/005_opclass_damage.pl
index 1eea215227..0db58dee5b 100644
--- a/src/bin/pg_amcheck/t/005_opclass_damage.pl
+++ b/src/bin/pg_amcheck/t/005_opclass_damage.pl
@@ -90,7 +90,7 @@ $node->safe_psql(
 
 # We should get no corruptions
 $node->command_like(
-	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	[ 'pg_amcheck', '--check-unique', '-p', $node->port, 'postgres' ],
 	qr/^$/,
 	'pg_amcheck all schemas, tables and indexes reports no corruption');
 
@@ -116,7 +116,7 @@ $node->safe_psql(
 
 # Unique index corruption should now be reported
 $node->command_checks_all(
-	[ 'pg_amcheck', '--checkunique', '-p', $node->port, 'postgres' ],
+	[ 'pg_amcheck', '--check-unique', '-p', $node->port, 'postgres' ],
 	2,
 	[qr/index uniqueness is violated for index "bttest_unique_idx"/],
 	[],
-- 
2.34.1

From 50f38667510057beea9c88bff8b7ee8cda825939 Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Fri, 10 May 2024 03:08:15 +0300
Subject: [PATCH v2 3/4] Amcheck: Don't load rightpage into BtreeCheckState

For cross-page unique constraint check use a local variable in the
similar way as implemented in bt_right_page_check_scankey().

Reported-by: Peter Geoghegan
---
 contrib/amcheck/verify_nbtree.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index b433cb33254..977f8b6799d 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -1910,6 +1910,8 @@ bt_target_page_check(BtreeCheckState *state)
 				/* The first key on the next page is the same */
 				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
 				{
+					Page		rightpage;
+
 					/*
 					 * Do the bt_entry_unique_check() call if it was
 					 * postponed.
@@ -1918,19 +1920,21 @@ bt_target_page_check(BtreeCheckState *state)
 						bt_entry_unique_check(state, itup, state->targetblock, offset, &lVis);
 
 					elog(DEBUG2, "cross page equal keys");
-					state->target = palloc_btree_page(state,
-													  rightblock_number);
-					topaque = BTPageGetOpaque(state->target);
+					rightpage = palloc_btree_page(state,
+												  rightblock_number);
+					topaque = BTPageGetOpaque(rightpage);
 
 					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
 						break;
 
 					itemid = PageGetItemIdCareful(state, rightblock_number,
-												  state->target,
+												  rightpage,
 												  rightfirstoffset);
-					itup = (IndexTuple) PageGetItem(state->target, itemid);
+					itup = (IndexTuple) PageGetItem(rightpage, itemid);
 
 					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset, &lVis);
+
+					pfree(rightpage);
 				}
 			}
 		}
-- 
2.39.3 (Apple Git-145)

From 9ae515266a041b30d14913000eb00dd9363fc1f8 Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Fri, 10 May 2024 03:08:07 +0300
Subject: [PATCH v2 2/4] amcheck: Refactoring the storage of the last visible
 entry

This commit introduces a new data structure BtreeLastVisibleEntry comprising
information about the last visible heap entry with the current value of key.
Usage of this data structure allows us to avoid passing all this information
as individual function arguments.

Reported-by: Alexander Korotkov
Discussion: https://www.postgresql.org/message-id/CAPpHfdsVbB9ToriaB1UHuOKwjKxiZmTFQcEF%3DjuzzC_nby31uA%40mail.gmail.com
Author: Pavel Borisov, Alexander Korotkov
---
 contrib/amcheck/verify_nbtree.c  | 125 +++++++++++++++----------------
 src/tools/pgindent/typedefs.list |   1 +
 2 files changed, 61 insertions(+), 65 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index c7be785f88b..b433cb33254 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -145,6 +145,19 @@ typedef struct BtreeLevel
 	bool		istruerootlevel;
 } BtreeLevel;
 
+/*
+ * Information about the last visible entry with current B-tree key.  Used
+ * for validation of the unique constraint.
+ */
+typedef struct BtreeLastVisibleEntry
+{
+	BlockNumber blkno;			/* Index block */
+	OffsetNumber offset;		/* Offset on index block */
+	int			postingIndex;	/* Number in the posting list (-1 for
+								 * non-deduplicated tuples) */
+	ItemPointer tid;			/* Heap tid */
+} BtreeLastVisibleEntry;
+
 PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
@@ -165,17 +178,13 @@ static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
 static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
-static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
-								BlockNumber block, OffsetNumber offset,
-								int posting, ItemPointer nexttid,
+static void bt_report_duplicate(BtreeCheckState *state, BtreeLastVisibleEntry *lVis,
+								ItemPointer nexttid,
 								BlockNumber nblock, OffsetNumber noffset,
 								int nposting);
 static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 								  BlockNumber targetblock,
-								  OffsetNumber offset, int *lVis_i,
-								  ItemPointer *lVis_tid,
-								  OffsetNumber *lVis_offset,
-								  BlockNumber *lVis_block);
+								  OffsetNumber offset, BtreeLastVisibleEntry *lVis);
 static void bt_target_page_check(BtreeCheckState *state);
 static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
 												OffsetNumber *rightfirstoffset);
@@ -997,8 +1006,7 @@ heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
  */
 static void
 bt_report_duplicate(BtreeCheckState *state,
-					ItemPointer tid, BlockNumber block, OffsetNumber offset,
-					int posting,
+					BtreeLastVisibleEntry *lVis,
 					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
 					int nposting)
 {
@@ -1010,18 +1018,18 @@ bt_report_duplicate(BtreeCheckState *state,
 			   *pnposting = "";
 
 	htid = psprintf("tid=(%u,%u)",
-					ItemPointerGetBlockNumberNoCheck(tid),
-					ItemPointerGetOffsetNumberNoCheck(tid));
+					ItemPointerGetBlockNumberNoCheck(lVis->tid),
+					ItemPointerGetOffsetNumberNoCheck(lVis->tid));
 	nhtid = psprintf("tid=(%u,%u)",
 					 ItemPointerGetBlockNumberNoCheck(nexttid),
 					 ItemPointerGetOffsetNumberNoCheck(nexttid));
-	itid = psprintf("tid=(%u,%u)", block, offset);
+	itid = psprintf("tid=(%u,%u)", lVis->blkno, lVis->offset);
 
-	if (nblock != block || noffset != offset)
+	if (nblock != lVis->blkno || noffset != lVis->offset)
 		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
 
-	if (posting >= 0)
-		pposting = psprintf(" posting %u", posting);
+	if (lVis->postingIndex >= 0)
+		pposting = psprintf(" posting %u", lVis->postingIndex);
 
 	if (nposting >= 0)
 		pnposting = psprintf(" posting %u", nposting);
@@ -1038,9 +1046,7 @@ bt_report_duplicate(BtreeCheckState *state,
 /* Check if current nbtree leaf entry complies with UNIQUE constraint */
 static void
 bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
-					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i,
-					  ItemPointer *lVis_tid, OffsetNumber *lVis_offset,
-					  BlockNumber *lVis_block)
+					  BlockNumber targetblock, OffsetNumber offset, BtreeLastVisibleEntry *lVis)
 {
 	ItemPointer tid;
 	bool		has_visible_entry = false;
@@ -1049,7 +1055,7 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 
 	/*
 	 * Current tuple has posting list. Report duplicate if TID of any posting
-	 * list entry is visible and lVis_tid is valid.
+	 * list entry is visible and lVis->tid is valid.
 	 */
 	if (BTreeTupleIsPosting(itup))
 	{
@@ -1059,11 +1065,10 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 			if (heap_entry_is_visible(state, tid))
 			{
 				has_visible_entry = true;
-				if (ItemPointerIsValid(*lVis_tid))
+				if (ItemPointerIsValid(lVis->tid))
 				{
 					bt_report_duplicate(state,
-										*lVis_tid, *lVis_block,
-										*lVis_offset, *lVis_i,
+										lVis,
 										tid, targetblock,
 										offset, i);
 				}
@@ -1073,13 +1078,13 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 				 * between the posting list entries of the first tuple on the
 				 * page after cross-page check.
 				 */
-				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+				if (lVis->blkno != targetblock && ItemPointerIsValid(lVis->tid))
 					return;
 
-				*lVis_i = i;
-				*lVis_tid = tid;
-				*lVis_offset = offset;
-				*lVis_block = targetblock;
+				lVis->blkno = targetblock;
+				lVis->offset = offset;
+				lVis->postingIndex = i;
+				lVis->tid = tid;
 			}
 		}
 	}
@@ -1087,7 +1092,7 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 	/*
 	 * Current tuple has no posting list. If TID is visible save info about it
 	 * for the next comparisons in the loop in bt_target_page_check(). Report
-	 * duplicate if lVis_tid is already valid.
+	 * duplicate if lVis->tid is already valid.
 	 */
 	else
 	{
@@ -1095,37 +1100,38 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 		if (heap_entry_is_visible(state, tid))
 		{
 			has_visible_entry = true;
-			if (ItemPointerIsValid(*lVis_tid))
+			if (ItemPointerIsValid(lVis->tid))
 			{
 				bt_report_duplicate(state,
-									*lVis_tid, *lVis_block,
-									*lVis_offset, *lVis_i,
+									lVis,
 									tid, targetblock,
 									offset, -1);
 			}
-			*lVis_i = -1;
-			*lVis_tid = tid;
-			*lVis_offset = offset;
-			*lVis_block = targetblock;
+
+			lVis->blkno = targetblock;
+			lVis->offset = offset;
+			lVis->tid = tid;
+			lVis->postingIndex = -1;
 		}
 	}
 
-	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
-		*lVis_block != targetblock)
+	if (!has_visible_entry &&
+		lVis->blkno != InvalidBlockNumber &&
+		lVis->blkno != targetblock)
 	{
 		char	   *posting = "";
 
-		if (*lVis_i >= 0)
-			posting = psprintf(" posting %u", *lVis_i);
+		if (lVis->postingIndex >= 0)
+			posting = psprintf(" posting %u", lVis->postingIndex);
 		ereport(DEBUG1,
 				(errcode(ERRCODE_NO_DATA),
 				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) in index \"%s\"",
 						targetblock, offset,
 						RelationGetRelationName(state->rel)),
 				 errdetail("It doesn't have visible heap tids and key is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)).",
-						   *lVis_block, *lVis_offset, posting,
-						   ItemPointerGetBlockNumberNoCheck(*lVis_tid),
-						   ItemPointerGetOffsetNumberNoCheck(*lVis_tid)),
+						   lVis->blkno, lVis->offset, posting,
+						   ItemPointerGetBlockNumberNoCheck(lVis->tid),
+						   ItemPointerGetOffsetNumberNoCheck(lVis->tid)),
 				 errhint("VACUUM the table and repeat the check.")));
 	}
 }
@@ -1372,12 +1378,8 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
-	/* last visible entry info for checking indexes with unique constraint */
-	int			lVis_i = -1;	/* the position of last visible item for
-								 * posting tuple. for non-posting tuple (-1) */
-	ItemPointer lVis_tid = NULL;
-	BlockNumber lVis_block = InvalidBlockNumber;
-	OffsetNumber lVis_offset = InvalidOffsetNumber;
+	/* Last visible entry info for checking indexes with unique constraint */
+	BtreeLastVisibleEntry lVis = {InvalidBlockNumber, InvalidOffsetNumber, -1, NULL};
 
 	topaque = BTPageGetOpaque(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
@@ -1784,11 +1786,9 @@ bt_target_page_check(BtreeCheckState *state)
 		 */
 		if (state->checkunique && state->indexinfo->ii_Unique &&
 			P_ISLEAF(topaque) && !skey->anynullkeys &&
-			(BTreeTupleIsPosting(itup) || ItemPointerIsValid(lVis_tid)))
+			(BTreeTupleIsPosting(itup) || ItemPointerIsValid(lVis.tid)))
 		{
-			bt_entry_unique_check(state, itup, state->targetblock, offset,
-								  &lVis_i, &lVis_tid, &lVis_offset,
-								  &lVis_block);
+			bt_entry_unique_check(state, itup, state->targetblock, offset, &lVis);
 			unique_checked = true;
 		}
 
@@ -1816,17 +1816,16 @@ bt_target_page_check(BtreeCheckState *state)
 			if (_bt_compare(state->rel, skey, state->target,
 							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
 			{
-				lVis_i = -1;
-				lVis_tid = NULL;
-				lVis_block = InvalidBlockNumber;
-				lVis_offset = InvalidOffsetNumber;
+				lVis.blkno = InvalidBlockNumber;
+				lVis.offset = InvalidOffsetNumber;
+				lVis.postingIndex = -1;
+				lVis.tid = NULL;
 			}
 			else if (!unique_checked)
 			{
-				bt_entry_unique_check(state, itup, state->targetblock, offset,
-									  &lVis_i, &lVis_tid, &lVis_offset,
-									  &lVis_block);
+				bt_entry_unique_check(state, itup, state->targetblock, offset, &lVis);
 			}
+
 			skey->scantid = scantid;	/* Restore saved scan key state */
 		}
 
@@ -1916,9 +1915,7 @@ bt_target_page_check(BtreeCheckState *state)
 					 * postponed.
 					 */
 					if (!unique_checked)
-						bt_entry_unique_check(state, itup, state->targetblock, offset,
-											  &lVis_i, &lVis_tid, &lVis_offset,
-											  &lVis_block);
+						bt_entry_unique_check(state, itup, state->targetblock, offset, &lVis);
 
 					elog(DEBUG2, "cross page equal keys");
 					state->target = palloc_btree_page(state,
@@ -1933,9 +1930,7 @@ bt_target_page_check(BtreeCheckState *state)
 												  rightfirstoffset);
 					itup = (IndexTuple) PageGetItem(state->target, itemid);
 
-					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
-										  &lVis_i, &lVis_tid, &lVis_offset,
-										  &lVis_block);
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset, &lVis);
 				}
 			}
 		}
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 2311f82d81e..7535dc6d692 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -315,6 +315,7 @@ BrinStatsData
 BrinTuple
 BrinValues
 BtreeCheckState
+BtreeLastVisibleEntry
 BtreeLevel
 Bucket
 BufFile
-- 
2.39.3 (Apple Git-145)

From 0c8a911657e68f77255f849897319c227160bd02 Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Fri, 10 May 2024 03:07:53 +0300
Subject: [PATCH v2 1/4] amcheck: Optimize speed of checking for unique
 constraint violation

Currently, when amcheck validates a unique constraint, it visits the heap for
each index tuple.  This commit implements skipping keys, which have only one
non-dedeuplicated index tuple (quite common case for unique indexes). That
gives substantial economy on index checking time.

Reported-by: Noah Misch
Discussion: https://postgr.es/m/20240325020323.fd.nmisch%40google.com
Author: Alexander Korotkov, Pavel Borisov
---
 contrib/amcheck/verify_nbtree.c | 35 +++++++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 70f65b645a6..c7be785f88b 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -1429,6 +1429,13 @@ bt_target_page_check(BtreeCheckState *state)
 		bool		lowersizelimit;
 		ItemPointer scantid;
 
+		/*
+		 * True if we already called bt_entry_unique_check() for the current
+		 * item.  This helps to avoid visiting the heap for keys, which are
+		 * anyway presented only once and can't comprise a unique violation.
+		 */
+		bool		unique_checked = false;
+
 		CHECK_FOR_INTERRUPTS();
 
 		itemid = PageGetItemIdCareful(state, state->targetblock,
@@ -1771,13 +1778,19 @@ bt_target_page_check(BtreeCheckState *state)
 
 		/*
 		 * If the index is unique verify entries uniqueness by checking the
-		 * heap tuples visibility.
+		 * heap tuples visibility.  Immediately check posting tuples and
+		 * tuples with repeated keys.  Postpone check for keys, which have the
+		 * first appearance.
 		 */
 		if (state->checkunique && state->indexinfo->ii_Unique &&
-			P_ISLEAF(topaque) && !skey->anynullkeys)
+			P_ISLEAF(topaque) && !skey->anynullkeys &&
+			(BTreeTupleIsPosting(itup) || ItemPointerIsValid(lVis_tid)))
+		{
 			bt_entry_unique_check(state, itup, state->targetblock, offset,
 								  &lVis_i, &lVis_tid, &lVis_offset,
 								  &lVis_block);
+			unique_checked = true;
+		}
 
 		if (state->checkunique && state->indexinfo->ii_Unique &&
 			P_ISLEAF(topaque) && OffsetNumberNext(offset) <= max)
@@ -1796,6 +1809,9 @@ bt_target_page_check(BtreeCheckState *state)
 			 * data (whole index tuple or last posting in index tuple). Key
 			 * containing null value does not violate unique constraint and
 			 * treated as different to any other key.
+			 *
+			 * If the next key is the same as the previous one, do the
+			 * bt_entry_unique_check() call if it was postponed.
 			 */
 			if (_bt_compare(state->rel, skey, state->target,
 							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
@@ -1805,6 +1821,12 @@ bt_target_page_check(BtreeCheckState *state)
 				lVis_block = InvalidBlockNumber;
 				lVis_offset = InvalidOffsetNumber;
 			}
+			else if (!unique_checked)
+			{
+				bt_entry_unique_check(state, itup, state->targetblock, offset,
+									  &lVis_i, &lVis_tid, &lVis_offset,
+									  &lVis_block);
+			}
 			skey->scantid = scantid;	/* Restore saved scan key state */
 		}
 
@@ -1889,6 +1911,15 @@ bt_target_page_check(BtreeCheckState *state)
 				/* The first key on the next page is the same */
 				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
 				{
+					/*
+					 * Do the bt_entry_unique_check() call if it was
+					 * postponed.
+					 */
+					if (!unique_checked)
+						bt_entry_unique_check(state, itup, state->targetblock, offset,
+											  &lVis_i, &lVis_tid, &lVis_offset,
+											  &lVis_block);
+
 					elog(DEBUG2, "cross page equal keys");
 					state->target = palloc_btree_page(state,
 													  rightblock_number);
-- 
2.39.3 (Apple Git-145)

From 9b36cf33f719107c899863f15ed5d0938a99b029 Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Fri, 10 May 2024 03:07:22 +0300
Subject: [PATCH v2 4/4] amcheck: Report an error when the next page to a leaf
 is not a leaf

This is a very unlikely condition during checking a B-tree unique constraint,
meaning that the index structure is violated badly, and we shouldn't continue
checking to avoid endless loops, etc.  So it's worth immediately throwing an
error.

Reported-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-Wzk%2B2116uOXdOViA27SHcr31WKPgmjsxXLBs_aTxAeThzg%40mail.gmail.com
Author: Pavel Borisov
---
 contrib/amcheck/verify_nbtree.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 977f8b6799d..e9bbc18c4a5 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -1849,7 +1849,6 @@ bt_target_page_check(BtreeCheckState *state)
 		if (offset == max)
 		{
 			BTScanInsert rightkey;
-			BlockNumber rightblock_number;
 
 			/* first offset on a right index page (log only) */
 			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
@@ -1894,10 +1893,11 @@ bt_target_page_check(BtreeCheckState *state)
 			 * If index has unique constraint make sure that no more than one
 			 * found equal items is visible.
 			 */
-			rightblock_number = topaque->btpo_next;
 			if (state->checkunique && state->indexinfo->ii_Unique &&
-				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+				rightkey && P_ISLEAF(topaque) && !P_RIGHTMOST(topaque))
 			{
+				BlockNumber rightblock_number = topaque->btpo_next;
+
 				elog(DEBUG2, "check cross page unique condition");
 
 				/*
@@ -1924,9 +1924,19 @@ bt_target_page_check(BtreeCheckState *state)
 												  rightblock_number);
 					topaque = BTPageGetOpaque(rightpage);
 
-					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
-						break;
-
+					if (P_IGNORE(topaque))
+					{
+						if (unlikely(!P_ISLEAF(topaque)))
+							ereport(ERROR,
+									(errcode(ERRCODE_INDEX_CORRUPTED),
+									 errmsg("right block of leaf block is non-leaf for index \"%s\"",
+											RelationGetRelationName(state->rel)),
+									 errdetail_internal("Block=%u page lsn=%X/%X.",
+														state->targetblock,
+														LSN_FORMAT_ARGS(state->targetlsn))));
+						else
+							break;
+					}
 					itemid = PageGetItemIdCareful(state, rightblock_number,
 												  rightpage,
 												  rightfirstoffset);
-- 
2.39.3 (Apple Git-145)

From 7014fa433ba5c3c6ea645b13d433a11f4b5a0b15 Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Fri, 10 May 2024 03:08:07 +0300
Subject: [PATCH v3 2/4] amcheck: Refactoring the storage of the last visible
 entry

This commit introduces a new data structure BtreeLastVisibleEntry comprising
information about the last visible heap entry with the current value of key.
Usage of this data structure allows us to avoid passing all this information
as individual function arguments.

Reported-by: Alexander Korotkov
Discussion: https://www.postgresql.org/message-id/CAPpHfdsVbB9ToriaB1UHuOKwjKxiZmTFQcEF%3DjuzzC_nby31uA%40mail.gmail.com
Author: Pavel Borisov, Alexander Korotkov
---
 contrib/amcheck/verify_nbtree.c  | 125 +++++++++++++++----------------
 src/tools/pgindent/typedefs.list |   1 +
 2 files changed, 61 insertions(+), 65 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index c7be785f88b..b433cb33254 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -145,6 +145,19 @@ typedef struct BtreeLevel
 	bool		istruerootlevel;
 } BtreeLevel;
 
+/*
+ * Information about the last visible entry with current B-tree key.  Used
+ * for validation of the unique constraint.
+ */
+typedef struct BtreeLastVisibleEntry
+{
+	BlockNumber blkno;			/* Index block */
+	OffsetNumber offset;		/* Offset on index block */
+	int			postingIndex;	/* Number in the posting list (-1 for
+								 * non-deduplicated tuples) */
+	ItemPointer tid;			/* Heap tid */
+} BtreeLastVisibleEntry;
+
 PG_FUNCTION_INFO_V1(bt_index_check);
 PG_FUNCTION_INFO_V1(bt_index_parent_check);
 
@@ -165,17 +178,13 @@ static void bt_recheck_sibling_links(BtreeCheckState *state,
 									 BlockNumber btpo_prev_from_target,
 									 BlockNumber leftcurrent);
 static bool heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid);
-static void bt_report_duplicate(BtreeCheckState *state, ItemPointer tid,
-								BlockNumber block, OffsetNumber offset,
-								int posting, ItemPointer nexttid,
+static void bt_report_duplicate(BtreeCheckState *state, BtreeLastVisibleEntry *lVis,
+								ItemPointer nexttid,
 								BlockNumber nblock, OffsetNumber noffset,
 								int nposting);
 static void bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 								  BlockNumber targetblock,
-								  OffsetNumber offset, int *lVis_i,
-								  ItemPointer *lVis_tid,
-								  OffsetNumber *lVis_offset,
-								  BlockNumber *lVis_block);
+								  OffsetNumber offset, BtreeLastVisibleEntry *lVis);
 static void bt_target_page_check(BtreeCheckState *state);
 static BTScanInsert bt_right_page_check_scankey(BtreeCheckState *state,
 												OffsetNumber *rightfirstoffset);
@@ -997,8 +1006,7 @@ heap_entry_is_visible(BtreeCheckState *state, ItemPointer tid)
  */
 static void
 bt_report_duplicate(BtreeCheckState *state,
-					ItemPointer tid, BlockNumber block, OffsetNumber offset,
-					int posting,
+					BtreeLastVisibleEntry *lVis,
 					ItemPointer nexttid, BlockNumber nblock, OffsetNumber noffset,
 					int nposting)
 {
@@ -1010,18 +1018,18 @@ bt_report_duplicate(BtreeCheckState *state,
 			   *pnposting = "";
 
 	htid = psprintf("tid=(%u,%u)",
-					ItemPointerGetBlockNumberNoCheck(tid),
-					ItemPointerGetOffsetNumberNoCheck(tid));
+					ItemPointerGetBlockNumberNoCheck(lVis->tid),
+					ItemPointerGetOffsetNumberNoCheck(lVis->tid));
 	nhtid = psprintf("tid=(%u,%u)",
 					 ItemPointerGetBlockNumberNoCheck(nexttid),
 					 ItemPointerGetOffsetNumberNoCheck(nexttid));
-	itid = psprintf("tid=(%u,%u)", block, offset);
+	itid = psprintf("tid=(%u,%u)", lVis->blkno, lVis->offset);
 
-	if (nblock != block || noffset != offset)
+	if (nblock != lVis->blkno || noffset != lVis->offset)
 		nitid = psprintf(" tid=(%u,%u)", nblock, noffset);
 
-	if (posting >= 0)
-		pposting = psprintf(" posting %u", posting);
+	if (lVis->postingIndex >= 0)
+		pposting = psprintf(" posting %u", lVis->postingIndex);
 
 	if (nposting >= 0)
 		pnposting = psprintf(" posting %u", nposting);
@@ -1038,9 +1046,7 @@ bt_report_duplicate(BtreeCheckState *state,
 /* Check if current nbtree leaf entry complies with UNIQUE constraint */
 static void
 bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
-					  BlockNumber targetblock, OffsetNumber offset, int *lVis_i,
-					  ItemPointer *lVis_tid, OffsetNumber *lVis_offset,
-					  BlockNumber *lVis_block)
+					  BlockNumber targetblock, OffsetNumber offset, BtreeLastVisibleEntry *lVis)
 {
 	ItemPointer tid;
 	bool		has_visible_entry = false;
@@ -1049,7 +1055,7 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 
 	/*
 	 * Current tuple has posting list. Report duplicate if TID of any posting
-	 * list entry is visible and lVis_tid is valid.
+	 * list entry is visible and lVis->tid is valid.
 	 */
 	if (BTreeTupleIsPosting(itup))
 	{
@@ -1059,11 +1065,10 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 			if (heap_entry_is_visible(state, tid))
 			{
 				has_visible_entry = true;
-				if (ItemPointerIsValid(*lVis_tid))
+				if (ItemPointerIsValid(lVis->tid))
 				{
 					bt_report_duplicate(state,
-										*lVis_tid, *lVis_block,
-										*lVis_offset, *lVis_i,
+										lVis,
 										tid, targetblock,
 										offset, i);
 				}
@@ -1073,13 +1078,13 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 				 * between the posting list entries of the first tuple on the
 				 * page after cross-page check.
 				 */
-				if (*lVis_block != targetblock && ItemPointerIsValid(*lVis_tid))
+				if (lVis->blkno != targetblock && ItemPointerIsValid(lVis->tid))
 					return;
 
-				*lVis_i = i;
-				*lVis_tid = tid;
-				*lVis_offset = offset;
-				*lVis_block = targetblock;
+				lVis->blkno = targetblock;
+				lVis->offset = offset;
+				lVis->postingIndex = i;
+				lVis->tid = tid;
 			}
 		}
 	}
@@ -1087,7 +1092,7 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 	/*
 	 * Current tuple has no posting list. If TID is visible save info about it
 	 * for the next comparisons in the loop in bt_target_page_check(). Report
-	 * duplicate if lVis_tid is already valid.
+	 * duplicate if lVis->tid is already valid.
 	 */
 	else
 	{
@@ -1095,37 +1100,38 @@ bt_entry_unique_check(BtreeCheckState *state, IndexTuple itup,
 		if (heap_entry_is_visible(state, tid))
 		{
 			has_visible_entry = true;
-			if (ItemPointerIsValid(*lVis_tid))
+			if (ItemPointerIsValid(lVis->tid))
 			{
 				bt_report_duplicate(state,
-									*lVis_tid, *lVis_block,
-									*lVis_offset, *lVis_i,
+									lVis,
 									tid, targetblock,
 									offset, -1);
 			}
-			*lVis_i = -1;
-			*lVis_tid = tid;
-			*lVis_offset = offset;
-			*lVis_block = targetblock;
+
+			lVis->blkno = targetblock;
+			lVis->offset = offset;
+			lVis->tid = tid;
+			lVis->postingIndex = -1;
 		}
 	}
 
-	if (!has_visible_entry && *lVis_block != InvalidBlockNumber &&
-		*lVis_block != targetblock)
+	if (!has_visible_entry &&
+		lVis->blkno != InvalidBlockNumber &&
+		lVis->blkno != targetblock)
 	{
 		char	   *posting = "";
 
-		if (*lVis_i >= 0)
-			posting = psprintf(" posting %u", *lVis_i);
+		if (lVis->postingIndex >= 0)
+			posting = psprintf(" posting %u", lVis->postingIndex);
 		ereport(DEBUG1,
 				(errcode(ERRCODE_NO_DATA),
 				 errmsg("index uniqueness can not be checked for index tid=(%u,%u) in index \"%s\"",
 						targetblock, offset,
 						RelationGetRelationName(state->rel)),
 				 errdetail("It doesn't have visible heap tids and key is equal to the tid=(%u,%u)%s (points to heap tid=(%u,%u)).",
-						   *lVis_block, *lVis_offset, posting,
-						   ItemPointerGetBlockNumberNoCheck(*lVis_tid),
-						   ItemPointerGetOffsetNumberNoCheck(*lVis_tid)),
+						   lVis->blkno, lVis->offset, posting,
+						   ItemPointerGetBlockNumberNoCheck(lVis->tid),
+						   ItemPointerGetOffsetNumberNoCheck(lVis->tid)),
 				 errhint("VACUUM the table and repeat the check.")));
 	}
 }
@@ -1372,12 +1378,8 @@ bt_target_page_check(BtreeCheckState *state)
 	OffsetNumber max;
 	BTPageOpaque topaque;
 
-	/* last visible entry info for checking indexes with unique constraint */
-	int			lVis_i = -1;	/* the position of last visible item for
-								 * posting tuple. for non-posting tuple (-1) */
-	ItemPointer lVis_tid = NULL;
-	BlockNumber lVis_block = InvalidBlockNumber;
-	OffsetNumber lVis_offset = InvalidOffsetNumber;
+	/* Last visible entry info for checking indexes with unique constraint */
+	BtreeLastVisibleEntry lVis = {InvalidBlockNumber, InvalidOffsetNumber, -1, NULL};
 
 	topaque = BTPageGetOpaque(state->target);
 	max = PageGetMaxOffsetNumber(state->target);
@@ -1784,11 +1786,9 @@ bt_target_page_check(BtreeCheckState *state)
 		 */
 		if (state->checkunique && state->indexinfo->ii_Unique &&
 			P_ISLEAF(topaque) && !skey->anynullkeys &&
-			(BTreeTupleIsPosting(itup) || ItemPointerIsValid(lVis_tid)))
+			(BTreeTupleIsPosting(itup) || ItemPointerIsValid(lVis.tid)))
 		{
-			bt_entry_unique_check(state, itup, state->targetblock, offset,
-								  &lVis_i, &lVis_tid, &lVis_offset,
-								  &lVis_block);
+			bt_entry_unique_check(state, itup, state->targetblock, offset, &lVis);
 			unique_checked = true;
 		}
 
@@ -1816,17 +1816,16 @@ bt_target_page_check(BtreeCheckState *state)
 			if (_bt_compare(state->rel, skey, state->target,
 							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
 			{
-				lVis_i = -1;
-				lVis_tid = NULL;
-				lVis_block = InvalidBlockNumber;
-				lVis_offset = InvalidOffsetNumber;
+				lVis.blkno = InvalidBlockNumber;
+				lVis.offset = InvalidOffsetNumber;
+				lVis.postingIndex = -1;
+				lVis.tid = NULL;
 			}
 			else if (!unique_checked)
 			{
-				bt_entry_unique_check(state, itup, state->targetblock, offset,
-									  &lVis_i, &lVis_tid, &lVis_offset,
-									  &lVis_block);
+				bt_entry_unique_check(state, itup, state->targetblock, offset, &lVis);
 			}
+
 			skey->scantid = scantid;	/* Restore saved scan key state */
 		}
 
@@ -1916,9 +1915,7 @@ bt_target_page_check(BtreeCheckState *state)
 					 * postponed.
 					 */
 					if (!unique_checked)
-						bt_entry_unique_check(state, itup, state->targetblock, offset,
-											  &lVis_i, &lVis_tid, &lVis_offset,
-											  &lVis_block);
+						bt_entry_unique_check(state, itup, state->targetblock, offset, &lVis);
 
 					elog(DEBUG2, "cross page equal keys");
 					state->target = palloc_btree_page(state,
@@ -1933,9 +1930,7 @@ bt_target_page_check(BtreeCheckState *state)
 												  rightfirstoffset);
 					itup = (IndexTuple) PageGetItem(state->target, itemid);
 
-					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset,
-										  &lVis_i, &lVis_tid, &lVis_offset,
-										  &lVis_block);
+					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset, &lVis);
 				}
 			}
 		}
diff --git a/src/tools/pgindent/typedefs.list b/src/tools/pgindent/typedefs.list
index 34ec87a85eb..8d24418fe3c 100644
--- a/src/tools/pgindent/typedefs.list
+++ b/src/tools/pgindent/typedefs.list
@@ -315,6 +315,7 @@ BrinStatsData
 BrinTuple
 BrinValues
 BtreeCheckState
+BtreeLastVisibleEntry
 BtreeLevel
 Bucket
 BufFile
-- 
2.39.3 (Apple Git-145)

From b4645780074cdb0a3abdb7c84435d6abad4e3bec Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Fri, 10 May 2024 03:07:22 +0300
Subject: [PATCH v3 4/4] amcheck: Report an error when the next page to a leaf
 is not a leaf

This is a very unlikely condition during checking a B-tree unique constraint,
meaning that the index structure is violated badly, and we shouldn't continue
checking to avoid endless loops, etc.  So it's worth immediately throwing an
error.

Reported-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-Wzk%2B2116uOXdOViA27SHcr31WKPgmjsxXLBs_aTxAeThzg%40mail.gmail.com
Author: Pavel Borisov
---
 contrib/amcheck/verify_nbtree.c | 22 ++++++++++++++++------
 1 file changed, 16 insertions(+), 6 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 977f8b6799d..e9bbc18c4a5 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -1849,7 +1849,6 @@ bt_target_page_check(BtreeCheckState *state)
 		if (offset == max)
 		{
 			BTScanInsert rightkey;
-			BlockNumber rightblock_number;
 
 			/* first offset on a right index page (log only) */
 			OffsetNumber rightfirstoffset = InvalidOffsetNumber;
@@ -1894,10 +1893,11 @@ bt_target_page_check(BtreeCheckState *state)
 			 * If index has unique constraint make sure that no more than one
 			 * found equal items is visible.
 			 */
-			rightblock_number = topaque->btpo_next;
 			if (state->checkunique && state->indexinfo->ii_Unique &&
-				rightkey && P_ISLEAF(topaque) && rightblock_number != P_NONE)
+				rightkey && P_ISLEAF(topaque) && !P_RIGHTMOST(topaque))
 			{
+				BlockNumber rightblock_number = topaque->btpo_next;
+
 				elog(DEBUG2, "check cross page unique condition");
 
 				/*
@@ -1924,9 +1924,19 @@ bt_target_page_check(BtreeCheckState *state)
 												  rightblock_number);
 					topaque = BTPageGetOpaque(rightpage);
 
-					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
-						break;
-
+					if (P_IGNORE(topaque))
+					{
+						if (unlikely(!P_ISLEAF(topaque)))
+							ereport(ERROR,
+									(errcode(ERRCODE_INDEX_CORRUPTED),
+									 errmsg("right block of leaf block is non-leaf for index \"%s\"",
+											RelationGetRelationName(state->rel)),
+									 errdetail_internal("Block=%u page lsn=%X/%X.",
+														state->targetblock,
+														LSN_FORMAT_ARGS(state->targetlsn))));
+						else
+							break;
+					}
 					itemid = PageGetItemIdCareful(state, rightblock_number,
 												  rightpage,
 												  rightfirstoffset);
-- 
2.39.3 (Apple Git-145)

From e9041f10384250472a54bd75a1cf28f89ec15e32 Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Mon, 13 May 2024 04:18:56 +0300
Subject: [PATCH v3 3/4] amcheck: Don't load the right sibling page into
 BtreeCheckState

5ae2087202 implemented a cross-page unique constraint check by loading
the right sibling to the BtreeCheckState.target variable.  This is wrong,
because bt_target_page_check() shouldn't change the target page.  Also,
BtreeCheckState.target shouldn't be changed alone without
BtreeCheckState.targetblock.

However, the above didn't cause any visible bugs for the following reasons.
1. We do a cross-page unique constraint check only for leaf index pages.
2. The only way target page get accessed after a cross-page unique constraint
   check is loading of the lowkey.
3. The only place lowkey is used is bt_child_highkey_check(), and that applies
   only to non-leafs.

The reasons above don't diminish the fact that changing BtreeCheckState.target
for a cross-page unique constraint check is wrong.  This commit changes this
check to temporarily store the right sibling to the local variable.

Reported-by: Peter Geoghegan
Discussion: https://postgr.es/m/CAH2-Wzk%2B2116uOXdOViA27SHcr31WKPgmjsxXLBs_aTxAeThzg%40mail.gmail.com
Author: Pavel Borisov
---
 contrib/amcheck/verify_nbtree.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index b433cb33254..977f8b6799d 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -1910,6 +1910,8 @@ bt_target_page_check(BtreeCheckState *state)
 				/* The first key on the next page is the same */
 				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
 				{
+					Page		rightpage;
+
 					/*
 					 * Do the bt_entry_unique_check() call if it was
 					 * postponed.
@@ -1918,19 +1920,21 @@ bt_target_page_check(BtreeCheckState *state)
 						bt_entry_unique_check(state, itup, state->targetblock, offset, &lVis);
 
 					elog(DEBUG2, "cross page equal keys");
-					state->target = palloc_btree_page(state,
-													  rightblock_number);
-					topaque = BTPageGetOpaque(state->target);
+					rightpage = palloc_btree_page(state,
+												  rightblock_number);
+					topaque = BTPageGetOpaque(rightpage);
 
 					if (P_IGNORE(topaque) || !P_ISLEAF(topaque))
 						break;
 
 					itemid = PageGetItemIdCareful(state, rightblock_number,
-												  state->target,
+												  rightpage,
 												  rightfirstoffset);
-					itup = (IndexTuple) PageGetItem(state->target, itemid);
+					itup = (IndexTuple) PageGetItem(rightpage, itemid);
 
 					bt_entry_unique_check(state, itup, rightblock_number, rightfirstoffset, &lVis);
+
+					pfree(rightpage);
 				}
 			}
 		}
-- 
2.39.3 (Apple Git-145)

From f801783ade88e524b73acfa19f1b06e5b54608df Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Fri, 10 May 2024 03:07:53 +0300
Subject: [PATCH v3 1/4] amcheck: Optimize speed of checking for unique
 constraint violation

Currently, when amcheck validates a unique constraint, it visits the heap for
each index tuple.  This commit implements skipping keys, which have only one
non-dedeuplicated index tuple (quite common case for unique indexes). That
gives substantial economy on index checking time.

Reported-by: Noah Misch
Discussion: https://postgr.es/m/20240325020323.fd.nmisch%40google.com
Author: Alexander Korotkov, Pavel Borisov
---
 contrib/amcheck/verify_nbtree.c | 35 +++++++++++++++++++++++++++++++--
 1 file changed, 33 insertions(+), 2 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 70f65b645a6..c7be785f88b 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -1429,6 +1429,13 @@ bt_target_page_check(BtreeCheckState *state)
 		bool		lowersizelimit;
 		ItemPointer scantid;
 
+		/*
+		 * True if we already called bt_entry_unique_check() for the current
+		 * item.  This helps to avoid visiting the heap for keys, which are
+		 * anyway presented only once and can't comprise a unique violation.
+		 */
+		bool		unique_checked = false;
+
 		CHECK_FOR_INTERRUPTS();
 
 		itemid = PageGetItemIdCareful(state, state->targetblock,
@@ -1771,13 +1778,19 @@ bt_target_page_check(BtreeCheckState *state)
 
 		/*
 		 * If the index is unique verify entries uniqueness by checking the
-		 * heap tuples visibility.
+		 * heap tuples visibility.  Immediately check posting tuples and
+		 * tuples with repeated keys.  Postpone check for keys, which have the
+		 * first appearance.
 		 */
 		if (state->checkunique && state->indexinfo->ii_Unique &&
-			P_ISLEAF(topaque) && !skey->anynullkeys)
+			P_ISLEAF(topaque) && !skey->anynullkeys &&
+			(BTreeTupleIsPosting(itup) || ItemPointerIsValid(lVis_tid)))
+		{
 			bt_entry_unique_check(state, itup, state->targetblock, offset,
 								  &lVis_i, &lVis_tid, &lVis_offset,
 								  &lVis_block);
+			unique_checked = true;
+		}
 
 		if (state->checkunique && state->indexinfo->ii_Unique &&
 			P_ISLEAF(topaque) && OffsetNumberNext(offset) <= max)
@@ -1796,6 +1809,9 @@ bt_target_page_check(BtreeCheckState *state)
 			 * data (whole index tuple or last posting in index tuple). Key
 			 * containing null value does not violate unique constraint and
 			 * treated as different to any other key.
+			 *
+			 * If the next key is the same as the previous one, do the
+			 * bt_entry_unique_check() call if it was postponed.
 			 */
 			if (_bt_compare(state->rel, skey, state->target,
 							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
@@ -1805,6 +1821,12 @@ bt_target_page_check(BtreeCheckState *state)
 				lVis_block = InvalidBlockNumber;
 				lVis_offset = InvalidOffsetNumber;
 			}
+			else if (!unique_checked)
+			{
+				bt_entry_unique_check(state, itup, state->targetblock, offset,
+									  &lVis_i, &lVis_tid, &lVis_offset,
+									  &lVis_block);
+			}
 			skey->scantid = scantid;	/* Restore saved scan key state */
 		}
 
@@ -1889,6 +1911,15 @@ bt_target_page_check(BtreeCheckState *state)
 				/* The first key on the next page is the same */
 				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
 				{
+					/*
+					 * Do the bt_entry_unique_check() call if it was
+					 * postponed.
+					 */
+					if (!unique_checked)
+						bt_entry_unique_check(state, itup, state->targetblock, offset,
+											  &lVis_i, &lVis_tid, &lVis_offset,
+											  &lVis_block);
+
 					elog(DEBUG2, "cross page equal keys");
 					state->target = palloc_btree_page(state,
 													  rightblock_number);
-- 
2.39.3 (Apple Git-145)

From 9e3903fd2497c967aa001010b20167363963017a Mon Sep 17 00:00:00 2001
From: Pavel Borisov <pashkin.elfe@gmail.com>
Date: Mon, 13 May 2024 15:19:16 +0400
Subject: [PATCH] amcheck: Get lowkey only for internal pages of btree index

state->lowkey is used only for checking child pages in btree index inside
bt_child_check()/bt_child_highkey_check() All calls of these functions
possible only from internal pages level. So there is no reason of getting
page lowkey for leaf pages just to invalidate it next without actual usage.
---
 contrib/amcheck/verify_nbtree.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 70f65b645a..8e18a19deb 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -938,13 +938,13 @@ nextpage:
 		 * falls to the boundary of pages on the target level.  See
 		 * bt_child_highkey_check() for details.  So, typically we won't end
 		 * up doing anything with low key, but it's simpler for general case
-		 * high key verification to always have it available.
+		 * high key verification to have it available for all non-leaf pages.
 		 *
 		 * The correctness of managing low key in the case of concurrent
 		 * splits wasn't investigated yet.  Thankfully we only need low key
 		 * for readonly verification and concurrent splits won't happen.
 		 */
-		if (state->readonly && !P_RIGHTMOST(opaque))
+		if (state->readonly && !P_RIGHTMOST(opaque) && !P_ISLEAF(opaque))
 		{
 			IndexTuple	itup;
 			ItemId		itemid;
-- 
2.34.1

From cc7bf7991ff827c4b41c0cd7888a073b91f66827 Mon Sep 17 00:00:00 2001
From: Mark Dilger <mark.dilger@enterprisedb.com>
Date: Fri, 17 May 2024 10:24:11 -0700
Subject: [PATCH v1] Add a WIP corruption checker

To help analyze Alexander Korotkov's v3 series of patches, add a
corruption checker that runs an infinite loop corrupting an index
and seeing if the corruption is detected.

THIS IS NOT FOR COMMIT.
---
 contrib/amcheck/t/006_corrupt_idx.pl | 136 +++++++++++++++++++++++++++
 1 file changed, 136 insertions(+)
 create mode 100644 contrib/amcheck/t/006_corrupt_idx.pl

diff --git a/contrib/amcheck/t/006_corrupt_idx.pl b/contrib/amcheck/t/006_corrupt_idx.pl
new file mode 100644
index 0000000000..d27b0ff9b6
--- /dev/null
+++ b/contrib/amcheck/t/006_corrupt_idx.pl
@@ -0,0 +1,136 @@
+
+# Copyright (c) 2023-2024, PostgreSQL Global Development Group
+
+# This regression test checks the behavior of the btree validation in the
+# presence of breaking sort order changes.
+#
+use strict;
+use warnings FATAL => 'all';
+use PostgreSQL::Test::Cluster;
+use PostgreSQL::Test::Utils;
+use Test::More;
+use Fcntl 'SEEK_SET';
+
+my $node = PostgreSQL::Test::Cluster->new('test');
+$node->init;
+$node->append_conf('postgresql.conf', 'autovacuum = off');
+$node->start;
+
+# Create tables and indexes over some values
+$node->safe_psql('postgres',qq(
+CREATE EXTENSION amcheck;
+CREATE TABLE tbl (i TEXT);
+INSERT INTO tbl (SELECT 'MAGIC_' || gs::TEXT FROM generate_series(1,100000) gs ORDER BY RANDOM());
+CREATE UNIQUE INDEX idx ON tbl (i);
+));	
+
+my @blocks;
+
+while (1)
+{
+	my ($result, $stdout, $stderr);
+
+	$result = $node->safe_psql(
+		'postgres', q(
+		SELECT bt_index_parent_check('idx', true, true, true);
+	));
+	is($result, '', 'run amcheck on non-broken idx');
+
+	my $pgdata = $node->data_dir;
+	my $rel = $node->safe_psql('postgres',
+		qq(SELECT pg_relation_filepath('public.idx')));
+	my $relpath = "$pgdata/$rel";
+	$node->stop;
+
+	my ($blksize, @blocks) = read_blocks($relpath);
+
+	my $ttl = 1000;
+	my $corrupted_blkno = undef;
+	while (!defined($corrupted_blkno) && $ttl--)
+	{
+		my $blkno = int(rand(scalar(@blocks)-1));
+
+		if ($blocks[$blkno] =~ m/.*MAGIC_(\d+)/)
+		{
+			my $magic = $1;
+			my $corrupted_block = $blocks[$blkno];
+
+			my $next_magic = $magic + 1;
+			my $prev_magic = $magic - 1;
+			if ($next_magic >= 1 && $next_magic <= 100000 && $blocks[$blkno+1] =~ m/MAGIC_$next_magic/)
+			{
+				if ($corrupted_block =~ s/MAGIC_$magic/MAGIC_$next_magic/)
+				{
+					write_block($relpath, $blksize, $blkno, $corrupted_block);
+					$corrupted_blkno = $blkno;
+				}
+			}
+			elsif ($prev_magic >= 1 && $prev_magic <= 100000 && $blocks[$blkno+1] =~ m/MAGIC_$prev_magic/)
+			{
+				if ($corrupted_block =~ s/MAGIC_$magic/MAGIC_$prev_magic/)
+				{
+					write_block($relpath, $blksize, $blkno, $corrupted_block);
+					$corrupted_blkno = $blkno;
+				}
+			}
+		}
+	}
+
+	BAIL_OUT("Failed to corrupt anything") unless($ttl > 0);
+
+	# Ok, we've corrupted the file.  Restart the node and see if the
+	# corruption checker notices anything.
+	$node->start;
+
+	($result, $stdout, $stderr) = $node->psql(
+		'postgres', q(
+		SELECT bt_index_parent_check('idx', true, true, true);
+	));
+	ok( $stderr =~ /item order invariant violated for index "idx"|index uniqueness is violated for index "idx"|could not find tuple using search from root page in index "idx"|mismatch between parent key and child high key in index "idx"|detected uniqueness violation for index "idx"/);
+
+	# Repair the damage.
+	$node->stop;
+	write_block($relpath, $blksize, $corrupted_blkno, $blocks[$corrupted_blkno]);
+
+	# Restart the database and confirm the index is back to passing
+	$node->start;
+ 	$result = $node->safe_psql(
+		'postgres', q(
+		SELECT bt_index_check('idx', true, true);
+	));
+	is($result, '', 'run amcheck on non-broken idx');
+}
+
+# Not reached
+done_testing();
+
+sub read_blocks
+{
+	my ($relpath) = @_;
+	my $file;
+	open($file, '+<', $relpath)
+		or BAIL_OUT("open failed: $!");
+	binmode $file;
+	my ($dev, $ino, $mode, $nlink, $uid, $gid, $rdev, $size, $atime, $mtime, $ctime, $blksize, $blocks) = stat($file);
+
+	my @result;
+	for (my $blkno = 0; $blkno < $blocks; $blkno++)
+	{
+		sysseek($file, $blkno * $blksize, SEEK_SET);
+		sysread($file, $result[$blkno], $blksize);
+	}
+	close($file);
+	return ($blksize, @result);
+}
+
+sub write_block
+{
+	my ($relpath, $blksize, $blkno, $block) = @_;
+	my $file;
+	open($file, '+<', $relpath)
+		or BAIL_OUT("open failed: $!");
+	binmode $file;
+	sysseek($file, $blkno * $blksize, SEEK_SET);
+	syswrite($file, $block);
+	close($file);
+}
-- 
2.39.3 (Apple Git-145)

From 324ab164ef7e32dd40e120bc22a79711a82cd77b Mon Sep 17 00:00:00 2001
From: Alexander Korotkov <akorotkov@postgresql.org>
Date: Fri, 10 May 2024 03:07:53 +0300
Subject: [PATCH v4] amcheck: Optimize speed of checking for unique constraint
 violation

Currently, when amcheck validates a unique constraint, it visits the heap for
each index tuple.  This commit implements skipping keys, which have only one
non-dedeuplicated index tuple (quite common case for unique indexes). That
gives substantial economy on index checking time.

Reported-by: Noah Misch
Discussion: https://postgr.es/m/20240325020323.fd.nmisch%40google.com
Author: Alexander Korotkov, Pavel Borisov
---
 contrib/amcheck/verify_nbtree.c | 36 ++++++++++++++++++++++++++++++---
 1 file changed, 33 insertions(+), 3 deletions(-)

diff --git a/contrib/amcheck/verify_nbtree.c b/contrib/amcheck/verify_nbtree.c
index 34990c5cea3..7cfb136763f 100644
--- a/contrib/amcheck/verify_nbtree.c
+++ b/contrib/amcheck/verify_nbtree.c
@@ -1433,6 +1433,13 @@ bt_target_page_check(BtreeCheckState *state)
 		bool		lowersizelimit;
 		ItemPointer scantid;
 
+		/*
+		 * True if we already called bt_entry_unique_check() for the current
+		 * item.  This helps to avoid visiting the heap for keys, which are
+		 * anyway presented only once and can't comprise a unique violation.
+		 */
+		bool		unique_checked = false;
+
 		CHECK_FOR_INTERRUPTS();
 
 		itemid = PageGetItemIdCareful(state, state->targetblock,
@@ -1775,12 +1782,18 @@ bt_target_page_check(BtreeCheckState *state)
 
 		/*
 		 * If the index is unique verify entries uniqueness by checking the
-		 * heap tuples visibility.
+		 * heap tuples visibility.  Immediately check posting tuples and
+		 * tuples with repeated keys.  Postpone check for keys, which have the
+		 * first appearance.
 		 */
 		if (state->checkunique && state->indexinfo->ii_Unique &&
-			P_ISLEAF(topaque) && !skey->anynullkeys)
+			P_ISLEAF(topaque) && !skey->anynullkeys &&
+			(BTreeTupleIsPosting(itup) || ItemPointerIsValid(lVis.tid)))
+		{
 			bt_entry_unique_check(state, itup, state->targetblock, offset,
 								  &lVis);
+			unique_checked = true;
+		}
 
 		if (state->checkunique && state->indexinfo->ii_Unique &&
 			P_ISLEAF(topaque) && OffsetNumberNext(offset) <= max)
@@ -1799,6 +1812,9 @@ bt_target_page_check(BtreeCheckState *state)
 			 * data (whole index tuple or last posting in index tuple). Key
 			 * containing null value does not violate unique constraint and
 			 * treated as different to any other key.
+			 *
+			 * If the next key is the same as the previous one, do the
+			 * bt_entry_unique_check() call if it was postponed.
 			 */
 			if (_bt_compare(state->rel, skey, state->target,
 							OffsetNumberNext(offset)) != 0 || skey->anynullkeys)
@@ -1808,6 +1824,11 @@ bt_target_page_check(BtreeCheckState *state)
 				lVis.postingIndex = -1;
 				lVis.tid = NULL;
 			}
+			else if (!unique_checked)
+			{
+				bt_entry_unique_check(state, itup, state->targetblock, offset,
+									  &lVis);
+			}
 			skey->scantid = scantid;	/* Restore saved scan key state */
 		}
 
@@ -1890,10 +1911,19 @@ bt_target_page_check(BtreeCheckState *state)
 				rightkey->scantid = NULL;
 
 				/* The first key on the next page is the same */
-				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 && !rightkey->anynullkeys)
+				if (_bt_compare(state->rel, rightkey, state->target, max) == 0 &&
+					!rightkey->anynullkeys)
 				{
 					Page		rightpage;
 
+					/*
+					 * Do the bt_entry_unique_check() call if it was
+					 * postponed.
+					 */
+					if (!unique_checked)
+						bt_entry_unique_check(state, itup, state->targetblock,
+											  offset, &lVis);
+
 					elog(DEBUG2, "cross page equal keys");
 					rightpage = palloc_btree_page(state,
 												  rightblock_number);
-- 
2.39.3 (Apple Git-145)