Set all variable-length fields of pg_attribute to null on column drop

From d587588df6d2bea8ea9b9e13b897e4206b1a0884 Mon Sep 17 00:00:00 2001
From: Peter Eisentraut <peter@eisentraut.org>
Date: Thu, 30 Nov 2023 12:19:20 +0100
Subject: [PATCH v1] Set all variable-length fields of pg_attribute to null on
 column drop

When a column is dropped, the fields attacl, attoptions, and
attfdwoptions were kept unchanged.  This is probably harmless, but it
seems wasteful, and leaves potentially dangling data lying around (for
example, attacl could contain references to users that are later also
dropped).

Change this to set those fields to null when a column is marked as
dropped.
---
 src/backend/catalog/heap.c | 39 ++++++++++++++++++++------------------
 1 file changed, 21 insertions(+), 18 deletions(-)

diff --git a/src/backend/catalog/heap.c b/src/backend/catalog/heap.c
index 7224d96695..b93894889d 100644
--- a/src/backend/catalog/heap.c
+++ b/src/backend/catalog/heap.c
@@ -1647,6 +1647,9 @@ RemoveAttributeById(Oid relid, AttrNumber attnum)
 	HeapTuple	tuple;
 	Form_pg_attribute attStruct;
 	char		newattname[NAMEDATALEN];
+	Datum		valuesAtt[Natts_pg_attribute] = {0};
+	bool		nullsAtt[Natts_pg_attribute] = {0};
+	bool		replacesAtt[Natts_pg_attribute] = {0};
 
 	/*
 	 * Grab an exclusive lock on the target table, which we will NOT release
@@ -1695,24 +1698,24 @@ RemoveAttributeById(Oid relid, AttrNumber attnum)
 			 "........pg.dropped.%d........", attnum);
 	namestrcpy(&(attStruct->attname), newattname);
 
-	/* clear the missing value if any */
-	if (attStruct->atthasmissing)
-	{
-		Datum		valuesAtt[Natts_pg_attribute] = {0};
-		bool		nullsAtt[Natts_pg_attribute] = {0};
-		bool		replacesAtt[Natts_pg_attribute] = {0};
-
-		/* update the tuple - set atthasmissing and attmissingval */
-		valuesAtt[Anum_pg_attribute_atthasmissing - 1] =
-			BoolGetDatum(false);
-		replacesAtt[Anum_pg_attribute_atthasmissing - 1] = true;
-		valuesAtt[Anum_pg_attribute_attmissingval - 1] = (Datum) 0;
-		nullsAtt[Anum_pg_attribute_attmissingval - 1] = true;
-		replacesAtt[Anum_pg_attribute_attmissingval - 1] = true;
-
-		tuple = heap_modify_tuple(tuple, RelationGetDescr(attr_rel),
-								  valuesAtt, nullsAtt, replacesAtt);
-	}
+	/* Clear the missing value */
+	attStruct->atthasmissing = false;
+	nullsAtt[Anum_pg_attribute_attmissingval - 1] = true;
+	replacesAtt[Anum_pg_attribute_attmissingval - 1] = true;
+
+	/*
+	 * Clear the other variable-length fields.  This saves some space in
+	 * pg_attribute and removes no longer useful information.
+	 */
+	nullsAtt[Anum_pg_attribute_attacl - 1] = true;
+	replacesAtt[Anum_pg_attribute_attacl - 1] = true;
+	nullsAtt[Anum_pg_attribute_attoptions - 1] = true;
+	replacesAtt[Anum_pg_attribute_attoptions - 1] = true;
+	nullsAtt[Anum_pg_attribute_attfdwoptions - 1] = true;
+	replacesAtt[Anum_pg_attribute_attfdwoptions - 1] = true;
+
+	tuple = heap_modify_tuple(tuple, RelationGetDescr(attr_rel),
+							  valuesAtt, nullsAtt, replacesAtt);
 
 	CatalogTupleUpdate(attr_rel, &tuple->t_self, tuple);
 
-- 
2.43.0