From 2ae09254584340b7700e31680d752822a782be75 Mon Sep 17 00:00:00 2001
From: Maksim Kita <kitaetoya@gmail.com>
Date: Wed, 7 Oct 2020 17:18:01 +0300
Subject: [PATCH] Allow alter type delete value in enum

---
 src/backend/catalog/pg_enum.c   | 60 +++++++++++++++++++++++++++++++++
 src/backend/commands/typecmds.c |  8 +++--
 src/backend/parser/gram.y       | 11 ++++++
 src/include/catalog/pg_enum.h   |  1 +
 4 files changed, 78 insertions(+), 2 deletions(-)

diff --git a/src/backend/catalog/pg_enum.c b/src/backend/catalog/pg_enum.c
index 27e4100a6f..c547dfd22c 100644
--- a/src/backend/catalog/pg_enum.c
+++ b/src/backend/catalog/pg_enum.c
@@ -583,6 +583,66 @@ RenameEnumLabel(Oid enumTypeOid,
 	table_close(pg_enum, RowExclusiveLock);
 }
 
+extern void RemoveEnumLabel(Oid enumTypeOid, const char *val)
+{
+	Relation	pg_enum;
+	HeapTuple	enum_tup;
+	Form_pg_enum en;
+	CatCList   *list;
+	int			nelems;
+	HeapTuple	old_tup;
+	int			i;
+
+	/* check length of new label is ok */
+	if (strlen(val) > (NAMEDATALEN - 1))
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_NAME),
+				 errmsg("invalid enum label \"%s\"", val),
+				 errdetail("Labels must be %d characters or less.",
+						   NAMEDATALEN - 1)));
+
+	/*
+	 * Acquire a lock on the enum type, which we won't release until commit.
+	 * This ensures that two backends aren't concurrently modifying the same
+	 * enum type.  Since we are not changing the type's sort order, this is
+	 * probably not really necessary, but there seems no reason not to take
+	 * the lock to be sure.
+	 */
+	LockDatabaseObject(TypeRelationId, enumTypeOid, 0, ExclusiveLock);
+
+	pg_enum = table_open(EnumRelationId, RowExclusiveLock);
+
+	/* Get the list of existing members of the enum */
+	list = SearchSysCacheList1(ENUMTYPOIDNAME,
+							   ObjectIdGetDatum(enumTypeOid));
+	nelems = list->n_members;
+
+	/*
+	 * Check if the label is in use.  (The unique index on pg_enum would catch that anyway, but we
+	 * prefer a friendlier error message.)
+	 */
+	for (i = 0; i < nelems; i++)
+	{
+		enum_tup = &(list->members[i]->tuple);
+		en = (Form_pg_enum) GETSTRUCT(enum_tup);
+		if (strcmp(NameStr(en->enumlabel), val) == 0)
+		{
+			old_tup = enum_tup;
+			break;
+		}
+	}
+	if (!old_tup)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+				 errmsg("\"%s\" is not an existing enum label",
+						val)));
+
+	ReleaseCatCacheList(list);
+
+	CatalogTupleDelete(pg_enum, &old_tup->t_self);
+
+	table_close(pg_enum, RowExclusiveLock);
+}
 
 /*
  * Test if the given enum value is on the blacklist
diff --git a/src/backend/commands/typecmds.c b/src/backend/commands/typecmds.c
index 483bb65ddc..b9b3e18a94 100644
--- a/src/backend/commands/typecmds.c
+++ b/src/backend/commands/typecmds.c
@@ -1232,13 +1232,17 @@ AlterEnum(AlterEnumStmt *stmt)
 
 	ReleaseSysCache(tup);
 
-	if (stmt->oldVal)
+	if (stmt->oldVal && stmt->newVal)
 	{
 		/* Rename an existing label */
 		RenameEnumLabel(enum_type_oid, stmt->oldVal, stmt->newVal);
 	}
-	else
+	else if (stmt->oldVal)
 	{
+		/* Delete an existing label */
+		RemoveEnumLabel(enum_type_oid, stmt->oldVal);
+	}
+	else {
 		/* Add a new label */
 		AddEnumLabel(enum_type_oid, stmt->newVal,
 					 stmt->newValNeighbor, stmt->newValIsAfter,
diff --git a/src/backend/parser/gram.y b/src/backend/parser/gram.y
index 0d101d8171..82dd5147fa 100644
--- a/src/backend/parser/gram.y
+++ b/src/backend/parser/gram.y
@@ -5792,6 +5792,17 @@ AlterEnumStmt:
 				n->skipIfNewValExists = false;
 				$$ = (Node *) n;
 			}
+		 | ALTER TYPE_P any_name DELETE_P VALUE_P Sconst
+		 	{
+				AlterEnumStmt *n = makeNode(AlterEnumStmt);
+				n->typeName = $3;
+				n->oldVal = $6;
+				n->newVal = NULL;
+				n->newValNeighbor = NULL;
+				n->newValIsAfter = false;
+				n->skipIfNewValExists = false;
+				$$ = (Node *) n;
+			 }
 		 ;
 
 opt_if_not_exists: IF_P NOT EXISTS              { $$ = true; }
diff --git a/src/include/catalog/pg_enum.h b/src/include/catalog/pg_enum.h
index b28d441ba7..3dd81cd892 100644
--- a/src/include/catalog/pg_enum.h
+++ b/src/include/catalog/pg_enum.h
@@ -53,6 +53,7 @@ extern void AddEnumLabel(Oid enumTypeOid, const char *newVal,
 						 bool skipIfExists);
 extern void RenameEnumLabel(Oid enumTypeOid,
 							const char *oldVal, const char *newVal);
+extern void RemoveEnumLabel(Oid enumTypeOid, const char *val);
 extern bool EnumBlacklisted(Oid enum_id);
 extern Size EstimateEnumBlacklistSpace(void);
 extern void SerializeEnumBlacklist(void *space, Size size);
-- 
2.25.1