preserve-CLUSTER-ON-during-ALTER-TABLE.patch

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index f599393473..ea9941278e 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -490,6 +490,7 @@ static void ATPostAlterTypeParse(Oid oldId, Oid oldRelId, Oid refRelId,
 static void RebuildConstraintComment(AlteredTableInfo *tab, int pass,
 									 Oid objid, Relation rel, List *domname,
 									 const char *conname);
+static void PreserveClusterOn(AlteredTableInfo *tab, int pass, Oid indoid);
 static void TryReuseIndex(Oid oldId, IndexStmt *stmt);
 static void TryReuseForeignKey(Oid oldId, Constraint *con);
 static ObjectAddress ATExecAlterColumnGenericOptions(Relation rel, const char *colName,
@@ -11832,6 +11833,9 @@ ATPostAlterTypeParse(Oid oldId, Oid oldRelId, Oid refRelId, char *cmd,
 			newcmd->def = (Node *) stmt;
 			tab->subcmds[AT_PASS_OLD_INDEX] =
 				lappend(tab->subcmds[AT_PASS_OLD_INDEX], newcmd);
+
+			/* Preserve index's indisclustered property, if set. */
+			PreserveClusterOn(tab, AT_PASS_OLD_INDEX, oldId);
 		}
 		else if (IsA(stm, AlterTableStmt))
 		{
@@ -11868,6 +11872,9 @@ ATPostAlterTypeParse(Oid oldId, Oid oldRelId, Oid refRelId, char *cmd,
 											 rel,
 											 NIL,
 											 indstmt->idxname);
+
+					/* Preserve index's indisclustered property, if set. */
+					PreserveClusterOn(tab, AT_PASS_OLD_INDEX, indoid);
 				}
 				else if (cmd->subtype == AT_AddConstraint)
 				{
@@ -11990,6 +11997,37 @@ RebuildConstraintComment(AlteredTableInfo *tab, int pass, Oid objid,
 	tab->subcmds[pass] = lappend(tab->subcmds[pass], newcmd);
 }
 
+/*
+ * For an index on table that's being recreated due PostAlterType processing,
+ * preserve its indisclustered property by issuing ALTER TABLE CLUSTER ON on
+ * table to run after the command to re-create the index.
+ */
+static void
+PreserveClusterOn(AlteredTableInfo *tab, int pass, Oid indoid)
+{
+	HeapTuple	indexTuple;
+	Form_pg_index indexForm;
+
+	Assert(OidIsValid(indoid));
+	Assert(pass == AT_PASS_OLD_INDEX);
+
+	indexTuple = SearchSysCache1(INDEXRELID, ObjectIdGetDatum(indoid));
+	if (!HeapTupleIsValid(indexTuple))
+		elog(ERROR, "cache lookup failed for index %u", indoid);
+	indexForm = (Form_pg_index) GETSTRUCT(indexTuple);
+
+	if (indexForm->indisclustered)
+	{
+		AlterTableCmd *newcmd = makeNode(AlterTableCmd);
+
+		newcmd->subtype = AT_ClusterOn;
+		newcmd->name = get_rel_name(indoid);
+		tab->subcmds[pass] = lappend(tab->subcmds[pass], newcmd);
+	}
+
+	ReleaseSysCache(indexTuple);
+}
+
 /*
  * Subroutine for ATPostAlterTypeParse().  Calls out to CheckIndexCompatible()
  * for the real analysis, then mutates the IndexStmt based on that verdict.
diff --git a/src/test/regress/expected/alter_table.out b/src/test/regress/expected/alter_table.out
index e73ce4b6f3..010a7aa98b 100644
--- a/src/test/regress/expected/alter_table.out
+++ b/src/test/regress/expected/alter_table.out
@@ -4258,3 +4258,37 @@ alter table at_test_sql_partop attach partition at_test_sql_partop_1 for values
 drop table at_test_sql_partop;
 drop operator class at_test_sql_partop using btree;
 drop function at_test_sql_partop;
+-- alter type shouldn't lose clustered index
+create table alttype_cluster (a int);
+create index alttype_cluster_a on alttype_cluster (a);
+alter table alttype_cluster cluster on alttype_cluster_a;
+select indisclustered from pg_index where indrelid = 'alttype_cluster'::regclass;
+ indisclustered 
+----------------
+ t
+(1 row)
+
+alter table alttype_cluster alter a type bigint;
+select indisclustered from pg_index where indrelid = 'alttype_cluster'::regclass;
+ indisclustered 
+----------------
+ t
+(1 row)
+
+drop index alttype_cluster_a;
+alter table alttype_cluster add primary key (a);
+alter table alttype_cluster cluster on alttype_cluster_pkey;
+select indisclustered from pg_index where indrelid = 'alttype_cluster'::regclass;
+ indisclustered 
+----------------
+ t
+(1 row)
+
+alter table alttype_cluster alter a type int;
+select indisclustered from pg_index where indrelid = 'alttype_cluster'::regclass;
+ indisclustered 
+----------------
+ t
+(1 row)
+
+drop table alttype_cluster;
diff --git a/src/test/regress/sql/alter_table.sql b/src/test/regress/sql/alter_table.sql
index a16e4c9a29..d619a98de7 100644
--- a/src/test/regress/sql/alter_table.sql
+++ b/src/test/regress/sql/alter_table.sql
@@ -2801,3 +2801,18 @@ alter table at_test_sql_partop attach partition at_test_sql_partop_1 for values
 drop table at_test_sql_partop;
 drop operator class at_test_sql_partop using btree;
 drop function at_test_sql_partop;
+
+-- alter type shouldn't lose clustered index
+create table alttype_cluster (a int);
+create index alttype_cluster_a on alttype_cluster (a);
+alter table alttype_cluster cluster on alttype_cluster_a;
+select indisclustered from pg_index where indrelid = 'alttype_cluster'::regclass;
+alter table alttype_cluster alter a type bigint;
+select indisclustered from pg_index where indrelid = 'alttype_cluster'::regclass;
+drop index alttype_cluster_a;
+alter table alttype_cluster add primary key (a);
+alter table alttype_cluster cluster on alttype_cluster_pkey;
+select indisclustered from pg_index where indrelid = 'alttype_cluster'::regclass;
+alter table alttype_cluster alter a type int;
+select indisclustered from pg_index where indrelid = 'alttype_cluster'::regclass;
+drop table alttype_cluster;