From f93bbd6c30e883068f46ff86def28d0e66aea4f5 Mon Sep 17 00:00:00 2001
From: Justin Pryzby <pryzbyj@telsasoft.com>
Date: Fri, 7 Feb 2020 22:06:57 -0600
Subject: [PATCH v1 1/3] Preserve CLUSTER ON index during ALTER rewrite

Amit Langote and Justin Pryzby

https://www.postgresql.org/message-id/flat/20200202161718.GI13621%40telsasoft.com
---
 src/backend/commands/tablecmds.c | 39 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 39 insertions(+)

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index b7c8d66..642a85c 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,
@@ -11838,6 +11839,9 @@ 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))
 		{
@@ -11874,6 +11878,9 @@ 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)
 				{
@@ -11997,6 +12004,38 @@ RebuildConstraintComment(AlteredTableInfo *tab, int pass, Oid objid,
 }
 
 /*
+ * For a table's index that is to be recreated due to PostAlterType
+ * processing, preserve its indisclustered property by issuing ALTER TABLE
+ * CLUSTER ON command on the table that will run after the command to recreate
+ * 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.
  */
-- 
2.7.4