From a6da0047158b8a227f883aeed19eb7fcfbef11fb Mon Sep 17 00:00:00 2001
From: Alvaro Herrera <alvherre@alvh.no-ip.org>
Date: Wed, 20 Mar 2019 18:18:50 -0300
Subject: [PATCH] Add index_get_partition convenience function

This new function simplifies some existing coding, as well as supports
future patches.

Discussion: https://postgr.es/m/201901222145.t6wws6t6vrcu@alvherre.pgsql
Reviewed-by: Amit Langote, Jesper Pedersen
---
 src/backend/catalog/partition.c  | 36 ++++++++++++++++++++++++++++
 src/backend/commands/tablecmds.c | 40 +++++++++-----------------------
 src/include/catalog/partition.h  |  1 +
 3 files changed, 48 insertions(+), 29 deletions(-)

diff --git a/src/backend/catalog/partition.c b/src/backend/catalog/partition.c
index 3ccdaff8c4..8ea7a62418 100644
--- a/src/backend/catalog/partition.c
+++ b/src/backend/catalog/partition.c
@@ -145,6 +145,42 @@ get_partition_ancestors_worker(Relation inhRel, Oid relid, List **ancestors)
 	get_partition_ancestors_worker(inhRel, parentOid, ancestors);
 }
 
+/*
+ * index_get_partition
+ *		Return the OID of index of the given partition that is a child
+ *		of the given index, or InvalidOid if there isn't one.
+ */
+Oid
+index_get_partition(Relation partition, Oid indexId)
+{
+	List	   *idxlist = RelationGetIndexList(partition);
+	ListCell   *l;
+
+	foreach(l, idxlist)
+	{
+		Oid			partIdx = lfirst_oid(l);
+		HeapTuple	tup;
+		Form_pg_class classForm;
+		bool		ispartition;
+
+		tup = SearchSysCache1(RELOID, ObjectIdGetDatum(partIdx));
+		if (!tup)
+			elog(ERROR, "cache lookup failed for relation %u", partIdx);
+		classForm = (Form_pg_class) GETSTRUCT(tup);
+		ispartition = classForm->relispartition;
+		ReleaseSysCache(tup);
+		if (!ispartition)
+			continue;
+		if (get_partition_parent(lfirst_oid(l)) == indexId)
+		{
+			list_free(idxlist);
+			return partIdx;
+		}
+	}
+
+	return InvalidOid;
+}
+
 /*
  * map_partition_varattnos - maps varattno of any Vars in expr from the
  * attno's of 'from_rel' to the attno's of 'to_rel' partition, each of which
diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 515c29072c..3183b2aaa1 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -15649,36 +15649,18 @@ ATExecAttachPartitionIdx(List **wqueue, Relation parentIdx, RangeVar *name)
 static void
 refuseDupeIndexAttach(Relation parentIdx, Relation partIdx, Relation partitionTbl)
 {
-	Relation	pg_inherits;
-	ScanKeyData key;
-	HeapTuple	tuple;
-	SysScanDesc scan;
+	Oid			existingIdx;
 
-	pg_inherits = table_open(InheritsRelationId, AccessShareLock);
-	ScanKeyInit(&key, Anum_pg_inherits_inhparent,
-				BTEqualStrategyNumber, F_OIDEQ,
-				ObjectIdGetDatum(RelationGetRelid(parentIdx)));
-	scan = systable_beginscan(pg_inherits, InheritsParentIndexId, true,
-							  NULL, 1, &key);
-	while (HeapTupleIsValid(tuple = systable_getnext(scan)))
-	{
-		Form_pg_inherits inhForm;
-		Oid			tab;
-
-		inhForm = (Form_pg_inherits) GETSTRUCT(tuple);
-		tab = IndexGetRelation(inhForm->inhrelid, false);
-		if (tab == RelationGetRelid(partitionTbl))
-			ereport(ERROR,
-					(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
-					 errmsg("cannot attach index \"%s\" as a partition of index \"%s\"",
-							RelationGetRelationName(partIdx),
-							RelationGetRelationName(parentIdx)),
-					 errdetail("Another index is already attached for partition \"%s\".",
-							   RelationGetRelationName(partitionTbl))));
-	}
-
-	systable_endscan(scan);
-	table_close(pg_inherits, AccessShareLock);
+	existingIdx = index_get_partition(partitionTbl,
+									  RelationGetRelid(parentIdx));
+	if (OidIsValid(existingIdx))
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("cannot attach index \"%s\" as a partition of index \"%s\"",
+						RelationGetRelationName(partIdx),
+						RelationGetRelationName(parentIdx)),
+				 errdetail("Another index is already attached for partition \"%s\".",
+						   RelationGetRelationName(partitionTbl))));
 }
 
 /*
diff --git a/src/include/catalog/partition.h b/src/include/catalog/partition.h
index d84e325983..616e18af30 100644
--- a/src/include/catalog/partition.h
+++ b/src/include/catalog/partition.h
@@ -21,6 +21,7 @@
 
 extern Oid	get_partition_parent(Oid relid);
 extern List *get_partition_ancestors(Oid relid);
+extern Oid	index_get_partition(Relation partition, Oid indexId);
 extern List *map_partition_varattnos(List *expr, int fromrel_varno,
 						Relation to_rel, Relation from_rel,
 						bool *found_whole_row);