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

@@ -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

src/backend/commands/tablecmds.c

@@ -15649,25 +15649,11 @@ 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))
+	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\"",
@@ -15677,10 +15663,6 @@ refuseDupeIndexAttach(Relation parentIdx, Relation partIdx, Relation partitionTb
 						   RelationGetRelationName(partitionTbl))));
 }
 
-	systable_endscan(scan);
-	table_close(pg_inherits, AccessShareLock);
-}
-
 /*
  * Verify whether the set of attached partition indexes to a parent index on
  * a partitioned table is complete.  If it is, mark the parent index valid.

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);