Reset relhassubclass upon attaching table as a partition

We don't allow inheritance parents as partitions, and have checks to
prevent this; but if a table _was_ in the past an inheritance parents
and all their children are removed, the pg_class.relhassubclass flag
may remain set, which confuses the partition pruning code (most
obviously, it results in an assertion failure; in production builds it
may be worse.)

Fix by resetting relhassubclass on attach.

Backpatch to all supported versions.

Reported-by: Alexander Lakhin <exclusion@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/18550-d5e047e9a897a889@postgresql.org

src/backend/catalog/heap.c

@@ -3520,6 +3520,14 @@ StorePartitionBound(Relation rel, Relation parent, PartitionBoundSpec *bound)
 								 new_val, new_null, new_repl);
 	/* Also set the flag */
 	((Form_pg_class) GETSTRUCT(newtuple))->relispartition = true;
+
+	/*
+	 * We already checked for no inheritance children, but reset
+	 * relhassubclass in case it was left over.
+	 */
+	if (rel->rd_rel->relkind == RELKIND_RELATION && rel->rd_rel->relhassubclass)
+		((Form_pg_class) GETSTRUCT(newtuple))->relhassubclass = false;
+
 	CatalogTupleUpdate(classRel, &newtuple->t_self, newtuple);
 	heap_freetuple(newtuple);
 	table_close(classRel, RowExclusiveLock);

src/test/regress/expected/alter_table.out

@@ -3950,8 +3950,16 @@ ALTER TABLE list_parted ATTACH PARTITION child FOR VALUES IN (1);
 ERROR:  cannot attach inheritance child as partition
 ALTER TABLE list_parted ATTACH PARTITION parent FOR VALUES IN (1);
 ERROR:  cannot attach inheritance parent as partition
+DROP TABLE child;
+-- now it should work, with a little tweak
+ALTER TABLE parent ADD CONSTRAINT check_a CHECK (a > 0);
+ALTER TABLE list_parted ATTACH PARTITION parent FOR VALUES IN (1);
+-- test insert/update, per bug #18550
+INSERT INTO parent VALUES (1);
+UPDATE parent SET a = 2 WHERE a = 1;
+ERROR:  new row for relation "parent" violates partition constraint
+DETAIL:  Failing row contains (2, null).
 DROP TABLE parent CASCADE;
-NOTICE:  drop cascades to table child
 -- check any TEMP-ness
 CREATE TEMP TABLE temp_parted (a int) PARTITION BY LIST (a);
 CREATE TABLE perm_part (a int);

src/test/regress/sql/alter_table.sql

@@ -2425,6 +2425,13 @@ CREATE TABLE parent (LIKE list_parted);
 CREATE TABLE child () INHERITS (parent);
 ALTER TABLE list_parted ATTACH PARTITION child FOR VALUES IN (1);
 ALTER TABLE list_parted ATTACH PARTITION parent FOR VALUES IN (1);
+DROP TABLE child;
+-- now it should work, with a little tweak
+ALTER TABLE parent ADD CONSTRAINT check_a CHECK (a > 0);
+ALTER TABLE list_parted ATTACH PARTITION parent FOR VALUES IN (1);
+-- test insert/update, per bug #18550
+INSERT INTO parent VALUES (1);
+UPDATE parent SET a = 2 WHERE a = 1;
 DROP TABLE parent CASCADE;
 
 -- check any TEMP-ness