Make pg_partition_tree return no rows on unsupported and undefined objects

The function was tweaked so as it returned one row full of NULLs when
working on an unsupported relkind or an undefined object as of cc53123b,
and after discussion with Amit and Álvaro it looks more natural to make
it return no rows.

Author: Michael Paquier
Reviewed-by: Álvaro Herrera, Amit Langote
Discussion: https://postgr.es/m/20190227184808.GA17357@alvherre.pgsql

src/backend/utils/adt/partitionfuncs.c

@@ -69,9 +69,6 @@ pg_partition_tree(PG_FUNCTION_ARGS)
 	FuncCallContext *funcctx;
 	ListCell  **next;
 
-	if (!check_rel_can_be_partition(rootrelid))
-		PG_RETURN_NULL();
-
 	/* stuff done only on the first call of the function */
 	if (SRF_IS_FIRSTCALL())
 	{
@@ -82,6 +79,9 @@ pg_partition_tree(PG_FUNCTION_ARGS)
 		/* create a function context for cross-call persistence */
 		funcctx = SRF_FIRSTCALL_INIT();
 
+		if (!check_rel_can_be_partition(rootrelid))
+			SRF_RETURN_DONE(funcctx);
+
 		/* switch to memory context appropriate for multiple function calls */
 		oldcxt = MemoryContextSwitchTo(funcctx->multi_call_memory_ctx);
 

src/test/regress/expected/partition_info.out

@@ -9,8 +9,7 @@ SELECT * FROM pg_partition_tree(NULL);
 SELECT * FROM pg_partition_tree(0);
  relid | parentrelid | isleaf | level 
 -------+-------------+--------+-------
-       |             |        |      
-(1 row)
+(0 rows)
 
 SELECT pg_partition_root(NULL);
  pg_partition_root 
@@ -163,14 +162,12 @@ CREATE MATERIALIZED VIEW ptif_test_matview AS SELECT 1;
 SELECT * FROM pg_partition_tree('ptif_test_view');
  relid | parentrelid | isleaf | level 
 -------+-------------+--------+-------
-       |             |        |      
-(1 row)
+(0 rows)
 
 SELECT * FROM pg_partition_tree('ptif_test_matview');
  relid | parentrelid | isleaf | level 
 -------+-------------+--------+-------
-       |             |        |      
-(1 row)
+(0 rows)
 
 SELECT pg_partition_root('ptif_test_view');
  pg_partition_root