Fix buggy recursion in flatten_rtes_walker().

Must save-and-restore the context we are modifying.
Oversight in commit a61b1f748.

Tender Wang

Discussion: https://postgr.es/m/CAHewXNnnNySD_YcKNuFpQDV2gxWA7_YLWqHmYVcyoOYxn8kY2A@mail.gmail.com
Discussion: https://postgr.es/m/20230212233711.GA1316@telsasoft.com

src/backend/optimizer/plan/setrefs.c

@@ -536,11 +536,16 @@ flatten_rtes_walker(Node *node, flatten_rtes_walker_context *cxt)
 		 * Recurse into subselects.  Must update cxt->query to this query so
 		 * that the rtable and rteperminfos correspond with each other.
 		 */
+		Query	   *save_query = cxt->query;
+		bool		result;
+
 		cxt->query = (Query *) node;
-		return query_tree_walker((Query *) node,
+		result = query_tree_walker((Query *) node,
 								   flatten_rtes_walker,
 								   (void *) cxt,
 								   QTW_EXAMINE_RTES_BEFORE);
+		cxt->query = save_query;
+		return result;
 	}
 	return expression_tree_walker(node, flatten_rtes_walker,
 								  (void *) cxt);

src/test/regress/expected/join.out

@@ -5569,6 +5569,18 @@ select atts.relid::regclass, s.* from pg_stats s join
 ERROR:  column atts.relid does not exist
 LINE 1: select atts.relid::regclass, s.* from pg_stats s join
                ^
+-- Test bug in rangetable flattening
+explain (verbose, costs off)
+select 1 from
+  (select * from int8_tbl where q1 <> (select 42) offset 0) ss
+where false;
+        QUERY PLAN        
+--------------------------
+ Result
+   Output: 1
+   One-Time Filter: false
+(3 rows)
+
 --
 -- Test LATERAL
 --

src/test/regress/sql/join.sql

@@ -2086,6 +2086,12 @@ select atts.relid::regclass, s.* from pg_stats s join
     indexrelid from pg_index i) atts on atts.attnum = a.attnum where
     schemaname != 'pg_catalog';
 
+-- Test bug in rangetable flattening
+explain (verbose, costs off)
+select 1 from
+  (select * from int8_tbl where q1 <> (select 42) offset 0) ss
+where false;
+
 --
 -- Test LATERAL
 --