Treat JsonConstructorExpr as non-strict

Richard Guo · guofengrichard · Richard Guo · commit d29a3f4b46a0 · 2025-09-16T18:43:57.000+09:00
JsonConstructorExpr can produce non-NULL output with a NULL input, so it should be treated as a non-strict construct. Failing to do so can lead to incorrect query behavior. For example, in the reported case, when pulling up a subquery that is under an outer join, if the subquery's target list contains a JsonConstructorExpr that uses subquery variables and it is mistakenly treated as strict, it will be pulled up without being wrapped in a PlaceHolderVar. As a result, the expression will be evaluated at the wrong place and will not be forced to null when the outer join should do so. Back-patch to v16 where JsonConstructorExpr was introduced. Bug: #19046 Reported-by: Runyuan He <runyuan@berkeley.edu> Author: Tender Wang <tndrwang@gmail.com> Co-authored-by: Richard Guo <guofenglinux@gmail.com> Discussion: https://postgr.es/m/19046-765b6602b0a8cfdf@postgresql.org Backpatch-through: 16
diff --git a/src/backend/optimizer/util/clauses.c b/src/backend/optimizer/util/clauses.c
@@ -1112,6 +1112,8 @@ contain_nonstrict_functions_walker(Node *node, void *context)
 		return true;
 	if (IsA(node, BooleanTest))
 		return true;
+	if (IsA(node, JsonConstructorExpr))
+		return true;
 
 	/* Check other function-containing nodes */
 	if (check_functions_in_node(node, contain_nonstrict_functions_checker,
diff --git a/src/test/regress/expected/subselect.out b/src/test/regress/expected/subselect.out
@@ -1783,6 +1783,34 @@ fetch backward all in c1;
 
 commit;
 --
+-- Check that JsonConstructorExpr is treated as non-strict, and thus can be
+-- wrapped in a PlaceHolderVar
+--
+begin;
+create temp table json_tab (a int);
+insert into json_tab values (1);
+explain (verbose, costs off)
+select * from json_tab t1 left join (select json_array(1, a) from json_tab t2) s on false;
+                    QUERY PLAN                     
+---------------------------------------------------
+ Nested Loop Left Join
+   Output: t1.a, (JSON_ARRAY(1, a RETURNING json))
+   Join Filter: false
+   ->  Seq Scan on pg_temp.json_tab t1
+         Output: t1.a
+   ->  Result
+         Output: JSON_ARRAY(1, a RETURNING json)
+         One-Time Filter: false
+(8 rows)
+
+select * from json_tab t1 left join (select json_array(1, a) from json_tab t2) s on false;
+ a | json_array 
+---+------------
+ 1 | 
+(1 row)
+
+rollback;
+--
 -- Verify that we correctly flatten cases involving a subquery output
 -- expression that doesn't need to be wrapped in a PlaceHolderVar
 --
diff --git a/src/test/regress/sql/subselect.sql b/src/test/regress/sql/subselect.sql
@@ -929,6 +929,23 @@ fetch backward all in c1;
 
 commit;
 
+--
+-- Check that JsonConstructorExpr is treated as non-strict, and thus can be
+-- wrapped in a PlaceHolderVar
+--
+
+begin;
+
+create temp table json_tab (a int);
+insert into json_tab values (1);
+
+explain (verbose, costs off)
+select * from json_tab t1 left join (select json_array(1, a) from json_tab t2) s on false;
+
+select * from json_tab t1 left join (select json_array(1, a) from json_tab t2) s on false;
+
+rollback;
+
 --
 -- Verify that we correctly flatten cases involving a subquery output
 -- expression that doesn't need to be wrapped in a PlaceHolderVar
