Fix possible crash in add_paths_to_append_rel()

david-rowley · david-rowley · commit fc4089f3c65a · 2023-10-10T16:50:03.000+13:00
While working on a8a968a, I failed to consider that cheapest_startup_path can be NULL when there is no non-parameterized path in the pathlist. This is well documented in set_cheapest(), I just failed to notice. Here we adjust the code to just check if the RelOptInfo has a cheapest_startup_path set before adding it to the startup_subpaths list. Reported-by: Richard Guo Author: Richard Guo Discussion: https://postgr.es/m/CAMbWs49w3t03V69XhdCuw+GDwivny4uQUxrkVp6Gejaspt0wMQ@mail.gmail.com
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
@@ -1350,14 +1350,17 @@ add_paths_to_append_rel(PlannerInfo *root, RelOptInfo *rel,
 
 		/*
 		 * When the planner is considering cheap startup plans, we'll also
-		 * collect all the cheapest_startup_paths and build an AppendPath
-		 * containing those as subpaths.
+		 * collect all the cheapest_startup_paths (if set) and build an
+		 * AppendPath containing those as subpaths.
 		 */
-		if (rel->consider_startup && childrel->pathlist != NIL &&
-			childrel->cheapest_startup_path->param_info == NULL)
+		if (rel->consider_startup && childrel->cheapest_startup_path != NULL)
+		{
+			/* cheapest_startup_path must not be a parameterized path. */
+			Assert(childrel->cheapest_startup_path->param_info == NULL);
 			accumulate_append_subpath(childrel->cheapest_startup_path,
 									  &startup_subpaths,
 									  NULL);
+		}
 		else
 			startup_subpaths_valid = false;
 
diff --git a/src/test/regress/expected/union.out b/src/test/regress/expected/union.out
@@ -1453,3 +1453,19 @@ inner join tenk2 t2 on t1.tenthous = t2.tenthous
          ->  Result
 (8 rows)
 
+-- Ensure there is no problem if cheapest_startup_path is NULL
+explain (costs off)
+select * from tenk1 t1
+left join lateral
+  (select t1.tenthous from tenk2 t2 union all (values(1)))
+on true limit 1;
+                            QUERY PLAN                             
+-------------------------------------------------------------------
+ Limit
+   ->  Nested Loop Left Join
+         ->  Seq Scan on tenk1 t1
+         ->  Append
+               ->  Index Only Scan using tenk2_hundred on tenk2 t2
+               ->  Result
+(6 rows)
+
diff --git a/src/test/regress/sql/union.sql b/src/test/regress/sql/union.sql
@@ -550,4 +550,11 @@ explain (costs off)
 select t1.unique1 from tenk1 t1
 inner join tenk2 t2 on t1.tenthous = t2.tenthous
    union all
-(values(1)) limit 1;
+(values(1)) limit 1;
+
+-- Ensure there is no problem if cheapest_startup_path is NULL
+explain (costs off)
+select * from tenk1 t1
+left join lateral
+  (select t1.tenthous from tenk2 t2 union all (values(1)))
+on true limit 1;
