Fix mis-planning of repeated application of a projection.

create_projection_plan contains a hidden assumption (here made
explicit by an Assert) that a projection-capable Path will yield a
projection-capable Plan.  Unfortunately, that assumption is violated
only a few lines away, by create_projection_plan itself.  This means
that two stacked ProjectionPaths can yield an outcome where we try to
jam the upper path's tlist into a non-projection-capable child node,
resulting in an invalid plan.

There isn't any good reason to have stacked ProjectionPaths; indeed the
whole concept is faulty, since the set of Vars/Aggs/etc needed by the
upper one wouldn't necessarily be available in the output of the lower
one, nor could the lower one create such values if they weren't
available from its input.  Hence, we can fix this by adjusting
create_projection_path to strip any top-level ProjectionPath from the
subpath it's given.  (This amounts to saying "oh, we changed our
minds about what we need to project here".)

The test case added here only fails in v13 and HEAD; before that, we
don't attempt to shove the Sort into the parallel part of the plan,
for reasons that aren't entirely clear to me.  However, all the
directly-related code looks generally the same as far back as v11,
where the hazard was introduced (by d7c19e62a).  So I've got no faith
that the same type of bug doesn't exist in v11 and v12, given the
right test case.  Hence, back-patch the code changes, but not the
irrelevant test case, into those branches.

Per report from Bas Poot.

Discussion: https://postgr.es/m/534fca83789c4a378c7de379e9067d4f@politie.nl

diff --git a/src/backend/optimizer/plan/createplan.c b/src/backend/optimizer/plan/createplan.c
index b02f7809c966290f062c4a6bbb2fb3e60d35afbc..439e6b6426c8df120e1199d2ca54cd7b2a2f9a34 100644 (file)
--- a/src/backend/optimizer/plan/createplan.c
+++ b/src/backend/optimizer/plan/createplan.c
@@ -1976,6 +1976,7 @@ create_projection_plan(PlannerInfo *root, ProjectionPath *best_path, int flags)
         */
        subplan = create_plan_recurse(root, best_path->subpath,
                                      CP_IGNORE_TLIST);
+       Assert(is_projection_capable_plan(subplan));
        tlist = build_path_tlist(root, &best_path->path);
    }
    else

diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c
index b248b038e034967773d48e26b4c928ee477a7a6f..9ce5f95e3b1371dcb054bf479778f54a64f6068f 100644 (file)
--- a/src/backend/optimizer/util/pathnode.c
+++ b/src/backend/optimizer/util/pathnode.c
@@ -2632,7 +2632,23 @@ create_projection_path(PlannerInfo *root,
                       PathTarget *target)
 {
    ProjectionPath *pathnode = makeNode(ProjectionPath);
-   PathTarget *oldtarget = subpath->pathtarget;
+   PathTarget *oldtarget;
+
+   /*
+    * We mustn't put a ProjectionPath directly above another; it's useless
+    * and will confuse create_projection_plan.  Rather than making sure all
+    * callers handle that, let's implement it here, by stripping off any
+    * ProjectionPath in what we're given.  Given this rule, there won't be
+    * more than one.
+    */
+   if (IsA(subpath, ProjectionPath))
+   {
+       ProjectionPath *subpp = (ProjectionPath *) subpath;
+
+       Assert(subpp->path.parent == rel);
+       subpath = subpp->subpath;
+       Assert(!IsA(subpath, ProjectionPath));
+   }
 
    pathnode->path.pathtype = T_Result;
    pathnode->path.parent = rel;
@@ -2658,6 +2674,7 @@ create_projection_path(PlannerInfo *root,
     * Note: in the latter case, create_projection_plan has to recheck our
     * conclusion; see comments therein.
     */
+   oldtarget = subpath->pathtarget;
    if (is_projection_capable_path(subpath) ||
        equal(oldtarget->exprs, target->exprs))
    {

diff --git a/src/test/regress/expected/select_parallel.out b/src/test/regress/expected/select_parallel.out
index 05ebcb284a533b612b6853e261abfda4e1102036..4ea1aa7dfd4943c94436d0c43476ffb6c2c1c916 100644 (file)
--- a/src/test/regress/expected/select_parallel.out
+++ b/src/test/regress/expected/select_parallel.out
@@ -1126,6 +1126,29 @@ ORDER BY 1, 2, 3;
 ------------------------------+---------------------------+-------------+--------------
 (0 rows)
 
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT generate_series(1, two), array(select generate_series(1, two))
+  FROM tenk1 ORDER BY tenthous;
+                              QUERY PLAN                              
+----------------------------------------------------------------------
+ ProjectSet
+   Output: generate_series(1, tenk1.two), (SubPlan 1), tenk1.tenthous
+   ->  Gather Merge
+         Output: tenk1.two, tenk1.tenthous
+         Workers Planned: 4
+         ->  Result
+               Output: tenk1.two, tenk1.tenthous
+               ->  Sort
+                     Output: tenk1.tenthous, tenk1.two
+                     Sort Key: tenk1.tenthous
+                     ->  Parallel Seq Scan on public.tenk1
+                           Output: tenk1.tenthous, tenk1.two
+   SubPlan 1
+     ->  ProjectSet
+           Output: generate_series(1, tenk1.two)
+           ->  Result
+(16 rows)
+
 -- test passing expanded-value representations to workers
 CREATE FUNCTION make_some_array(int,int) returns int[] as
 $$declare x int[];

diff --git a/src/test/regress/sql/select_parallel.sql b/src/test/regress/sql/select_parallel.sql
index d31e290ec227e00ec11f23c9fe1f3f542c52051e..f9247312484ef9d25245ecc72637a5441bdf638d 100644 (file)
--- a/src/test/regress/sql/select_parallel.sql
+++ b/src/test/regress/sql/select_parallel.sql
@@ -429,6 +429,10 @@ ORDER BY 1;
 SELECT * FROM information_schema.foreign_data_wrapper_options
 ORDER BY 1, 2, 3;
 
+EXPLAIN (VERBOSE, COSTS OFF)
+SELECT generate_series(1, two), array(select generate_series(1, two))
+  FROM tenk1 ORDER BY tenthous;
+
 -- test passing expanded-value representations to workers
 CREATE FUNCTION make_some_array(int,int) returns int[] as
 $$declare x int[];