Fix incorrect is-this-the-topmost-join tests in parallel planning.

Two callers of generate_useful_gather_paths were testing the wrong
thing when deciding whether to call that function: they checked for
being at the top of the current join subproblem, rather than being at
the actual top join.  This'd result in failing to construct parallel
paths for a sub-join for which they might be useful.

While set_rel_pathlist() isn't actively broken, it seems best to
make its identical-in-intention test for this be like the other two.

This has been wrong all along, but given the lack of field complaints
I'm hesitant to back-patch into stable branches; we usually prefer
to avoid non-bug-fix changes in plan choices in minor releases.
It seems not too late for v15 though.

Richard Guo, reviewed by Antonin Houska and Tom Lane

Discussion: https://postgr.es/m/CAMbWs4-mH8Zf87-w+3P2J=nJB+5OyicO28ia9q_9o=Lamf_VHg@mail.gmail.com

diff --git a/src/backend/optimizer/geqo/geqo_eval.c b/src/backend/optimizer/geqo/geqo_eval.c
index de695c0a789b48ef6cd1f079b1f6da58f85a3686..004481d6082c77aefe0016eb2cafd8df2bb0c7a2 100644 (file)
--- a/src/backend/optimizer/geqo/geqo_eval.c
+++ b/src/backend/optimizer/geqo/geqo_eval.c
@@ -273,7 +273,7 @@ merge_clump(PlannerInfo *root, List *clumps, Clump *new_clump, int num_gene,
                 * rel once we know the final targetlist (see
                 * grouping_planner).
                 */
-               if (old_clump->size + new_clump->size < num_gene)
+               if (!bms_equal(joinrel->relids, root->all_baserels))
                    generate_useful_gather_paths(root, joinrel, false);
 
                /* Find and save the cheapest paths for this joinrel */

diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c
index 63f0f6b79c7613bb62cfe4743b4abc51d582d430..b23cc8ee82ddb1000e91378a4e7955202b9b14c3 100644 (file)
--- a/src/backend/optimizer/path/allpaths.c
+++ b/src/backend/optimizer/path/allpaths.c
@@ -554,12 +554,11 @@ set_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
     * its own pool of workers.  Instead, we'll consider gathering partial
     * paths for the parent appendrel.
     *
-    * Also, if this is the topmost scan/join rel (that is, the only baserel),
-    * we postpone gathering until the final scan/join targetlist is available
-    * (see grouping_planner).
+    * Also, if this is the topmost scan/join rel, we postpone gathering until
+    * the final scan/join targetlist is available (see grouping_planner).
     */
    if (rel->reloptkind == RELOPT_BASEREL &&
-       bms_membership(root->all_baserels) != BMS_SINGLETON)
+       !bms_equal(rel->relids, root->all_baserels))
        generate_useful_gather_paths(root, rel, false);
 
    /* Now find the cheapest of the paths for this rel */
@@ -3435,7 +3434,7 @@ standard_join_search(PlannerInfo *root, int levels_needed, List *initial_rels)
             * partial paths.  We'll do the same for the topmost scan/join rel
             * once we know the final targetlist (see grouping_planner).
             */
-           if (lev < levels_needed)
+           if (!bms_equal(rel->relids, root->all_baserels))
                generate_useful_gather_paths(root, rel, false);
 
            /* Find and save the cheapest paths for this rel */