diff options
Diffstat (limited to 'src/backend/optimizer')
| -rw-r--r-- | src/backend/optimizer/plan/planner.c | 32 |
1 files changed, 19 insertions, 13 deletions
diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c index 0e78628bf01..fd77334e5fd 100644 --- a/src/backend/optimizer/plan/planner.c +++ b/src/backend/optimizer/plan/planner.c @@ -7906,17 +7906,23 @@ apply_scanjoin_target_to_paths(PlannerInfo *root, check_stack_depth(); /* - * If the rel is partitioned, we want to drop its existing paths and - * generate new ones. This function would still be correct if we kept the - * existing paths: we'd modify them to generate the correct target above - * the partitioning Append, and then they'd compete on cost with paths - * generating the target below the Append. However, in our current cost - * model the latter way is always the same or cheaper cost, so modifying - * the existing paths would just be useless work. Moreover, when the cost - * is the same, varying roundoff errors might sometimes allow an existing - * path to be picked, resulting in undesirable cross-platform plan - * variations. So we drop old paths and thereby force the work to be done - * below the Append, except in the case of a non-parallel-safe target. + * If the rel only has Append and MergeAppend paths, we want to drop its + * existing paths and generate new ones. This function would still be + * correct if we kept the existing paths: we'd modify them to generate the + * correct target above the partitioning Append, and then they'd compete + * on cost with paths generating the target below the Append. However, in + * our current cost model the latter way is always the same or cheaper + * cost, so modifying the existing paths would just be useless work. + * Moreover, when the cost is the same, varying roundoff errors might + * sometimes allow an existing path to be picked, resulting in undesirable + * cross-platform plan variations. So we drop old paths and thereby force + * the work to be done below the Append. + * + * However, there are several cases when this optimization is not safe. If + * the rel isn't partitioned, then none of the paths will be Append or + * MergeAppend paths, so we should definitely not do this. If it is + * parititoned but is a joinrel, it may have Append and MergeAppend paths, + * but it can also have join paths that we can't afford to discard. * * Some care is needed, because we have to allow * generate_useful_gather_paths to see the old partial paths in the next @@ -7924,7 +7930,7 @@ apply_scanjoin_target_to_paths(PlannerInfo *root, * generate_useful_gather_paths to add path(s) to the main list, and * finally zap the partial pathlist. */ - if (rel_is_partitioned) + if (rel_is_partitioned && IS_SIMPLE_REL(rel)) rel->pathlist = NIL; /* @@ -7950,7 +7956,7 @@ apply_scanjoin_target_to_paths(PlannerInfo *root, } /* Finish dropping old paths for a partitioned rel, per comment above */ - if (rel_is_partitioned) + if (rel_is_partitioned && IS_SIMPLE_REL(rel)) rel->partial_pathlist = NIL; /* Extract SRF-free scan/join target. */ |