From 95f0c954e460ca415d9fec62358239cd1dedb4e6 Mon Sep 17 00:00:00 2001 From: Yura Sokolov Date: Mon, 20 Dec 2021 11:48:10 +0300 Subject: [PATCH] Quick fix to duplicate result rows after Append path removal. It could happen Append path is created with "parallel_aware" flag, but its single child is not. Append path parent (Gather or Gather Merge) thinks its child is parallel_aware, but after Append path removal Gather's child become not parallel_aware. Then when Gather/Gather Merge diecides to run child in several workers or worker + leader participation, it gathers duplicate result rows from several child path invocations. With this fix Append path copies its single child parallel_aware / cost / workers values. Copied `num_workers == 0` triggers assert `num_workers > 0` in cost_gather_merge function. But changing this assert to `num_workers >= 0` doesn't lead to any runtime and/or logical error. Fixes bug 17335 https://postgr.es/m/flat/17335-4dc92e1aea3a78af%40postgresql.org --- src/backend/optimizer/path/costsize.c | 2 +- src/backend/optimizer/util/pathnode.c | 3 +++ 2 files changed, 4 insertions(+), 1 deletion(-) diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c index 1e4d404f024..9949c3ab555 100644 --- a/src/backend/optimizer/path/costsize.c +++ b/src/backend/optimizer/path/costsize.c @@ -440,7 +440,7 @@ cost_gather_merge(GatherMergePath *path, PlannerInfo *root, * be overgenerous since the leader will do less work than other workers * in typical cases, but we'll go with it for now. */ - Assert(path->num_workers > 0); + Assert(path->num_workers >= 0); N = (double) path->num_workers + 1; logN = LOG2(N); diff --git a/src/backend/optimizer/util/pathnode.c b/src/backend/optimizer/util/pathnode.c index af5e8df26b4..2ff4678937a 100644 --- a/src/backend/optimizer/util/pathnode.c +++ b/src/backend/optimizer/util/pathnode.c @@ -1340,6 +1340,9 @@ create_append_path(PlannerInfo *root, pathnode->path.startup_cost = child->startup_cost; pathnode->path.total_cost = child->total_cost; pathnode->path.pathkeys = child->pathkeys; + pathnode->path.parallel_aware = child->parallel_aware; + pathnode->path.parallel_safe = child->parallel_safe; + pathnode->path.parallel_workers = child->parallel_workers; } else cost_append(pathnode); -- 2.34.1