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| author | Amit Kapila <akapila@postgresql.org> | 2018-09-14 09:36:30 +0530 |
|---|---|---|
| committer | Amit Kapila <akapila@postgresql.org> | 2018-09-14 09:36:30 +0530 |
| commit | 75f9c4ca5a8047d7a9cfbc7d51a610933d04dc7f (patch) | |
| tree | 888a57df088ecaab838f08475d295ace7e30ae67 /src/backend/optimizer/path/allpaths.c | |
| parent | 0ba06e0bfb8cfd24ff17aca92aa72245ddd6c4d7 (diff) | |
| download | postgresql-75f9c4ca5a8047d7a9cfbc7d51a610933d04dc7f.tar.gz postgresql-75f9c4ca5a8047d7a9cfbc7d51a610933d04dc7f.zip | |
Don't allow LIMIT/OFFSET clause within sub-selects to be pushed to workers.
Allowing sub-select containing LIMIT/OFFSET in workers can lead to
inconsistent results at the top-level as there is no guarantee that the
row order will be fully deterministic. The fix is to prohibit pushing
LIMIT/OFFSET within sub-selects to workers.
Reported-by: Andrew Fletcher
Bug: 15324
Author: Amit Kapila
Reviewed-by: Dilip Kumar
Backpatch-through: 9.6
Discussion: https://postgr.es/m/153417684333.10284.11356259990921828616@wrigleys.postgresql.org
Diffstat (limited to 'src/backend/optimizer/path/allpaths.c')
| -rw-r--r-- | src/backend/optimizer/path/allpaths.c | 13 |
1 files changed, 13 insertions, 0 deletions
diff --git a/src/backend/optimizer/path/allpaths.c b/src/backend/optimizer/path/allpaths.c index 5db1688bf04..5f74d3b36d9 100644 --- a/src/backend/optimizer/path/allpaths.c +++ b/src/backend/optimizer/path/allpaths.c @@ -620,7 +620,20 @@ set_rel_consider_parallel(PlannerInfo *root, RelOptInfo *rel, * the SubqueryScanPath as not parallel-safe. (Note that * set_subquery_pathlist() might push some of these quals down * into the subquery itself, but that doesn't change anything.) + * + * We can't push sub-select containing LIMIT/OFFSET to workers as + * there is no guarantee that the row order will be fully + * deterministic, and applying LIMIT/OFFSET will lead to + * inconsistent results at the top-level. (In some cases, where + * the result is ordered, we could relax this restriction. But it + * doesn't currently seem worth expending extra effort to do so.) */ + { + Query *subquery = castNode(Query, rte->subquery); + + if (limit_needed(subquery)) + return; + } break; case RTE_JOIN: |