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Commitf4a3c0b

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Consider unsorted paths in generate_useful_gather_paths
generate_useful_gather_paths used to skip unsorted paths (without anypathkeys), but that is unnecessary - the later code actually can handlesuch paths just fine by adding a Sort node. This is clearly a thinko,preventing construction of useful plans.Backpatch to 13, where Incremental Sort was introduced.Author: James ColemanReviewed-by: Tomas VondraBackpatch-through: 13Discussion:https://postgr.es/m/CAAaqYe8cK3g5CfLC4w7bs=hC0mSksZC=H5M8LSchj5e5OxpTAg@mail.gmail.com
1 parent29f8f54 commitf4a3c0b

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3 files changed

+19
-9
lines changed

3 files changed

+19
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‎src/backend/optimizer/path/allpaths.c

Lines changed: 2 additions & 9 deletions
Original file line numberDiff line numberDiff line change
@@ -2900,7 +2900,8 @@ generate_useful_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_r
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cheapest_partial_path=linitial(rel->partial_pathlist);
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/*
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* Consider incremental sort paths for each interesting ordering.
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* Consider sorted paths for each interesting ordering. We generate both
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* incremental and full sort.
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*/
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foreach(lc,useful_pathkeys_list)
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{
@@ -2914,14 +2915,6 @@ generate_useful_gather_paths(PlannerInfo *root, RelOptInfo *rel, bool override_r
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Path*subpath= (Path*)lfirst(lc2);
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GatherMergePath*path;
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2917-
/*
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* If the path has no ordering at all, then we can't use either
2919-
* incremental sort or rely on implicit sorting with a gather
2920-
* merge.
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*/
2922-
if (subpath->pathkeys==NIL)
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continue;
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is_sorted=pathkeys_count_contained_in(useful_pathkeys,
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subpath->pathkeys,
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&presorted_keys);

‎src/test/regress/expected/incremental_sort.out

Lines changed: 13 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -1468,6 +1468,19 @@ explain (costs off) select * from t union select * from t order by 1,3;
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-> Parallel Seq Scan on t t_1
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(13 rows)
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-- Full sort, not just incremental sort can be pushed below a gather merge path
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-- by generate_useful_gather_paths.
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explain (costs off) select distinct a,b from t;
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QUERY PLAN
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------------------------------------------
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Unique
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-> Gather Merge
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Workers Planned: 2
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-> Sort
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Sort Key: a, b
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-> Parallel Seq Scan on t
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(6 rows)
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drop table t;
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-- Sort pushdown can't go below where expressions are part of the rel target.
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-- In particular this is interesting for volatile expressions which have to

‎src/test/regress/sql/incremental_sort.sql

Lines changed: 4 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -220,6 +220,10 @@ explain (costs off) select a,b,sum(c) from t group by 1,2 order by 1,2,3 limit 1
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set enable_hashagg to off;
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explain (costs off)select*from tunionselect*from torder by1,3;
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-- Full sort, not just incremental sort can be pushed below a gather merge path
224+
-- by generate_useful_gather_paths.
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explain (costs off)select distinct a,bfrom t;
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droptable t;
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-- Sort pushdown can't go below where expressions are part of the rel target.

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