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Allow Gather Merge in more cases for parallel DISTINCT

Here we adjust the partial path generation for parallel DISTINCT queriesto add Sort nodes on top of any unsorted partial distinct paths.This increases the likelihood of the planner pushing a Sort below a GatherMerge which enables the final phase of the parallel distinct to beimplemented using a Unique node in more cases.Sorting the partial distinct paths is particularly useful when theDISTINCT query has an ORDER BY and LIMIT clause as this can allow cheaperplans by having the workers Hash Aggregate then Sort before feeding theresults into the Gather Merge. The non-parallel portion of the plan thenbecomes very cheap as it leaves only Unique and Limit to do in the leaderprocess.Author: Richard GuoReviewed-by: David RowleyDiscussion:https://postgr.es/m/CAMbWs48u9VoVOouJsys1qOaC9WVGVmBa+wT1dx8KvxF5GPzezA@mail.gmail.com

1 parent0b6517a commit7e0ade0Copy full SHA for 7e0ade0

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src
- backend/optimizer/plan
  - planner.c
- test/regress/expected
  - select_distinct.out

2 files changed

-5

lines changed

`‎src/backend/optimizer/plan/planner.c`

Lines changed: 1 addition & 1 deletion

Original file line number	Diff line number	Diff line change
`@@ -4819,7 +4819,7 @@ create_partial_distinct_paths(PlannerInfo root, RelOptInfo input_rel,`
`4819`	`4819`
`4820`	`4820`	`if (partial_distinct_rel->partial_pathlist!=NIL)`
`4821`	`4821`	`{`
`4822`		`-generate_gather_paths(root,partial_distinct_rel, true);`
	`4822`	`+generate_useful_gather_paths(root,partial_distinct_rel, true);`
`4823`	`4823`	`set_cheapest(partial_distinct_rel);`
`4824`	`4824`
`4825`	`4825`	`/*`

`‎src/test/regress/expected/select_distinct.out`

Lines changed: 4 additions & 4 deletions

Original file line number	Diff line number	Diff line change
`@@ -235,10 +235,10 @@ SELECT DISTINCT four FROM tenk1;`
`235`	`235`	`QUERY PLAN`
`236`	`236`	`----------------------------------------------------`
`237`	`237`	`Unique`
`238`		`- ->Sort`
`239`		`-Sort Key: four`
`240`		`- ->Gather`
`241`		`-Workers Planned: 2`
	`238`	`+ ->Gather Merge`
	`239`	`+Workers Planned: 2`
	`240`	`+ ->Sort`
	`241`	`+Sort Key: four`
`242`	`242`	`-> HashAggregate`
`243`	`243`	`Group Key: four`
`244`	`244`	`-> Parallel Seq Scan on tenk1`

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Commit7e0ade0

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2 files changed

2 files changed

`‎src/backend/optimizer/plan/planner.c`

`‎src/test/regress/expected/select_distinct.out`

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