postgres.git / summary / log / commit / refs

commit    7e0ade0ffe0a76b1926a4af39ecdf799c96ef1ba
Author:   David Rowley <drowley@postgresql.org>
Date:     Fri Feb 02 11:20:18 2024 +0000
First released in: 17.0

    Allow Gather Merge in more cases for parallel DISTINCT
    
    Here we adjust the partial path generation for parallel DISTINCT queries
    to add Sort nodes on top of any unsorted partial distinct paths.
    
    This increases the likelihood of the planner pushing a Sort below a Gather
    Merge which enables the final phase of the parallel distinct to be
    implemented using a Unique node in more cases.
    
    Sorting the partial distinct paths is particularly useful when the
    DISTINCT query has an ORDER BY and LIMIT clause as this can allow cheaper
    plans by having the workers Hash Aggregate then Sort before feeding the
    results into the Gather Merge.  The non-parallel portion of the plan then
    becomes very cheap as it leaves only Unique and Limit to do in the leader
    process.
    
    Author: Richard Guo
    Reviewed-by: David Rowley
    Discussion: https://postgr.es/m/CAMbWs48u9VoVOouJsys1qOaC9WVGVmBa+wT1dx8KvxF5GPzezA@mail.gmail.com


 src/backend/optimizer/plan/planner.c          | 2 +-
 src/test/regress/expected/select_distinct.out | 8 ++++----
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 342f5ad8d0a..acc324122fd 100644
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -4819,7 +4819,7 @@ create_partial_distinct_paths(PlannerInfo *root, RelOptInfo *input_rel,
 
 	if (partial_distinct_rel->partial_pathlist != NIL)
 	{
-		generate_gather_paths(root, partial_distinct_rel, true);
+		generate_useful_gather_paths(root, partial_distinct_rel, true);
 		set_cheapest(partial_distinct_rel);
 
 		/*
diff --git a/src/test/regress/expected/select_distinct.out b/src/test/regress/expected/select_distinct.out
index 1f72756ccb4..82b8e54f5f1 100644
--- a/src/test/regress/expected/select_distinct.out
+++ b/src/test/regress/expected/select_distinct.out
@@ -235,10 +235,10 @@ SELECT DISTINCT four FROM tenk1;
                      QUERY PLAN                     
 ----------------------------------------------------
  Unique
-   ->  Sort
-         Sort Key: four
-         ->  Gather
-               Workers Planned: 2
+   ->  Gather Merge
+         Workers Planned: 2
+         ->  Sort
+               Sort Key: four
                ->  HashAggregate
                      Group Key: four
                      ->  Parallel Seq Scan on tenk1


[parent: 0b6517a3b79a]