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

@@ -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);
 
 		/*

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