Push tuple limits through Gather and Gather Merge.

If we only need, say, 10 tuples in total, then we certainly don't need
more than 10 tuples from any single process.  Pushing down the limit
lets workers exit early when possible.  For Gather Merge, there is
an additional benefit: a Sort immediately below the Gather Merge can
be done as a bounded sort if there is an applicable limit.

Robert Haas and Tom Lane

Discussion: http://postgr.es/m/CA+TgmoYa3QKKrLj5rX7UvGqhH73G1Li4B-EKxrmASaca2tFu9Q@mail.gmail.com

2 files changed, 31 insertions, 2 deletions

diff --git a/src/test/regress/expected/select_parallel.out b/src/test/regress/expected/select_parallel.out
index 084f0f0c8e1..ccad18e978f 100644
--- a/src/test/regress/expected/select_parallel.out
+++ b/src/test/regress/expected/select_parallel.out
@@ -300,6 +300,29 @@ select count(*) from tenk1 group by twenty;
    500
 (20 rows)
 
+reset enable_hashagg;
+-- gather merge test with a LIMIT
+explain (costs off)
+  select fivethous from tenk1 order by fivethous limit 4;
+                  QUERY PLAN                  
+----------------------------------------------
+ Limit
+   ->  Gather Merge
+         Workers Planned: 4
+         ->  Sort
+               Sort Key: fivethous
+               ->  Parallel Seq Scan on tenk1
+(6 rows)
+
+select fivethous from tenk1 order by fivethous limit 4;
+ fivethous 
+-----------
+         0
+         0
+         1
+         1
+(4 rows)
+
 -- gather merge test with 0 worker
 set max_parallel_workers = 0;
 explain (costs off)
@@ -325,7 +348,6 @@ select string4 from tenk1 order by string4 limit 5;
 (5 rows)
 
 reset max_parallel_workers;
-reset enable_hashagg;
 SAVEPOINT settings;
 SET LOCAL force_parallel_mode = 1;
 explain (costs off)
diff --git a/src/test/regress/sql/select_parallel.sql b/src/test/regress/sql/select_parallel.sql
index 58c3f598905..c0debddbcd1 100644
--- a/src/test/regress/sql/select_parallel.sql
+++ b/src/test/regress/sql/select_parallel.sql
@@ -118,13 +118,20 @@ explain (costs off)
 
 select count(*) from tenk1 group by twenty;
 
+reset enable_hashagg;
+
+-- gather merge test with a LIMIT
+explain (costs off)
+  select fivethous from tenk1 order by fivethous limit 4;
+
+select fivethous from tenk1 order by fivethous limit 4;
+
 -- gather merge test with 0 worker
 set max_parallel_workers = 0;
 explain (costs off)
    select string4 from tenk1 order by string4 limit 5;
 select string4 from tenk1 order by string4 limit 5;
 reset max_parallel_workers;
-reset enable_hashagg;
 
 SAVEPOINT settings;
 SET LOCAL force_parallel_mode = 1;