Expand coverage of parallel gather merge a bit.

Previously paths reaching heap_compare_slots weren't covered.

Author: Rushabh Lathia
Reviewed-By: Andres Freund
Discussion:
	https://postgr.es/m/CAGPqQf3C+3PBujb+7m=ceWeii4-vBY=XS99LjzrpkpefvzJbFg@mail.gmail.com
	https://postgr.es/m/[email protected]
Backpatch: 10, where gather merge was introduced

1 files changed, 10 insertions, 4 deletions

diff --git a/src/test/regress/sql/select_parallel.sql b/src/test/regress/sql/select_parallel.sql
index d2d262c7249..e717f92e530 100644
--- a/src/test/regress/sql/select_parallel.sql
+++ b/src/test/regress/sql/select_parallel.sql
@@ -110,14 +110,20 @@ select  count(*) from tenk1, tenk2 where tenk1.unique1 = tenk2.unique1;
 reset enable_hashjoin;
 reset enable_nestloop;
 
---test gather merge
-set enable_hashagg to off;
+-- test gather merge
+set enable_hashagg = false;
 
 explain (costs off)
-   select  string4, count((unique2)) from tenk1 group by string4 order by string4;
+   select count(*) from tenk1 group by twenty;
 
-select  string4, count((unique2)) from tenk1 group by string4 order by string4;
+select count(*) from tenk1 group by twenty;
 
+-- 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;
 
 set force_parallel_mode=1;