Fix UNION/INTERSECT/EXCEPT over no columns.

Since 9.4, we've allowed the syntax "select union select" and variants
of that.  However, the planner wasn't expecting a no-column set operation
and ended up treating the set operation as if it were UNION ALL.

Turns out it's trivial to fix in v10 and later; we just need to be careful
about not generating a Sort node with no sort keys.  However, since a weird
corner case like this is never going to be exercised by developers, we'd
better have thorough regression tests if we want to consider it supported.

Per report from Victor Yegorov.

Discussion: https://postgr.es/m/CAGnEbojGJrRSOgJwNGM7JSJZpVAf8xXcVPbVrGdhbVEHZ-BUMw@mail.gmail.com

2 files changed, 158 insertions, 0 deletions

diff --git a/src/test/regress/expected/union.out b/src/test/regress/expected/union.out
index ee26b163f7..92d427a690 100644
--- a/src/test/regress/expected/union.out
+++ b/src/test/regress/expected/union.out
@@ -553,6 +553,121 @@ SELECT q1 FROM int8_tbl EXCEPT (((SELECT q2 FROM int8_tbl ORDER BY q2 LIMIT 1)))
 (5 rows)
 
 --
+-- Check behavior with empty select list (allowed since 9.4)
+--
+select union select;
+--
+(1 row)
+
+select intersect select;
+--
+(1 row)
+
+select except select;
+--
+(0 rows)
+
+-- check hashed implementation
+set enable_hashagg = true;
+set enable_sort = false;
+explain (costs off)
+select from generate_series(1,5) union select from generate_series(1,3);
+                           QUERY PLAN                           
+----------------------------------------------------------------
+ HashAggregate
+   ->  Append
+         ->  Function Scan on generate_series
+         ->  Function Scan on generate_series generate_series_1
+(4 rows)
+
+explain (costs off)
+select from generate_series(1,5) intersect select from generate_series(1,3);
+                              QUERY PLAN                              
+----------------------------------------------------------------------
+ HashSetOp Intersect
+   ->  Append
+         ->  Subquery Scan on "*SELECT* 1"
+               ->  Function Scan on generate_series
+         ->  Subquery Scan on "*SELECT* 2"
+               ->  Function Scan on generate_series generate_series_1
+(6 rows)
+
+select from generate_series(1,5) union select from generate_series(1,3);
+--
+(1 row)
+
+select from generate_series(1,5) union all select from generate_series(1,3);
+--
+(8 rows)
+
+select from generate_series(1,5) intersect select from generate_series(1,3);
+--
+(1 row)
+
+select from generate_series(1,5) intersect all select from generate_series(1,3);
+--
+(3 rows)
+
+select from generate_series(1,5) except select from generate_series(1,3);
+--
+(0 rows)
+
+select from generate_series(1,5) except all select from generate_series(1,3);
+--
+(2 rows)
+
+-- check sorted implementation
+set enable_hashagg = false;
+set enable_sort = true;
+explain (costs off)
+select from generate_series(1,5) union select from generate_series(1,3);
+                           QUERY PLAN                           
+----------------------------------------------------------------
+ Unique
+   ->  Append
+         ->  Function Scan on generate_series
+         ->  Function Scan on generate_series generate_series_1
+(4 rows)
+
+explain (costs off)
+select from generate_series(1,5) intersect select from generate_series(1,3);
+                              QUERY PLAN                              
+----------------------------------------------------------------------
+ SetOp Intersect
+   ->  Append
+         ->  Subquery Scan on "*SELECT* 1"
+               ->  Function Scan on generate_series
+         ->  Subquery Scan on "*SELECT* 2"
+               ->  Function Scan on generate_series generate_series_1
+(6 rows)
+
+select from generate_series(1,5) union select from generate_series(1,3);
+--
+(1 row)
+
+select from generate_series(1,5) union all select from generate_series(1,3);
+--
+(8 rows)
+
+select from generate_series(1,5) intersect select from generate_series(1,3);
+--
+(1 row)
+
+select from generate_series(1,5) intersect all select from generate_series(1,3);
+--
+(3 rows)
+
+select from generate_series(1,5) except select from generate_series(1,3);
+--
+(0 rows)
+
+select from generate_series(1,5) except all select from generate_series(1,3);
+--
+(2 rows)
+
+reset enable_hashagg;
+reset enable_sort;
+--
 -- Check handling of a case with unknown constants.  We don't guarantee
 -- an undecorated constant will work in all cases, but historically this
 -- usage has worked, so test we don't break it.
diff --git a/src/test/regress/sql/union.sql b/src/test/regress/sql/union.sql
index c0317cccb4..eed7c8d34b 100644
--- a/src/test/regress/sql/union.sql
+++ b/src/test/regress/sql/union.sql
@@ -191,6 +191,49 @@ SELECT q1 FROM int8_tbl EXCEPT (((SELECT q2 FROM int8_tbl ORDER BY q2 LIMIT 1)))
 (((((select * from int8_tbl)))));
 
 --
+-- Check behavior with empty select list (allowed since 9.4)
+--
+
+select union select;
+select intersect select;
+select except select;
+
+-- check hashed implementation
+set enable_hashagg = true;
+set enable_sort = false;
+
+explain (costs off)
+select from generate_series(1,5) union select from generate_series(1,3);
+explain (costs off)
+select from generate_series(1,5) intersect select from generate_series(1,3);
+
+select from generate_series(1,5) union select from generate_series(1,3);
+select from generate_series(1,5) union all select from generate_series(1,3);
+select from generate_series(1,5) intersect select from generate_series(1,3);
+select from generate_series(1,5) intersect all select from generate_series(1,3);
+select from generate_series(1,5) except select from generate_series(1,3);
+select from generate_series(1,5) except all select from generate_series(1,3);
+
+-- check sorted implementation
+set enable_hashagg = false;
+set enable_sort = true;
+
+explain (costs off)
+select from generate_series(1,5) union select from generate_series(1,3);
+explain (costs off)
+select from generate_series(1,5) intersect select from generate_series(1,3);
+
+select from generate_series(1,5) union select from generate_series(1,3);
+select from generate_series(1,5) union all select from generate_series(1,3);
+select from generate_series(1,5) intersect select from generate_series(1,3);
+select from generate_series(1,5) intersect all select from generate_series(1,3);
+select from generate_series(1,5) except select from generate_series(1,3);
+select from generate_series(1,5) except all select from generate_series(1,3);
+
+reset enable_hashagg;
+reset enable_sort;
+
+--
 -- Check handling of a case with unknown constants.  We don't guarantee
 -- an undecorated constant will work in all cases, but historically this
 -- usage has worked, so test we don't break it.