Teach planner to short-circuit EXCEPT/INTERSECT with dummy inputs

When either inputs of an INTERSECT [ALL] operator are proven not to return
any results (a dummy rel), then mark the entire INTERSECT operation as
dummy.

Likewise, if an EXCEPT [ALL] operation's left input is proven empty, then
mark the entire operation as dummy.

With EXCEPT ALL, we can easily handle the right input being dummy as
we can return the left input without any processing.  That can lead to
significant performance gains during query execution.  We can't easily
handle dummy right inputs for EXCEPT (without ALL), as that would require
deduplication of the left input.  Wiring up those Paths is likely more
complex than it's worth as the gains during execution aren't that great,
so let's leave that one to be handled by the normal Path generation code.

Author: David Rowley <dgrowleyml@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAApHDvri53PPF76c3M94_QNWbJfXjyCnjXuj_2=LYM-0m8WZtw@mail.gmail.com

2 files changed, 122 insertions, 2 deletions

diff --git a/src/test/regress/expected/union.out b/src/test/regress/expected/union.out
index 15931beea3a..fb77d108337 100644
--- a/src/test/regress/expected/union.out
+++ b/src/test/regress/expected/union.out
@@ -1217,7 +1217,7 @@ select event_id
 drop table events_child, events, other_events;
 reset enable_indexonlyscan;
 --
--- Test handling of UNION with provably empty inputs
+-- Test handling of UNION / EXCEPT / INTERSECT with provably empty inputs
 --
 -- Ensure the empty UNION input is pruned and de-duplication is done for the
 -- remaining relation.
@@ -1271,6 +1271,90 @@ ORDER BY 1;
          One-Time Filter: false
 (7 rows)
 
+-- Ensure the planner provides a const-false Result node
+EXPLAIN (COSTS OFF, VERBOSE)
+SELECT two FROM tenk1 WHERE 1=2
+INTERSECT
+SELECT four FROM tenk1
+ORDER BY 1;
+                             QUERY PLAN                              
+---------------------------------------------------------------------
+ Sort
+   Output: unnamed_subquery.two
+   Sort Key: unnamed_subquery.two
+   ->  Result
+         Output: unnamed_subquery.two
+         Replaces: Aggregate on unnamed_subquery, unnamed_subquery_1
+         One-Time Filter: false
+(7 rows)
+
+-- As above, with the inputs swapped
+EXPLAIN (COSTS OFF, VERBOSE)
+SELECT four FROM tenk1
+INTERSECT
+SELECT two FROM tenk1 WHERE 1=2
+ORDER BY 1;
+                             QUERY PLAN                              
+---------------------------------------------------------------------
+ Sort
+   Output: unnamed_subquery.four
+   Sort Key: unnamed_subquery.four
+   ->  Result
+         Output: unnamed_subquery.four
+         Replaces: Aggregate on unnamed_subquery, unnamed_subquery_1
+         One-Time Filter: false
+(7 rows)
+
+-- Try with both inputs dummy
+EXPLAIN (COSTS OFF, VERBOSE)
+SELECT four FROM tenk1 WHERE 1=2
+INTERSECT
+SELECT two FROM tenk1 WHERE 1=2
+ORDER BY 1;
+                             QUERY PLAN                              
+---------------------------------------------------------------------
+ Sort
+   Output: unnamed_subquery.four
+   Sort Key: unnamed_subquery.four
+   ->  Result
+         Output: unnamed_subquery.four
+         Replaces: Aggregate on unnamed_subquery, unnamed_subquery_1
+         One-Time Filter: false
+(7 rows)
+
+-- Ensure the planner provides a const-false Result node when the left input
+-- is empty
+EXPLAIN (COSTS OFF, VERBOSE)
+SELECT two FROM tenk1 WHERE 1=2
+EXCEPT
+SELECT four FROM tenk1
+ORDER BY 1;
+                             QUERY PLAN                              
+---------------------------------------------------------------------
+ Sort
+   Output: unnamed_subquery.two
+   Sort Key: unnamed_subquery.two
+   ->  Result
+         Output: unnamed_subquery.two
+         Replaces: Aggregate on unnamed_subquery, unnamed_subquery_1
+         One-Time Filter: false
+(7 rows)
+
+-- Ensure the planner only scans the left input when right input is empty
+EXPLAIN (COSTS OFF, VERBOSE)
+SELECT two FROM tenk1
+EXCEPT ALL
+SELECT four FROM tenk1 WHERE 1=2
+ORDER BY 1;
+           QUERY PLAN           
+--------------------------------
+ Sort
+   Output: tenk1.two
+   Sort Key: tenk1.two
+   ->  Seq Scan on public.tenk1
+         Output: tenk1.two
+(5 rows)
+
 -- Test constraint exclusion of UNION ALL subqueries
 explain (costs off)
  SELECT * FROM
diff --git a/src/test/regress/sql/union.sql b/src/test/regress/sql/union.sql
index e252316f69b..782cca23701 100644
--- a/src/test/regress/sql/union.sql
+++ b/src/test/regress/sql/union.sql
@@ -460,7 +460,7 @@ drop table events_child, events, other_events;
 reset enable_indexonlyscan;
 
 --
--- Test handling of UNION with provably empty inputs
+-- Test handling of UNION / EXCEPT / INTERSECT with provably empty inputs
 --
 
 -- Ensure the empty UNION input is pruned and de-duplication is done for the
@@ -487,6 +487,42 @@ UNION
 SELECT ten FROM tenk1 WHERE 1=2
 ORDER BY 1;
 
+-- Ensure the planner provides a const-false Result node
+EXPLAIN (COSTS OFF, VERBOSE)
+SELECT two FROM tenk1 WHERE 1=2
+INTERSECT
+SELECT four FROM tenk1
+ORDER BY 1;
+
+-- As above, with the inputs swapped
+EXPLAIN (COSTS OFF, VERBOSE)
+SELECT four FROM tenk1
+INTERSECT
+SELECT two FROM tenk1 WHERE 1=2
+ORDER BY 1;
+
+-- Try with both inputs dummy
+EXPLAIN (COSTS OFF, VERBOSE)
+SELECT four FROM tenk1 WHERE 1=2
+INTERSECT
+SELECT two FROM tenk1 WHERE 1=2
+ORDER BY 1;
+
+-- Ensure the planner provides a const-false Result node when the left input
+-- is empty
+EXPLAIN (COSTS OFF, VERBOSE)
+SELECT two FROM tenk1 WHERE 1=2
+EXCEPT
+SELECT four FROM tenk1
+ORDER BY 1;
+
+-- Ensure the planner only scans the left input when right input is empty
+EXPLAIN (COSTS OFF, VERBOSE)
+SELECT two FROM tenk1
+EXCEPT ALL
+SELECT four FROM tenk1 WHERE 1=2
+ORDER BY 1;
+
 -- Test constraint exclusion of UNION ALL subqueries
 explain (costs off)
  SELECT * FROM