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

diff --git a/src/backend/optimizer/prep/prepunion.c b/src/backend/optimizer/prep/prepunion.c
index da9431108a2a2035fa5928959ffd279d4a410a1f..f51977796840d3cd562cc3b3a893e05f63ecd23c 100644 (file)
--- a/src/backend/optimizer/prep/prepunion.c
+++ b/src/backend/optimizer/prep/prepunion.c
@@ -1185,6 +1185,69 @@ generate_nonunion_paths(SetOperationStmt *op, PlannerInfo *root,
                result_rel->reltarget = create_setop_pathtarget(root, tlist,
                                                                                                                list_make2(lpath, rpath));
 
+       /* Check for provably empty setop inputs and add short-circuit paths. */
+       if (op->op == SETOP_EXCEPT)
+       {
+               /*
+                * For EXCEPTs, if the left side is dummy then there's no need to
+                * inspect the right-hand side as scanning the right to find tuples to
+                * remove won't make the left-hand input any more empty.
+                */
+               if (is_dummy_rel(lrel))
+               {
+                       mark_dummy_rel(result_rel);
+
+                       return result_rel;
+               }
+
+               /* Handle EXCEPTs with dummy right input */
+               if (is_dummy_rel(rrel))
+               {
+                       if (op->all)
+                       {
+                               Path       *apath;
+
+                               /*
+                                * EXCEPT ALL: If the right-hand input is dummy then we can
+                                * simply scan the left-hand input.  To keep createplan.c
+                                * happy, use a single child Append to handle the translation
+                                * between the set op targetlist and the targetlist of the
+                                * left input.  The Append will be removed in setrefs.c.
+                                */
+                               apath = (Path *) create_append_path(root, result_rel, list_make1(lpath),
+                                                                                                       NIL, NIL, NULL, 0, false, -1);
+
+                               add_path(result_rel, apath);
+
+                               return result_rel;
+                       }
+                       else
+                       {
+                               /*
+                                * To make EXCEPT with a dummy RHS work means having to
+                                * deduplicate the left input.  That could be done with
+                                * AggPaths, but it doesn't seem worth the effort.  Let the
+                                * normal path generation code below handle this one.
+                                */
+                       }
+               }
+       }
+       else
+       {
+               /*
+                * For INTERSECT, if either input is a dummy rel then we can mark the
+                * result_rel as dummy since intersecting with an empty relation can
+                * never yield any results.  This is true regardless of INTERSECT or
+                * INTERSECT ALL.
+                */
+               if (is_dummy_rel(lrel) || is_dummy_rel(rrel))
+               {
+                       mark_dummy_rel(result_rel);
+
+                       return result_rel;
+               }
+       }
+
        /*
         * Estimate number of distinct groups that we'll need hashtable entries
         * for; this is the size of the left-hand input for EXCEPT, or the smaller

diff --git a/src/test/regress/expected/union.out b/src/test/regress/expected/union.out
index 15931beea3a7822fa76afcb04fa5ffa85f6a7452..fb77d108337ed9c8297528dfda4f25a6b4884002 100644 (file)
--- 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 e252316f69ba4b5bfd9df69798b6454aa388257b..782cca23701d4a8ce758de9be9bea06c3166c526 100644 (file)
--- 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