Avoid inserting Result nodes that only compute identity projections.

The planner sometimes inserts Result nodes to perform column projections
(ie, arbitrary scalar calculations) above plan nodes that lack projection
logic of their own.  However, we did that even if the lower plan node was
in fact producing the required column set already; which is a pretty common
case given the popularity of "SELECT * FROM ...".  Measurements show that
the useless plan node adds non-negligible overhead, especially when there
are many columns in the result.  So add a check to avoid inserting a Result
node unless there's something useful for it to do.

There are a couple of remaining places where unnecessary Result nodes
could get inserted, but they are (a) much less performance-critical,
and (b) coded in such a way that it's hard to avoid inserting a Result,
because the desired tlist is changed on-the-fly in subsequent logic.
We'll leave those alone for now.

Kyotaro Horiguchi; reviewed and further hacked on by Amit Kapila and
Tom Lane.

Author: tglsfdc

src/backend/optimizer/plan/createplan.c

@@ -937,10 +937,12 @@ create_unique_plan(PlannerInfo *root, UniquePath *best_path)
 	if (newitems || best_path->umethod == UNIQUE_PATH_SORT)
 	{
 		/*
-		 * If the top plan node can't do projections, we need to add a Result
-		 * node to help it along.
+		 * If the top plan node can't do projections and its existing target
+		 * list isn't already what we need, we need to add a Result node to
+		 * help it along.
 		 */
-		if (!is_projection_capable_plan(subplan))
+		if (!is_projection_capable_plan(subplan) &&
+			!tlist_same_exprs(newtlist, subplan->targetlist))
 			subplan = (Plan *) make_result(root, newtlist, NULL, subplan);
 		else
 			subplan->targetlist = newtlist;

src/backend/optimizer/plan/planner.c

@@ -1379,10 +1379,12 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 			{
 				/*
 				 * If the top-level plan node is one that cannot do expression
-				 * evaluation, we must insert a Result node to project the
+				 * evaluation and its existing target list isn't already what
+				 * we need, we must insert a Result node to project the
 				 * desired tlist.
 				 */
-				if (!is_projection_capable_plan(result_plan))
+				if (!is_projection_capable_plan(result_plan) &&
+					!tlist_same_exprs(sub_tlist, result_plan->targetlist))
 				{
 					result_plan = (Plan *) make_result(root,
 													   sub_tlist,
@@ -1542,10 +1544,13 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 			 * If the top-level plan node is one that cannot do expression
 			 * evaluation, we must insert a Result node to project the desired
 			 * tlist.  (In some cases this might not really be required, but
-			 * it's not worth trying to avoid it.)  Note that on second and
-			 * subsequent passes through the following loop, the top-level
-			 * node will be a WindowAgg which we know can project; so we only
-			 * need to check once.
+			 * it's not worth trying to avoid it.  In particular, think not to
+			 * skip adding the Result if the initial window_tlist matches the
+			 * top-level plan node's output, because we might change the tlist
+			 * inside the following loop.)  Note that on second and subsequent
+			 * passes through the following loop, the top-level node will be a
+			 * WindowAgg which we know can project; so we only need to check
+			 * once.
 			 */
 			if (!is_projection_capable_plan(result_plan))
 			{

src/backend/optimizer/util/tlist.c

@@ -158,6 +158,43 @@ get_tlist_exprs(List *tlist, bool includeJunk)
 }
 
 
+/*
+ * tlist_same_exprs
+ *		Check whether two target lists contain the same expressions
+ *
+ * Note: this function is used to decide whether it's safe to jam a new tlist
+ * into a non-projection-capable plan node.  Obviously we can't do that unless
+ * the node's tlist shows it already returns the column values we want.
+ * However, we can ignore the TargetEntry attributes resname, ressortgroupref,
+ * resorigtbl, resorigcol, and resjunk, because those are only labelings that
+ * don't affect the row values computed by the node.  (Moreover, if we didn't
+ * ignore them, we'd frequently fail to make the desired optimization, since
+ * the planner tends to not bother to make resname etc. valid in intermediate
+ * plan nodes.)  Note that on success, the caller must still jam the desired
+ * tlist into the plan node, else it won't have the desired labeling fields.
+ */
+bool
+tlist_same_exprs(List *tlist1, List *tlist2)
+{
+	ListCell   *lc1,
+			   *lc2;
+
+	if (list_length(tlist1) != list_length(tlist2))
+		return false;			/* not same length, so can't match */
+
+	forboth(lc1, tlist1, lc2, tlist2)
+	{
+		TargetEntry *tle1 = (TargetEntry *) lfirst(lc1);
+		TargetEntry *tle2 = (TargetEntry *) lfirst(lc2);
+
+		if (!equal(tle1->expr, tle2->expr))
+			return false;
+	}
+
+	return true;
+}
+
+
 /*
  * Does tlist have same output datatypes as listed in colTypes?
  *

src/include/optimizer/tlist.h

@@ -25,6 +25,9 @@ extern List *flatten_tlist(List *tlist, PVCAggregateBehavior aggbehavior,
 extern List *add_to_flat_tlist(List *tlist, List *exprs);
 
 extern List *get_tlist_exprs(List *tlist, bool includeJunk);
+
+extern bool tlist_same_exprs(List *tlist1, List *tlist2);
+
 extern bool tlist_same_datatypes(List *tlist, List *colTypes, bool junkOK);
 extern bool tlist_same_collations(List *tlist, List *colCollations, bool junkOK);
 

src/test/regress/expected/alter_table.out

@@ -385,61 +385,57 @@ create table nv_child_2011 () inherits (nv_parent);
 alter table nv_child_2010 add check (d between '2010-01-01'::date and '2010-12-31'::date) not valid;
 alter table nv_child_2011 add check (d between '2011-01-01'::date and '2011-12-31'::date) not valid;
 explain (costs off) select * from nv_parent where d between '2011-08-01' and '2011-08-31';
-                                   QUERY PLAN                                    
----------------------------------------------------------------------------------
- Result
-   ->  Append
-         ->  Seq Scan on nv_parent
-               Filter: ((d >= '08-01-2011'::date) AND (d <= '08-31-2011'::date))
-         ->  Seq Scan on nv_child_2010
-               Filter: ((d >= '08-01-2011'::date) AND (d <= '08-31-2011'::date))
-         ->  Seq Scan on nv_child_2011
-               Filter: ((d >= '08-01-2011'::date) AND (d <= '08-31-2011'::date))
-(8 rows)
+                                QUERY PLAN                                 
+---------------------------------------------------------------------------
+ Append
+   ->  Seq Scan on nv_parent
+         Filter: ((d >= '08-01-2011'::date) AND (d <= '08-31-2011'::date))
+   ->  Seq Scan on nv_child_2010
+         Filter: ((d >= '08-01-2011'::date) AND (d <= '08-31-2011'::date))
+   ->  Seq Scan on nv_child_2011
+         Filter: ((d >= '08-01-2011'::date) AND (d <= '08-31-2011'::date))
+(7 rows)
 
 create table nv_child_2009 (check (d between '2009-01-01'::date and '2009-12-31'::date)) inherits (nv_parent);
 explain (costs off) select * from nv_parent where d between '2011-08-01'::date and '2011-08-31'::date;
-                                   QUERY PLAN                                    
----------------------------------------------------------------------------------
- Result
-   ->  Append
-         ->  Seq Scan on nv_parent
-               Filter: ((d >= '08-01-2011'::date) AND (d <= '08-31-2011'::date))
-         ->  Seq Scan on nv_child_2010
-               Filter: ((d >= '08-01-2011'::date) AND (d <= '08-31-2011'::date))
-         ->  Seq Scan on nv_child_2011
-               Filter: ((d >= '08-01-2011'::date) AND (d <= '08-31-2011'::date))
-(8 rows)
+                                QUERY PLAN                                 
+---------------------------------------------------------------------------
+ Append
+   ->  Seq Scan on nv_parent
+         Filter: ((d >= '08-01-2011'::date) AND (d <= '08-31-2011'::date))
+   ->  Seq Scan on nv_child_2010
+         Filter: ((d >= '08-01-2011'::date) AND (d <= '08-31-2011'::date))
+   ->  Seq Scan on nv_child_2011
+         Filter: ((d >= '08-01-2011'::date) AND (d <= '08-31-2011'::date))
+(7 rows)
 
 explain (costs off) select * from nv_parent where d between '2009-08-01'::date and '2009-08-31'::date;
-                                   QUERY PLAN                                    
----------------------------------------------------------------------------------
- Result
-   ->  Append
-         ->  Seq Scan on nv_parent
-               Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
-         ->  Seq Scan on nv_child_2010
-               Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
-         ->  Seq Scan on nv_child_2011
-               Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
-         ->  Seq Scan on nv_child_2009
-               Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
-(10 rows)
+                                QUERY PLAN                                 
+---------------------------------------------------------------------------
+ Append
+   ->  Seq Scan on nv_parent
+         Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
+   ->  Seq Scan on nv_child_2010
+         Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
+   ->  Seq Scan on nv_child_2011
+         Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
+   ->  Seq Scan on nv_child_2009
+         Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
+(9 rows)
 
 -- after validation, the constraint should be used
 alter table nv_child_2011 VALIDATE CONSTRAINT nv_child_2011_d_check;
 explain (costs off) select * from nv_parent where d between '2009-08-01'::date and '2009-08-31'::date;
-                                   QUERY PLAN                                    
----------------------------------------------------------------------------------
- Result
-   ->  Append
-         ->  Seq Scan on nv_parent
-               Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
-         ->  Seq Scan on nv_child_2010
-               Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
-         ->  Seq Scan on nv_child_2009
-               Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
-(8 rows)
+                                QUERY PLAN                                 
+---------------------------------------------------------------------------
+ Append
+   ->  Seq Scan on nv_parent
+         Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
+   ->  Seq Scan on nv_child_2010
+         Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
+   ->  Seq Scan on nv_child_2009
+         Filter: ((d >= '08-01-2009'::date) AND (d <= '08-31-2009'::date))
+(7 rows)
 
 -- Foreign key adding test with mixed types
 -- Note: these tables are TEMP to avoid name conflicts when this test

src/test/regress/expected/inherit.out

@@ -1210,24 +1210,22 @@ select * from matest0 order by 1-id;
 reset enable_indexscan;
 set enable_seqscan = off;  -- plan with fewest seqscans should be merge
 explain (verbose, costs off) select * from matest0 order by 1-id;
-                               QUERY PLAN                               
-------------------------------------------------------------------------
- Result
-   Output: matest0.id, matest0.name, ((1 - matest0.id))
-   ->  Merge Append
-         Sort Key: ((1 - matest0.id))
-         ->  Index Scan using matest0i on public.matest0
-               Output: matest0.id, matest0.name, (1 - matest0.id)
-         ->  Index Scan using matest1i on public.matest1
-               Output: matest1.id, matest1.name, (1 - matest1.id)
-         ->  Sort
-               Output: matest2.id, matest2.name, ((1 - matest2.id))
-               Sort Key: ((1 - matest2.id))
-               ->  Seq Scan on public.matest2
-                     Output: matest2.id, matest2.name, (1 - matest2.id)
-         ->  Index Scan using matest3i on public.matest3
-               Output: matest3.id, matest3.name, (1 - matest3.id)
-(15 rows)
+                            QUERY PLAN                            
+------------------------------------------------------------------
+ Merge Append
+   Sort Key: ((1 - matest0.id))
+   ->  Index Scan using matest0i on public.matest0
+         Output: matest0.id, matest0.name, (1 - matest0.id)
+   ->  Index Scan using matest1i on public.matest1
+         Output: matest1.id, matest1.name, (1 - matest1.id)
+   ->  Sort
+         Output: matest2.id, matest2.name, ((1 - matest2.id))
+         Sort Key: ((1 - matest2.id))
+         ->  Seq Scan on public.matest2
+               Output: matest2.id, matest2.name, (1 - matest2.id)
+   ->  Index Scan using matest3i on public.matest3
+         Output: matest3.id, matest3.name, (1 - matest3.id)
+(13 rows)
 
 select * from matest0 order by 1-id;
  id |  name  
@@ -1258,48 +1256,45 @@ SELECT thousand, tenthous FROM tenk1
 UNION ALL
 SELECT thousand, thousand FROM tenk1
 ORDER BY thousand, tenthous;
-                                  QUERY PLAN                                   
--------------------------------------------------------------------------------
- Result
-   ->  Merge Append
-         Sort Key: tenk1.thousand, tenk1.tenthous
-         ->  Index Only Scan using tenk1_thous_tenthous on tenk1
-         ->  Sort
-               Sort Key: tenk1_1.thousand, tenk1_1.thousand
-               ->  Index Only Scan using tenk1_thous_tenthous on tenk1 tenk1_1
-(7 rows)
+                               QUERY PLAN                                
+-------------------------------------------------------------------------
+ Merge Append
+   Sort Key: tenk1.thousand, tenk1.tenthous
+   ->  Index Only Scan using tenk1_thous_tenthous on tenk1
+   ->  Sort
+         Sort Key: tenk1_1.thousand, tenk1_1.thousand
+         ->  Index Only Scan using tenk1_thous_tenthous on tenk1 tenk1_1
+(6 rows)
 
 explain (costs off)
 SELECT thousand, tenthous, thousand+tenthous AS x FROM tenk1
 UNION ALL
 SELECT 42, 42, hundred FROM tenk1
 ORDER BY thousand, tenthous;
-                               QUERY PLAN                               
-------------------------------------------------------------------------
- Result
-   ->  Merge Append
-         Sort Key: tenk1.thousand, tenk1.tenthous
-         ->  Index Only Scan using tenk1_thous_tenthous on tenk1
-         ->  Sort
-               Sort Key: (42), (42)
-               ->  Index Only Scan using tenk1_hundred on tenk1 tenk1_1
-(7 rows)
+                            QUERY PLAN                            
+------------------------------------------------------------------
+ Merge Append
+   Sort Key: tenk1.thousand, tenk1.tenthous
+   ->  Index Only Scan using tenk1_thous_tenthous on tenk1
+   ->  Sort
+         Sort Key: (42), (42)
+         ->  Index Only Scan using tenk1_hundred on tenk1 tenk1_1
+(6 rows)
 
 explain (costs off)
 SELECT thousand, tenthous FROM tenk1
 UNION ALL
 SELECT thousand, random()::integer FROM tenk1
 ORDER BY thousand, tenthous;
-                                  QUERY PLAN                                   
--------------------------------------------------------------------------------
- Result
-   ->  Merge Append
-         Sort Key: tenk1.thousand, tenk1.tenthous
-         ->  Index Only Scan using tenk1_thous_tenthous on tenk1
-         ->  Sort
-               Sort Key: tenk1_1.thousand, ((random())::integer)
-               ->  Index Only Scan using tenk1_thous_tenthous on tenk1 tenk1_1
-(7 rows)
+                               QUERY PLAN                                
+-------------------------------------------------------------------------
+ Merge Append
+   Sort Key: tenk1.thousand, tenk1.tenthous
+   ->  Index Only Scan using tenk1_thous_tenthous on tenk1
+   ->  Sort
+         Sort Key: tenk1_1.thousand, ((random())::integer)
+         ->  Index Only Scan using tenk1_thous_tenthous on tenk1 tenk1_1
+(6 rows)
 
 -- Check min/max aggregate optimization
 explain (costs off)
@@ -1345,16 +1340,15 @@ SELECT x, y FROM
    UNION ALL
    SELECT unique2 AS x, unique2 AS y FROM tenk1 b) s
 ORDER BY x, y;
-                            QUERY PLAN                             
--------------------------------------------------------------------
- Result
-   ->  Merge Append
-         Sort Key: a.thousand, a.tenthous
-         ->  Index Only Scan using tenk1_thous_tenthous on tenk1 a
-         ->  Sort
-               Sort Key: b.unique2, b.unique2
-               ->  Index Only Scan using tenk1_unique2 on tenk1 b
-(7 rows)
+                         QUERY PLAN                          
+-------------------------------------------------------------
+ Merge Append
+   Sort Key: a.thousand, a.tenthous
+   ->  Index Only Scan using tenk1_thous_tenthous on tenk1 a
+   ->  Sort
+         Sort Key: b.unique2, b.unique2
+         ->  Index Only Scan using tenk1_unique2 on tenk1 b
+(6 rows)
 
 reset enable_seqscan;
 reset enable_indexscan;

src/test/regress/expected/union.out

@@ -474,15 +474,14 @@ explain (costs off)
   UNION ALL
   SELECT * FROM t2) t
  WHERE ab = 'ab';
-                    QUERY PLAN                     
----------------------------------------------------
- Result
-   ->  Append
-         ->  Index Scan using t1_ab_idx on t1
-               Index Cond: ((a || b) = 'ab'::text)
-         ->  Index Only Scan using t2_pkey on t2
-               Index Cond: (ab = 'ab'::text)
-(6 rows)
+                 QUERY PLAN                  
+---------------------------------------------
+ Append
+   ->  Index Scan using t1_ab_idx on t1
+         Index Cond: ((a || b) = 'ab'::text)
+   ->  Index Only Scan using t2_pkey on t2
+         Index Cond: (ab = 'ab'::text)
+(5 rows)
 
 explain (costs off)
  SELECT * FROM
@@ -510,10 +509,9 @@ explain (costs off)
    UNION ALL
    SELECT 2 AS t, * FROM tenk1 b) c
  WHERE t = 2;
-           QUERY PLAN            
----------------------------------
- Result
-   ->  Append
-         ->  Seq Scan on tenk1 b
-(3 rows)
+        QUERY PLAN         
+---------------------------
+ Append
+   ->  Seq Scan on tenk1 b
+(2 rows)