Push limit through subqueries to underlying sort, where possible.

Douglas Doole, reviewed by Ashutosh Bapat and by me.  Minor formatting
change by me.

Discussion: http://postgr.es/m/CADE5jYLuugnEEUsyW6Q_4mZFYTxHxaVCQmGAsF0yiY8ZDggi-w@mail.gmail.com

src/backend/executor/nodeLimit.c

@@ -308,6 +308,9 @@ recompute_limits(LimitState *node)
  * since the MergeAppend surely need read no more than that many tuples from
  * any one input.  We also have to be prepared to look through a Result,
  * since the planner might stick one atop MergeAppend for projection purposes.
+ * We can also accept one or more levels of subqueries that have no quals or
+ * SRFs (that is, each subquery is just projecting columns) between the LIMIT
+ * and any of the above.
  *
  * This is a bit of a kluge, but we don't have any more-abstract way of
  * communicating between the two nodes; and it doesn't seem worth trying
@@ -320,6 +323,29 @@ recompute_limits(LimitState *node)
 static void
 pass_down_bound(LimitState *node, PlanState *child_node)
 {
+	/*
+	 * If the child is a subquery that does no filtering (no predicates)
+	 * and does not have any SRFs in the target list then we can potentially
+	 * push the limit through the subquery. It is possible that we could have
+	 * multiple subqueries, so tunnel through them all.
+	 */
+	while (IsA(child_node, SubqueryScanState))
+	{
+		SubqueryScanState *subqueryScanState;
+
+		subqueryScanState = (SubqueryScanState *) child_node;
+
+		/*
+		 * Non-empty predicates or an SRF means we cannot push down the limit.
+		 */
+		if (subqueryScanState->ss.ps.qual != NULL ||
+			expression_returns_set((Node *) child_node->plan->targetlist))
+			return;
+
+		/* Use the child in the following checks */
+		child_node = subqueryScanState->subplan;
+	}
+
 	if (IsA(child_node, SortState))
 	{
 		SortState  *sortState = (SortState *) child_node;

src/test/regress/expected/subselect.out

@@ -1041,3 +1041,55 @@ NOTICE:  x = 9, y = 13
 (3 rows)
 
 drop function tattle(x int, y int);
+--
+-- Test that LIMIT can be pushed to SORT through a subquery that just
+-- projects columns
+--
+create table sq_limit (pk int primary key, c1 int, c2 int);
+insert into sq_limit values
+	(1, 1, 1),
+	(2, 2, 2),
+	(3, 3, 3),
+	(4, 4, 4),
+	(5, 1, 1),
+	(6, 2, 2),
+	(7, 3, 3),
+	(8, 4, 4);
+-- The explain contains data that may not be invariant, so
+-- filter for just the interesting bits.  The goal here is that
+-- we should see three notices, in order:
+--   NOTICE: Limit
+--   NOTICE: Subquery
+--   NOTICE: Top-N Sort
+-- A missing step, or steps out of order means we have a problem.
+do $$
+	declare x text;
+	begin
+		for x in
+			explain (analyze, summary off, timing off, costs off)
+			select * from (select pk,c2 from sq_limit order by c1,pk) as x limit 3
+		loop
+			if (left(ltrim(x), 5) = 'Limit') then
+				raise notice 'Limit';
+			end if;
+			if (left(ltrim(x), 12) = '->  Subquery') then
+				raise notice 'Subquery';
+			end if;
+			if (left(ltrim(x), 18) = 'Sort Method: top-N') then
+				raise notice 'Top-N Sort';
+			end if;
+		end loop;
+	end;
+$$;
+NOTICE:  Limit
+NOTICE:  Subquery
+NOTICE:  Top-N Sort
+select * from (select pk,c2 from sq_limit order by c1,pk) as x limit 3;
+ pk | c2 
+----+----
+  1 |  1
+  5 |  1
+  2 |  2
+(3 rows)
+
+drop table sq_limit;

src/test/regress/sql/subselect.sql

@@ -540,3 +540,49 @@ select * from
   where tattle(x, u);
 
 drop function tattle(x int, y int);
+
+--
+-- Test that LIMIT can be pushed to SORT through a subquery that just
+-- projects columns
+--
+create table sq_limit (pk int primary key, c1 int, c2 int);
+insert into sq_limit values
+	(1, 1, 1),
+	(2, 2, 2),
+	(3, 3, 3),
+	(4, 4, 4),
+	(5, 1, 1),
+	(6, 2, 2),
+	(7, 3, 3),
+	(8, 4, 4);
+
+-- The explain contains data that may not be invariant, so
+-- filter for just the interesting bits.  The goal here is that
+-- we should see three notices, in order:
+--   NOTICE: Limit
+--   NOTICE: Subquery
+--   NOTICE: Top-N Sort
+-- A missing step, or steps out of order means we have a problem.
+do $$
+	declare x text;
+	begin
+		for x in
+			explain (analyze, summary off, timing off, costs off)
+			select * from (select pk,c2 from sq_limit order by c1,pk) as x limit 3
+		loop
+			if (left(ltrim(x), 5) = 'Limit') then
+				raise notice 'Limit';
+			end if;
+			if (left(ltrim(x), 12) = '->  Subquery') then
+				raise notice 'Subquery';
+			end if;
+			if (left(ltrim(x), 18) = 'Sort Method: top-N') then
+				raise notice 'Top-N Sort';
+			end if;
+		end loop;
+	end;
+$$;
+
+select * from (select pk,c2 from sq_limit order by c1,pk) as x limit 3;
+
+drop table sq_limit;