Code review for pushing LIMIT through subqueries.

Minor improvements for commit 1f6d515a.  We do not need the (rather
expensive) test for SRFs in the targetlist, because since v10 any
such SRFs would appear in separate ProjectSet nodes.  Also, make the
code look more like the existing cases by turning it into a simple
recursion --- the argument that there might be some performance
benefit to contorting the code seems unfounded to me, especially since
any good compiler should turn the tail-recursion into iteration anyway.

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

src/backend/executor/nodeLimit.c

@@ -303,14 +303,11 @@ recompute_limits(LimitState *node)
 
 /*
  * If we have a COUNT, and our input is a Sort node, notify it that it can
- * use bounded sort.  Also, if our input is a MergeAppend, we can apply the
- * same bound to any Sorts that are direct children of the MergeAppend,
- * 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.
+ * use bounded sort.  We can also pass down the bound through plan nodes
+ * that cannot remove or combine input rows; for example, if our input is a
+ * MergeAppend, we can apply the same bound to any Sorts that are direct
+ * children of the MergeAppend, since the MergeAppend surely need not read
+ * more than that many tuples from any one input.
  *
  * 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
@@ -324,27 +321,10 @@ 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.
+	 * Since this function recurses, in principle we should check stack depth
+	 * here.  In practice, it's probably pointless since the earlier node
+	 * initialization tree traversal would surely have consumed more stack.
 	 */
-	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))
 	{
@@ -365,6 +345,7 @@ pass_down_bound(LimitState *node, PlanState *child_node)
 	}
 	else if (IsA(child_node, MergeAppendState))
 	{
+		/* Pass down the bound through MergeAppend */
 		MergeAppendState *maState = (MergeAppendState *) child_node;
 		int			i;
 
@@ -374,6 +355,9 @@ pass_down_bound(LimitState *node, PlanState *child_node)
 	else if (IsA(child_node, ResultState))
 	{
 		/*
+		 * We also have to be prepared to look through a Result, since the
+		 * planner might stick one atop MergeAppend for projection purposes.
+		 *
 		 * If Result supported qual checking, we'd have to punt on seeing a
 		 * qual.  Note that having a resconstantqual is not a showstopper: if
 		 * that fails we're not getting any rows at all.
@@ -381,6 +365,24 @@ pass_down_bound(LimitState *node, PlanState *child_node)
 		if (outerPlanState(child_node))
 			pass_down_bound(node, outerPlanState(child_node));
 	}
+	else if (IsA(child_node, SubqueryScanState))
+	{
+		/*
+		 * We can also look through SubqueryScan, but only if it has no qual
+		 * (otherwise it might discard rows).
+		 */
+		SubqueryScanState *subqueryState = (SubqueryScanState *) child_node;
+
+		if (subqueryState->ss.ps.qual == NULL)
+			pass_down_bound(node, subqueryState->subplan);
+	}
+
+	/*
+	 * In principle we could look through any plan node type that is certain
+	 * not to discard or combine input rows.  In practice, there are not many
+	 * node types that the planner might put between Sort and Limit, so trying
+	 * to be very general is not worth the trouble.
+	 */
 }
 
 /* ----------------------------------------------------------------