Further fallout from the MergeAppend patch.

Fix things so that top-N sorting can be used in child Sort nodes of a
MergeAppend node, when there is a LIMIT and no intervening joins or
grouping.  Actually doing this on the executor side isn't too bad,
but it's a bit messier to get the planner to cost it properly.
Per gripe from Robert Haas.

In passing, fix an oversight in the original top-N-sorting patch:
query_planner should not assume that a LIMIT can be used to make an
explicit sort cheaper when there will be grouping or aggregation in
between.  Possibly this should be back-patched, but I'm not sure the
mistake is serious enough to be a real problem in practice.

src/backend/executor/nodeLimit.c

@@ -25,6 +25,7 @@
 #include "executor/nodeLimit.h"
 
 static void recompute_limits(LimitState *node);
+static void pass_down_bound(LimitState *node, PlanState *child_node);
 
 
 /* ----------------------------------------------------------------
@@ -293,26 +294,35 @@ recompute_limits(LimitState *node)
 	/* Set state-machine state */
 	node->lstate = LIMIT_RESCAN;
 
-	/*
-	 * If we have a COUNT, and our input is a Sort node, notify it that it can
-	 * use bounded sort.
-	 *
-	 * 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
-	 * to invent one without some more examples of special communication
-	 * needs.
-	 *
-	 * Note: it is the responsibility of nodeSort.c to react properly to
-	 * changes of these parameters.  If we ever do redesign this, it'd be a
-	 * good idea to integrate this signaling with the parameter-change
-	 * mechanism.
-	 */
-	if (IsA(outerPlanState(node), SortState))
+	/* Notify child node about limit, if useful */
+	pass_down_bound(node, outerPlanState(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.
+ *
+ * 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
+ * to invent one without some more examples of special communication needs.
+ *
+ * Note: it is the responsibility of nodeSort.c to react properly to
+ * changes of these parameters.  If we ever do redesign this, it'd be a
+ * good idea to integrate this signaling with the parameter-change mechanism.
+ */
+static void
+pass_down_bound(LimitState *node, PlanState *child_node)
+{
+	if (IsA(child_node, SortState))
 	{
-		SortState  *sortState = (SortState *) outerPlanState(node);
+		SortState  *sortState = (SortState *) child_node;
 		int64		tuples_needed = node->count + node->offset;
 
-		/* negative test checks for overflow */
+		/* negative test checks for overflow in sum */
 		if (node->noCount || tuples_needed < 0)
 		{
 			/* make sure flag gets reset if needed upon rescan */
@@ -324,6 +334,19 @@ recompute_limits(LimitState *node)
 			sortState->bound = tuples_needed;
 		}
 	}
+	else if (IsA(child_node, MergeAppendState))
+	{
+		MergeAppendState *maState = (MergeAppendState *) child_node;
+		int			i;
+
+		for (i = 0; i < maState->ms_nplans; i++)
+			pass_down_bound(node, maState->mergeplans[i]);
+	}
+	else if (IsA(child_node, ResultState))
+	{
+		if (outerPlanState(child_node))
+			pass_down_bound(node, outerPlanState(child_node));
+	}
 }
 
 /* ----------------------------------------------------------------

src/backend/nodes/outfuncs.c

@@ -1493,6 +1493,7 @@ _outMergeAppendPath(StringInfo str, MergeAppendPath *node)
 	_outPathInfo(str, (Path *) node);
 
 	WRITE_NODE_FIELD(subpaths);
+	WRITE_FLOAT_FIELD(limit_tuples, "%.0f");
 }
 
 static void
@@ -1611,6 +1612,7 @@ _outPlannerInfo(StringInfo str, PlannerInfo *node)
 	WRITE_NODE_FIELD(minmax_aggs);
 	WRITE_FLOAT_FIELD(total_table_pages, "%.0f");
 	WRITE_FLOAT_FIELD(tuple_fraction, "%.4f");
+	WRITE_FLOAT_FIELD(limit_tuples, "%.0f");
 	WRITE_BOOL_FIELD(hasInheritedTarget);
 	WRITE_BOOL_FIELD(hasJoinRTEs);
 	WRITE_BOOL_FIELD(hasHavingQual);

src/backend/optimizer/plan/createplan.c

@@ -714,7 +714,7 @@ create_merge_append_plan(PlannerInfo *root, MergeAppendPath *best_path)
 		if (!pathkeys_contained_in(pathkeys, subpath->pathkeys))
 			subplan = (Plan *) make_sort(root, subplan, numsortkeys,
 										 sortColIdx, sortOperators, nullsFirst,
-										 -1.0);
+										 best_path->limit_tuples);
 
 		subplans = lappend(subplans, subplan);
 	}

src/backend/optimizer/plan/planmain.c

@@ -101,8 +101,9 @@ query_planner(PlannerInfo *root, List *tlist,
 	ListCell   *lc;
 	double		total_pages;
 
-	/* Make tuple_fraction accessible to lower-level routines */
+	/* Make tuple_fraction, limit_tuples accessible to lower-level routines */
 	root->tuple_fraction = tuple_fraction;
+	root->limit_tuples = limit_tuples;
 
 	*num_groups = 1;			/* default result */
 
@@ -315,6 +316,9 @@ query_planner(PlannerInfo *root, List *tlist,
 			!pathkeys_contained_in(root->distinct_pathkeys, root->group_pathkeys) ||
 		 !pathkeys_contained_in(root->window_pathkeys, root->group_pathkeys))
 			tuple_fraction = 0.0;
+
+		/* In any case, limit_tuples shouldn't be specified here */
+		Assert(limit_tuples < 0);
 	}
 	else if (parse->hasAggs || root->hasHavingQual)
 	{
@@ -323,6 +327,9 @@ query_planner(PlannerInfo *root, List *tlist,
 		 * it will deliver a single result row (so leave *num_groups 1).
 		 */
 		tuple_fraction = 0.0;
+
+		/* limit_tuples shouldn't be specified here */
+		Assert(limit_tuples < 0);
 	}
 	else if (parse->distinctClause)
 	{
@@ -347,6 +354,9 @@ query_planner(PlannerInfo *root, List *tlist,
 		 */
 		if (tuple_fraction >= 1.0)
 			tuple_fraction /= *num_groups;
+
+		/* limit_tuples shouldn't be specified here */
+		Assert(limit_tuples < 0);
 	}
 	else
 	{

src/backend/optimizer/plan/planner.c

@@ -968,6 +968,7 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 	{
 		/* No set operations, do regular planning */
 		List	   *sub_tlist;
+		double		sub_limit_tuples;
 		AttrNumber *groupColIdx = NULL;
 		bool		need_tlist_eval = true;
 		QualCost	tlist_cost;
@@ -1119,13 +1120,28 @@ grouping_planner(PlannerInfo *root, double tuple_fraction)
 		else
 			root->query_pathkeys = NIL;
 
+		/*
+		 * Figure out whether there's a hard limit on the number of rows that
+		 * query_planner's result subplan needs to return.  Even if we know a
+		 * hard limit overall, it doesn't apply if the query has any
+		 * grouping/aggregation operations.
+		 */
+		if (parse->groupClause ||
+			parse->distinctClause ||
+			parse->hasAggs ||
+			parse->hasWindowFuncs ||
+			root->hasHavingQual)
+			sub_limit_tuples = -1.0;
+		else
+			sub_limit_tuples = limit_tuples;
+
 		/*
 		 * Generate the best unsorted and presorted paths for this Query (but
 		 * note there may not be any presorted path).  query_planner will also
 		 * estimate the number of groups in the query, and canonicalize all
 		 * the pathkeys.
 		 */
-		query_planner(root, sub_tlist, tuple_fraction, limit_tuples,
+		query_planner(root, sub_tlist, tuple_fraction, sub_limit_tuples,
 					  &cheapest_path, &sorted_path, &dNumGroups);
 
 		/*

src/backend/optimizer/util/pathnode.c

@@ -694,6 +694,35 @@ create_merge_append_path(PlannerInfo *root,
 	pathnode->path.pathkeys = pathkeys;
 	pathnode->subpaths = subpaths;
 
+	/*
+	 * Apply query-wide LIMIT if known and path is for sole base relation.
+	 * Finding out the latter at this low level is a bit klugy.
+	 */
+	pathnode->limit_tuples = root->limit_tuples;
+	if (pathnode->limit_tuples >= 0)
+	{
+		Index		rti;
+
+		for (rti = 1; rti < root->simple_rel_array_size; rti++)
+		{
+			RelOptInfo *brel = root->simple_rel_array[rti];
+
+			if (brel == NULL)
+				continue;
+
+			/* ignore RTEs that are "other rels" */
+			if (brel->reloptkind != RELOPT_BASEREL)
+				continue;
+
+			if (brel != rel)
+			{
+				/* Oops, it's a join query */
+				pathnode->limit_tuples = -1.0;
+				break;
+			}
+		}
+	}
+
 	/* Add up all the costs of the input paths */
 	input_startup_cost = 0;
 	input_total_cost = 0;
@@ -720,7 +749,7 @@ create_merge_append_path(PlannerInfo *root,
 					  subpath->parent->width,
 					  0.0,
 					  work_mem,
-					  -1.0);
+					  pathnode->limit_tuples);
 			input_startup_cost += sort_path.startup_cost;
 			input_total_cost += sort_path.total_cost;
 		}

src/include/nodes/relation.h

@@ -198,6 +198,7 @@ typedef struct PlannerInfo
 	double		total_table_pages;		/* # of pages in all tables of query */
 
 	double		tuple_fraction; /* tuple_fraction passed to query_planner */
+	double		limit_tuples;	/* limit_tuples passed to query_planner */
 
 	bool		hasInheritedTarget;		/* true if parse->resultRelation is an
 										 * inheritance child rel */
@@ -766,6 +767,7 @@ typedef struct MergeAppendPath
 {
 	Path		path;
 	List	   *subpaths;		/* list of component Paths */
+	double		limit_tuples;	/* hard limit on output tuples, or -1 */
 } MergeAppendPath;
 
 /*