Quick hack to allow the outer query's tuple_fraction to be passed down

to a subquery if the outer query is simple enough that the LIMIT can
be reflected directly to the subquery.  This didn't use to be very
interesting, because a subquery that couldn't have been flattened into
the upper query was usually not going to be very responsive to
tuple_fraction anyway.  But with new code that allows UNION ALL subqueries
to pay attention to tuple_fraction, this is useful to do.  In particular
this lets the optimization occur when the UNION ALL is directly inside
a view.

src/backend/optimizer/path/allpaths.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.133 2005/06/09 04:18:59 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/allpaths.c,v 1.134 2005/06/10 03:32:21 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -353,6 +353,28 @@ set_inherited_rel_pathlist(PlannerInfo *root, RelOptInfo *rel,
 	set_cheapest(rel);
 }
 
+/* quick-and-dirty test to see if any joining is needed */
+static bool
+has_multiple_baserels(PlannerInfo *root)
+{
+	int			num_base_rels = 0;
+	Index		rti;
+
+	for (rti = 1; rti < root->base_rel_array_size; rti++)
+	{
+		RelOptInfo *brel = root->base_rel_array[rti];
+
+		if (brel == NULL)
+			continue;
+
+		/* ignore RTEs that are "other rels" */
+		if (brel->reloptkind == RELOPT_BASEREL)
+			if (++num_base_rels > 1)
+				return true;
+	}
+	return false;
+}
+
 /*
  * set_subquery_pathlist
  *		Build the (single) access path for a subquery RTE
@@ -361,8 +383,10 @@ static void
 set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
 					  Index rti, RangeTblEntry *rte)
 {
+	Query	   *parse = root->parse;
 	Query	   *subquery = rte->subquery;
 	bool	   *differentTypes;
+	double		tuple_fraction;
 	List	   *pathkeys;
 	List	   *subquery_pathkeys;
 
@@ -416,8 +440,24 @@ set_subquery_pathlist(PlannerInfo *root, RelOptInfo *rel,
 
 	pfree(differentTypes);
 
+	/*
+	 * We can safely pass the outer tuple_fraction down to the subquery
+	 * if the outer level has no joining, aggregation, or sorting to do.
+	 * Otherwise we'd better tell the subquery to plan for full retrieval.
+	 * (XXX This could probably be made more intelligent ...)
+	 */
+	if (parse->hasAggs ||
+		parse->groupClause ||
+		parse->havingQual ||
+		parse->distinctClause ||
+		parse->sortClause ||
+		has_multiple_baserels(root))
+		tuple_fraction = 0.0;	/* default case */
+	else
+		tuple_fraction = root->tuple_fraction;
+
 	/* Generate the plan for the subquery */
-	rel->subplan = subquery_planner(subquery, 0.0 /* default case */,
+	rel->subplan = subquery_planner(subquery, tuple_fraction,
 									&subquery_pathkeys);
 
 	/* Copy number of output rows from subplan */

src/backend/optimizer/plan/planmain.c

@@ -14,7 +14,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planmain.c,v 1.84 2005/06/08 23:02:04 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planmain.c,v 1.85 2005/06/10 03:32:23 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -79,6 +79,9 @@ query_planner(PlannerInfo *root, List *tlist, double tuple_fraction,
 	Path	   *cheapestpath;
 	Path	   *sortedpath;
 
+	/* Make tuple_fraction accessible to lower-level routines */
+	root->tuple_fraction = tuple_fraction;
+
 	/*
 	 * If the query has an empty join tree, then it's something easy like
 	 * "SELECT 2+2;" or "INSERT ... VALUES()".	Fall through quickly.

src/include/nodes/relation.h

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2005, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/nodes/relation.h,v 1.113 2005/06/09 04:19:00 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/nodes/relation.h,v 1.114 2005/06/10 03:32:25 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -92,6 +92,8 @@ typedef struct PlannerInfo
 	List	   *query_pathkeys; /* desired pathkeys for query_planner(),
 								 * and actual pathkeys afterwards */
 
+	double		tuple_fraction;	/* tuple_fraction passed to query_planner */
+
 	bool		hasJoinRTEs;	/* true if any RTEs are RTE_JOIN kind */
 	bool		hasHavingQual;	/* true if havingQual was non-null */
 } PlannerInfo;