Add a concept of "placeholder" variables to the planner. These are variables

that represent some expression that we desire to compute below the top level
of the plan, and then let that value "bubble up" as though it were a plain
Var (ie, a column value).

The immediate application is to allow sub-selects to be flattened even when
they are below an outer join and have non-nullable output expressions.
Formerly we couldn't flatten because such an expression wouldn't properly
go to NULL when evaluated above the outer join.  Now, we wrap it in a
PlaceHolderVar and arrange for the actual evaluation to occur below the outer
join.  When the resulting Var bubbles up through the join, it will be set to
NULL if necessary, yielding the correct results.  This fixes a planner
limitation that's existed since 7.1.

In future we might want to use this mechanism to re-introduce some form of
Hellerstein's "expensive functions" optimization, ie place the evaluation of
an expensive function at the most suitable point in the plan tree.

1 files changed, 10 insertions, 8 deletions

diff --git a/src/backend/optimizer/plan/planmain.c b/src/backend/optimizer/plan/planmain.c
index 7dcdaf250f9..171724983dc 100644
--- a/src/backend/optimizer/plan/planmain.c
+++ b/src/backend/optimizer/plan/planmain.c
@@ -14,7 +14,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planmain.c,v 1.110 2008/08/14 18:47:59 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/planmain.c,v 1.111 2008/10/21 20:42:53 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -23,6 +23,7 @@
 #include "optimizer/cost.h"
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"
+#include "optimizer/placeholder.h"
 #include "optimizer/planmain.h"
 #include "optimizer/tlist.h"
 #include "utils/selfuncs.h"
@@ -132,7 +133,7 @@ query_planner(PlannerInfo *root, List *tlist,
 	 * for "simple" rels.
 	 *
 	 * NOTE: append_rel_list was set up by subquery_planner, so do not touch
-	 * here; eq_classes may contain data already, too.
+	 * here; ditto placeholder_list; eq_classes may contain data already, too.
 	 */
 	root->simple_rel_array_size = list_length(parse->rtable) + 1;
 	root->simple_rel_array = (RelOptInfo **)
@@ -204,12 +205,6 @@ query_planner(PlannerInfo *root, List *tlist,
 	 * added to appropriate lists belonging to the mentioned relations.  We
 	 * also build EquivalenceClasses for provably equivalent expressions, and
 	 * form a target joinlist for make_one_rel() to work from.
-	 *
-	 * Note: all subplan nodes will have "flat" (var-only) tlists. This
-	 * implies that all expression evaluations are done at the root of the
-	 * plan tree. Once upon a time there was code to try to push expensive
-	 * function calls down to lower plan nodes, but that's dead code and has
-	 * been for a long time...
 	 */
 	build_base_rel_tlists(root, tlist);
 
@@ -241,6 +236,13 @@ query_planner(PlannerInfo *root, List *tlist,
 	root->sort_pathkeys = canonicalize_pathkeys(root, root->sort_pathkeys);
 
 	/*
+	 * Examine any "placeholder" expressions generated during subquery pullup.
+	 * Make sure that we know what level to evaluate them at, and that the
+	 * Vars they need are marked as needed.
+	 */
+	fix_placeholder_eval_levels(root);
+
+	/*
 	 * Ready to do the primary planning.
 	 */
 	final_rel = make_one_rel(root, joinlist);