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.

2 files changed, 30 insertions, 2 deletions

diff --git a/src/include/optimizer/placeholder.h b/src/include/optimizer/placeholder.h
new file mode 100644
index 0000000000..4f39aedcfe
--- /dev/null
+++ b/src/include/optimizer/placeholder.h
@@ -0,0 +1,28 @@
+/*-------------------------------------------------------------------------
+ *
+ * placeholder.h
+ *	  prototypes for optimizer/util/placeholder.c.
+ *
+ *
+ * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * $PostgreSQL: pgsql/src/include/optimizer/placeholder.h,v 1.1 2008/10/21 20:42:53 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef PLACEHOLDER_H
+#define PLACEHOLDER_H
+
+#include "nodes/relation.h"
+
+
+extern PlaceHolderVar *make_placeholder_expr(PlannerInfo *root, Expr *expr,
+											 Relids phrels);
+extern PlaceHolderInfo *find_placeholder_info(PlannerInfo *root,
+											  PlaceHolderVar *phv);
+extern void fix_placeholder_eval_levels(PlannerInfo *root);
+extern void add_placeholders_to_joinrel(PlannerInfo *root,
+										RelOptInfo *joinrel);
+
+#endif   /* PLACEHOLDER_H */
diff --git a/src/include/optimizer/var.h b/src/include/optimizer/var.h
index 4a8f84e553..c478259bcd 100644
--- a/src/include/optimizer/var.h
+++ b/src/include/optimizer/var.h
@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- * $PostgreSQL: pgsql/src/include/optimizer/var.h,v 1.38 2008/09/01 20:42:45 tgl Exp $
+ * $PostgreSQL: pgsql/src/include/optimizer/var.h,v 1.39 2008/10/21 20:42:53 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -24,7 +24,7 @@ extern bool contain_vars_of_level(Node *node, int levelsup);
 extern int	locate_var_of_level(Node *node, int levelsup);
 extern int	locate_var_of_relation(Node *node, int relid, int levelsup);
 extern int	find_minimum_var_level(Node *node);
-extern List *pull_var_clause(Node *node, bool includeUpperVars);
+extern List *pull_var_clause(Node *node, bool includePlaceHolderVars);
 extern Node *flatten_join_alias_vars(PlannerInfo *root, Node *node);
 
 #endif   /* VAR_H */