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, 50 insertions, 38 deletions

diff --git a/src/backend/optimizer/path/costsize.c b/src/backend/optimizer/path/costsize.c
index fa580cf14e..ba33da829a 100644
--- a/src/backend/optimizer/path/costsize.c
+++ b/src/backend/optimizer/path/costsize.c
@@ -54,7 +54,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.199 2008/10/17 20:27:24 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/path/costsize.c,v 1.200 2008/10/21 20:42:52 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -69,6 +69,7 @@
 #include "optimizer/clauses.h"
 #include "optimizer/cost.h"
 #include "optimizer/pathnode.h"
+#include "optimizer/placeholder.h"
 #include "optimizer/planmain.h"
 #include "parser/parsetree.h"
 #include "utils/lsyscache.h"
@@ -2610,55 +2611,66 @@ set_rel_width(PlannerInfo *root, RelOptInfo *rel)
 {
 	Oid			reloid = planner_rt_fetch(rel->relid, root)->relid;
 	int32		tuple_width = 0;
-	ListCell   *tllist;
+	ListCell   *lc;
 
-	foreach(tllist, rel->reltargetlist)
+	foreach(lc, rel->reltargetlist)
 	{
-		Var		   *var = (Var *) lfirst(tllist);
-		int			ndx;
-		int32		item_width;
+		Node	   *node = (Node *) lfirst(lc);
 
-		/* For now, punt on whole-row child Vars */
-		if (!IsA(var, Var))
+		if (IsA(node, Var))
 		{
-			tuple_width += 32;	/* arbitrary */
-			continue;
-		}
-
-		Assert(var->varno == rel->relid);
-		Assert(var->varattno >= rel->min_attr);
-		Assert(var->varattno <= rel->max_attr);
+			Var		   *var = (Var *) node;
+			int			ndx;
+			int32		item_width;
 
-		ndx = var->varattno - rel->min_attr;
+			Assert(var->varno == rel->relid);
+			Assert(var->varattno >= rel->min_attr);
+			Assert(var->varattno <= rel->max_attr);
 
-		/*
-		 * The width probably hasn't been cached yet, but may as well check
-		 */
-		if (rel->attr_widths[ndx] > 0)
-		{
-			tuple_width += rel->attr_widths[ndx];
-			continue;
-		}
+			ndx = var->varattno - rel->min_attr;
 
-		if (reloid != InvalidOid)
-		{
-			item_width = get_attavgwidth(reloid, var->varattno);
-			if (item_width > 0)
+			/*
+			 * The width probably hasn't been cached yet, but may as well check
+			 */
+			if (rel->attr_widths[ndx] > 0)
 			{
-				rel->attr_widths[ndx] = item_width;
-				tuple_width += item_width;
+				tuple_width += rel->attr_widths[ndx];
 				continue;
 			}
+
+			/* Try to get column width from statistics */
+			if (reloid != InvalidOid)
+			{
+				item_width = get_attavgwidth(reloid, var->varattno);
+				if (item_width > 0)
+				{
+					rel->attr_widths[ndx] = item_width;
+					tuple_width += item_width;
+					continue;
+				}
+			}
+
+			/*
+			 * Not a plain relation, or can't find statistics for it. Estimate
+			 * using just the type info.
+			 */
+			item_width = get_typavgwidth(var->vartype, var->vartypmod);
+			Assert(item_width > 0);
+			rel->attr_widths[ndx] = item_width;
+			tuple_width += item_width;
 		}
+		else if (IsA(node, PlaceHolderVar))
+		{
+			PlaceHolderVar *phv = (PlaceHolderVar *) node;
+			PlaceHolderInfo *phinfo = find_placeholder_info(root, phv);
 
-		/*
-		 * Not a plain relation, or can't find statistics for it. Estimate
-		 * using just the type info.
-		 */
-		item_width = get_typavgwidth(var->vartype, var->vartypmod);
-		Assert(item_width > 0);
-		rel->attr_widths[ndx] = item_width;
-		tuple_width += item_width;
+			tuple_width += phinfo->ph_width;
+		}
+		else
+		{
+			/* For now, punt on whole-row child Vars */
+			tuple_width += 32;	/* arbitrary */
+		}
 	}
 	Assert(tuple_width >= 0);
 	rel->width = tuple_width;