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.

src/backend/optimizer/util/clauses.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/util/clauses.c,v 1.269 2008/10/09 19:27:40 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/util/clauses.c,v 1.270 2008/10/21 20:42:53 tgl Exp $
  *
  * HISTORY
  *	  AUTHOR			DATE			MAJOR EVENT
@@ -1188,6 +1188,12 @@ find_nonnullable_rels_walker(Node *node, bool top_level)
 			result = find_nonnullable_rels_walker((Node *) expr->quals,
 												  top_level);
 	}
+	else if (IsA(node, PlaceHolderVar))
+	{
+		PlaceHolderVar *phv = (PlaceHolderVar *) node;
+
+		result = find_nonnullable_rels_walker((Node *) phv->phexpr, top_level);
+	}
 	return result;
 }
 
@@ -1393,6 +1399,12 @@ find_nonnullable_vars_walker(Node *node, bool top_level)
 			result = find_nonnullable_vars_walker((Node *) expr->quals,
 												  top_level);
 	}
+	else if (IsA(node, PlaceHolderVar))
+	{
+		PlaceHolderVar *phv = (PlaceHolderVar *) node;
+
+		result = find_nonnullable_vars_walker((Node *) phv->phexpr, top_level);
+	}
 	return result;
 }
 
@@ -1921,6 +1933,7 @@ eval_const_expressions(PlannerInfo *root, Node *node)
  *	  constant.  This effectively means that we plan using the first supplied
  *	  value of the Param.
  * 2. Fold stable, as well as immutable, functions to constants.
+ * 3. Reduce PlaceHolderVar nodes to their contained expressions.
  *--------------------
  */
 Node *
@@ -2823,6 +2836,20 @@ eval_const_expressions_mutator(Node *node,
 		newfslink->quals = quals;
 		return (Node *) newfslink;
 	}
+	if (IsA(node, PlaceHolderVar) && context->estimate)
+	{
+		/*
+		 * In estimation mode, just strip the PlaceHolderVar node altogether;
+		 * this amounts to estimating that the contained value won't be forced
+		 * to null by an outer join.  In regular mode we just use the default
+		 * behavior (ie, simplify the expression but leave the PlaceHolderVar
+		 * node intact).
+		 */
+		PlaceHolderVar *phv = (PlaceHolderVar *) node;
+
+		return eval_const_expressions_mutator((Node *) phv->phexpr,
+											  context);
+	}
 
 	/*
 	 * For any node type not handled above, we recurse using