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, 35 insertions, 14 deletions

diff --git a/src/backend/optimizer/plan/initsplan.c b/src/backend/optimizer/plan/initsplan.c
index 49c2cdf2f5..1208262b8e 100644
--- a/src/backend/optimizer/plan/initsplan.c
+++ b/src/backend/optimizer/plan/initsplan.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/initsplan.c,v 1.142 2008/08/17 01:19:59 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/optimizer/plan/initsplan.c,v 1.143 2008/10/21 20:42:53 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -21,6 +21,7 @@
 #include "optimizer/joininfo.h"
 #include "optimizer/pathnode.h"
 #include "optimizer/paths.h"
+#include "optimizer/placeholder.h"
 #include "optimizer/planmain.h"
 #include "optimizer/prep.h"
 #include "optimizer/restrictinfo.h"
@@ -131,7 +132,7 @@ add_base_rels_to_query(PlannerInfo *root, Node *jtnode)
 void
 build_base_rel_tlists(PlannerInfo *root, List *final_tlist)
 {
-	List	   *tlist_vars = pull_var_clause((Node *) final_tlist, false);
+	List	   *tlist_vars = pull_var_clause((Node *) final_tlist, true);
 
 	if (tlist_vars != NIL)
 	{
@@ -146,6 +147,10 @@ build_base_rel_tlists(PlannerInfo *root, List *final_tlist)
  *	  relation's targetlist if not already present, and mark the variable
  *	  as being needed for the indicated join (or for final output if
  *	  where_needed includes "relation 0").
+ *
+ *	  The list may also contain PlaceHolderVars.  These don't necessarily
+ *	  have a single owning relation; we keep their attr_needed info in
+ *	  root->placeholder_list instead.
  */
 void
 add_vars_to_targetlist(PlannerInfo *root, List *vars, Relids where_needed)
@@ -156,20 +161,36 @@ add_vars_to_targetlist(PlannerInfo *root, List *vars, Relids where_needed)
 
 	foreach(temp, vars)
 	{
-		Var		   *var = (Var *) lfirst(temp);
-		RelOptInfo *rel = find_base_rel(root, var->varno);
-		int			attrno = var->varattno;
+		Node	   *node = (Node *) lfirst(temp);
+
+		if (IsA(node, Var))
+		{
+			Var		   *var = (Var *) node;
+			RelOptInfo *rel = find_base_rel(root, var->varno);
+			int			attno = var->varattno;
 
-		Assert(attrno >= rel->min_attr && attrno <= rel->max_attr);
-		attrno -= rel->min_attr;
-		if (bms_is_empty(rel->attr_needed[attrno]))
+			Assert(attno >= rel->min_attr && attno <= rel->max_attr);
+			attno -= rel->min_attr;
+			if (rel->attr_needed[attno] == NULL)
+			{
+				/* Variable not yet requested, so add to reltargetlist */
+				/* XXX is copyObject necessary here? */
+				rel->reltargetlist = lappend(rel->reltargetlist,
+											 copyObject(var));
+			}
+			rel->attr_needed[attno] = bms_add_members(rel->attr_needed[attno],
+													  where_needed);
+		}
+		else if (IsA(node, PlaceHolderVar))
 		{
-			/* Variable not yet requested, so add to reltargetlist */
-			/* XXX is copyObject necessary here? */
-			rel->reltargetlist = lappend(rel->reltargetlist, copyObject(var));
+			PlaceHolderVar *phv = (PlaceHolderVar *) node;
+			PlaceHolderInfo *phinfo = find_placeholder_info(root, phv);
+
+			phinfo->ph_needed = bms_add_members(phinfo->ph_needed,
+												where_needed);
 		}
-		rel->attr_needed[attrno] = bms_add_members(rel->attr_needed[attrno],
-												   where_needed);
+		else
+			elog(ERROR, "unrecognized node type: %d", (int) nodeTag(node));
 	}
 }
 
@@ -934,7 +955,7 @@ distribute_qual_to_rels(PlannerInfo *root, Node *clause,
 	 */
 	if (bms_membership(relids) == BMS_MULTIPLE)
 	{
-		List	   *vars = pull_var_clause(clause, false);
+		List	   *vars = pull_var_clause(clause, true);
 
 		add_vars_to_targetlist(root, vars, relids);
 		list_free(vars);