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, 37 insertions, 3 deletions

diff --git a/src/backend/nodes/nodeFuncs.c b/src/backend/nodes/nodeFuncs.c
index baf98b67634..7236360347c 100644
--- a/src/backend/nodes/nodeFuncs.c
+++ b/src/backend/nodes/nodeFuncs.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/nodes/nodeFuncs.c,v 1.34 2008/10/06 17:39:26 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/nodes/nodeFuncs.c,v 1.35 2008/10/21 20:42:52 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -220,6 +220,9 @@ exprType(Node *expr)
 		case T_CurrentOfExpr:
 			type = BOOLOID;
 			break;
+		case T_PlaceHolderVar:
+			type = exprType((Node *) ((PlaceHolderVar *) expr)->phexpr);
+			break;
 		default:
 			elog(ERROR, "unrecognized node type: %d", (int) nodeTag(expr));
 			type = InvalidOid;	/* keep compiler quiet */
@@ -420,6 +423,8 @@ exprTypmod(Node *expr)
 			return ((CoerceToDomainValue *) expr)->typeMod;
 		case T_SetToDefault:
 			return ((SetToDefault *) expr)->typeMod;
+		case T_PlaceHolderVar:
+			return exprTypmod((Node *) ((PlaceHolderVar *) expr)->phexpr);
 		default:
 			break;
 	}
@@ -876,6 +881,10 @@ exprLocation(Node *expr)
 		case T_CommonTableExpr:
 			loc = ((CommonTableExpr *) expr)->location;
 			break;
+		case T_PlaceHolderVar:
+			/* just use argument's location */
+			loc = exprLocation((Node *) ((PlaceHolderVar *) expr)->phexpr);
+			break;
 		default:
 			/* for any other node type it's just unknown... */
 			loc = -1;
@@ -1272,11 +1281,12 @@ expression_tree_walker(Node *node,
 			{
 				FlattenedSubLink *fslink = (FlattenedSubLink *) node;
 
-				if (expression_tree_walker((Node *) fslink->quals,
-										   walker, context))
+				if (walker(fslink->quals, context))
 					return true;
 			}
 			break;
+		case T_PlaceHolderVar:
+			return walker(((PlaceHolderVar *) node)->phexpr, context);
 		case T_AppendRelInfo:
 			{
 				AppendRelInfo *appinfo = (AppendRelInfo *) node;
@@ -1286,6 +1296,8 @@ expression_tree_walker(Node *node,
 					return true;
 			}
 			break;
+		case T_PlaceHolderInfo:
+			return walker(((PlaceHolderInfo *) node)->ph_var, context);
 		default:
 			elog(ERROR, "unrecognized node type: %d",
 				 (int) nodeTag(node));
@@ -1918,6 +1930,17 @@ expression_tree_mutator(Node *node,
 				return (Node *) newnode;
 			}
 			break;
+		case T_PlaceHolderVar:
+			{
+				PlaceHolderVar *phv = (PlaceHolderVar *) node;
+				PlaceHolderVar *newnode;
+
+				FLATCOPY(newnode, phv, PlaceHolderVar);
+				MUTATE(newnode->phexpr, phv->phexpr, Expr *);
+				/* Assume we need not copy the relids bitmapset */
+				return (Node *) newnode;
+			}
+			break;
 		case T_AppendRelInfo:
 			{
 				AppendRelInfo *appinfo = (AppendRelInfo *) node;
@@ -1928,6 +1951,17 @@ expression_tree_mutator(Node *node,
 				return (Node *) newnode;
 			}
 			break;
+		case T_PlaceHolderInfo:
+			{
+				PlaceHolderInfo *phinfo = (PlaceHolderInfo *) node;
+				PlaceHolderInfo *newnode;
+
+				FLATCOPY(newnode, phinfo, PlaceHolderInfo);
+				MUTATE(newnode->ph_var, phinfo->ph_var, PlaceHolderVar *);
+				/* Assume we need not copy the relids bitmapsets */
+				return (Node *) newnode;
+			}
+			break;
 		default:
 			elog(ERROR, "unrecognized node type: %d",
 				 (int) nodeTag(node));