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, 40 insertions, 1 deletions

diff --git a/src/backend/nodes/copyfuncs.c b/src/backend/nodes/copyfuncs.c
index f0a49155024..2dce7bab18f 100644
--- a/src/backend/nodes/copyfuncs.c
+++ b/src/backend/nodes/copyfuncs.c
@@ -15,7 +15,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/nodes/copyfuncs.c,v 1.408 2008/10/07 19:27:04 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/nodes/copyfuncs.c,v 1.409 2008/10/21 20:42:52 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -1593,6 +1593,22 @@ _copyFlattenedSubLink(FlattenedSubLink *from)
 }
 
 /*
+ * _copyPlaceHolderVar
+ */
+static PlaceHolderVar *
+_copyPlaceHolderVar(PlaceHolderVar *from)
+{
+	PlaceHolderVar *newnode = makeNode(PlaceHolderVar);
+
+	COPY_NODE_FIELD(phexpr);
+	COPY_BITMAPSET_FIELD(phrels);
+	COPY_SCALAR_FIELD(phid);
+	COPY_SCALAR_FIELD(phlevelsup);
+
+	return newnode;
+}
+
+/*
  * _copySpecialJoinInfo
  */
 static SpecialJoinInfo *
@@ -1631,6 +1647,23 @@ _copyAppendRelInfo(AppendRelInfo *from)
 	return newnode;
 }
 
+/*
+ * _copyPlaceHolderInfo
+ */
+static PlaceHolderInfo *
+_copyPlaceHolderInfo(PlaceHolderInfo *from)
+{
+	PlaceHolderInfo *newnode = makeNode(PlaceHolderInfo);
+
+	COPY_SCALAR_FIELD(phid);
+	COPY_NODE_FIELD(ph_var);
+	COPY_BITMAPSET_FIELD(ph_eval_at);
+	COPY_BITMAPSET_FIELD(ph_needed);
+	COPY_SCALAR_FIELD(ph_width);
+
+	return newnode;
+}
+
 /* ****************************************************************
  *					parsenodes.h copy functions
  * ****************************************************************
@@ -3438,12 +3471,18 @@ copyObject(void *from)
 		case T_FlattenedSubLink:
 			retval = _copyFlattenedSubLink(from);
 			break;
+		case T_PlaceHolderVar:
+			retval = _copyPlaceHolderVar(from);
+			break;
 		case T_SpecialJoinInfo:
 			retval = _copySpecialJoinInfo(from);
 			break;
 		case T_AppendRelInfo:
 			retval = _copyAppendRelInfo(from);
 			break;
+		case T_PlaceHolderInfo:
+			retval = _copyPlaceHolderInfo(from);
+			break;
 
 			/*
 			 * VALUE NODES