From a80a12247a99f0bccf47bed5786f28a35fc80845 Mon Sep 17 00:00:00 2001
From: Tom Lane <tgl@sss.pgh.pa.us>
Date: Tue, 28 Oct 2008 17:13:51 +0000
Subject: [PATCH] Change WorkTableScan to not support backward scan.  The
 apparent support didn't actually work, because nodeRecursiveunion.c creates
 the underlying tuplestore with backward scan disabled; which is a decision
 that we shouldn't reverse because of performance cost.  We could imagine
 adding signaling from WorkTableScan to RecursiveUnion about whether backward
 scan is needed ... but in practice it'd be a waste of effort, because there
 simply isn't any current or plausible future scenario where WorkTableScan
 would be called on to scan backward.  So just dike out the code that claims
 to support it.

---
 src/backend/executor/execAmi.c           |  3 +--
 src/backend/executor/nodeWorktablescan.c | 19 ++++++++++++-------
 2 files changed, 13 insertions(+), 9 deletions(-)

diff --git a/src/backend/executor/execAmi.c b/src/backend/executor/execAmi.c
index 9b2e32576e..ef4f685389 100644
--- a/src/backend/executor/execAmi.c
+++ b/src/backend/executor/execAmi.c
@@ -6,7 +6,7 @@
  * Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
  * Portions Copyright (c) 1994, Regents of the University of California
  *
- *	$PostgreSQL: pgsql/src/backend/executor/execAmi.c,v 1.100 2008/10/17 22:10:29 tgl Exp $
+ *	$PostgreSQL: pgsql/src/backend/executor/execAmi.c,v 1.101 2008/10/28 17:13:51 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -419,7 +419,6 @@ ExecSupportsBackwardScan(Plan *node)
 		case T_FunctionScan:
 		case T_ValuesScan:
 		case T_CteScan:
-		case T_WorkTableScan:
 			return TargetListSupportsBackwardScan(node->targetlist);
 
 		case T_IndexScan:
diff --git a/src/backend/executor/nodeWorktablescan.c b/src/backend/executor/nodeWorktablescan.c
index 49b7279aba..47f6e202ec 100644
--- a/src/backend/executor/nodeWorktablescan.c
+++ b/src/backend/executor/nodeWorktablescan.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/executor/nodeWorktablescan.c,v 1.3 2008/10/23 15:29:23 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/executor/nodeWorktablescan.c,v 1.4 2008/10/28 17:13:51 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -31,14 +31,21 @@ WorkTableScanNext(WorkTableScanState *node)
 {
 	TupleTableSlot *slot;
 	EState	   *estate;
-	ScanDirection direction;
 	Tuplestorestate *tuplestorestate;
 
 	/*
 	 * get information from the estate and scan state
+	 *
+	 * Note: we intentionally do not support backward scan.  Although it would
+	 * take only a couple more lines here, it would force nodeRecursiveunion.c
+	 * to create the tuplestore with backward scan enabled, which has a
+	 * performance cost.  In practice backward scan is never useful for a
+	 * worktable plan node, since it cannot appear high enough in the plan
+	 * tree of a scrollable cursor to be exposed to a backward-scan
+	 * requirement.  So it's not worth expending effort to support it.
 	 */
 	estate = node->ss.ps.state;
-	direction = estate->es_direction;
+	Assert(ScanDirectionIsForward(estate->es_direction));
 
 	tuplestorestate = node->rustate->working_table;
 
@@ -46,9 +53,7 @@ WorkTableScanNext(WorkTableScanState *node)
 	 * Get the next tuple from tuplestore. Return NULL if no more tuples.
 	 */
 	slot = node->ss.ss_ScanTupleSlot;
-	(void) tuplestore_gettupleslot(tuplestorestate,
-								   ScanDirectionIsForward(direction),
-								   slot);
+	(void) tuplestore_gettupleslot(tuplestorestate, true, slot);
 	return slot;
 }
 
@@ -114,7 +119,7 @@ ExecInitWorkTableScan(WorkTableScan *node, EState *estate, int eflags)
 	WorkTableScanState *scanstate;
 
 	/* check for unsupported flags */
-	Assert(!(eflags & EXEC_FLAG_MARK));
+	Assert(!(eflags & (EXEC_FLAG_BACKWARD | EXEC_FLAG_MARK)));
 
 	/*
 	 * WorkTableScan should not have any children.