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.

diff --git a/src/backend/executor/execAmi.c b/src/backend/executor/execAmi.c
index 9b2e32576e19139d5c51722da3edcfb1ce8e6023..ef4f6853899659ab94138bf2c5be939dc720ddc5 100644 (file)
--- 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 49b7279aba8b46d1906bbf5baa5928e1752a9c5c..47f6e202ec1fd68f6634f2bb7cd7247042292f76 100644 (file)
--- 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.