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Commita80a122

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Change WorkTableScan to not support backward scan. The apparent support
didn't actually work, because nodeRecursiveunion.c creates the underlyingtuplestore with backward scan disabled; which is a decision that we shouldn'treverse because of performance cost. We could imagine adding signaling fromWorkTableScan to RecursiveUnion about whether backward scan is needed ...but in practice it'd be a waste of effort, because there simply isn't anycurrent or plausible future scenario where WorkTableScan would be called onto scan backward. So just dike out the code that claims to support it.
1 parentd26bf23 commita80a122

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2 files changed

+13
-9
lines changed

2 files changed

+13
-9
lines changed

‎src/backend/executor/execAmi.c

Lines changed: 1 addition & 2 deletions
Original file line numberDiff line numberDiff line change
@@ -6,7 +6,7 @@
66
* Portions Copyright (c) 1996-2008, PostgreSQL Global Development Group
77
* Portions Copyright (c) 1994, Regents of the University of California
88
*
9-
*$PostgreSQL: pgsql/src/backend/executor/execAmi.c,v 1.100 2008/10/17 22:10:29 tgl Exp $
9+
*$PostgreSQL: pgsql/src/backend/executor/execAmi.c,v 1.101 2008/10/28 17:13:51 tgl Exp $
1010
*
1111
*-------------------------------------------------------------------------
1212
*/
@@ -419,7 +419,6 @@ ExecSupportsBackwardScan(Plan *node)
419419
caseT_FunctionScan:
420420
caseT_ValuesScan:
421421
caseT_CteScan:
422-
caseT_WorkTableScan:
423422
returnTargetListSupportsBackwardScan(node->targetlist);
424423

425424
caseT_IndexScan:

‎src/backend/executor/nodeWorktablescan.c

Lines changed: 12 additions & 7 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
99
*
1010
* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/executor/nodeWorktablescan.c,v 1.3 2008/10/23 15:29:23 tgl Exp $
11+
* $PostgreSQL: pgsql/src/backend/executor/nodeWorktablescan.c,v 1.4 2008/10/28 17:13:51 tgl Exp $
1212
*
1313
*-------------------------------------------------------------------------
1414
*/
@@ -31,24 +31,29 @@ WorkTableScanNext(WorkTableScanState *node)
3131
{
3232
TupleTableSlot*slot;
3333
EState*estate;
34-
ScanDirectiondirection;
3534
Tuplestorestate*tuplestorestate;
3635

3736
/*
3837
* get information from the estate and scan state
38+
*
39+
* Note: we intentionally do not support backward scan. Although it would
40+
* take only a couple more lines here, it would force nodeRecursiveunion.c
41+
* to create the tuplestore with backward scan enabled, which has a
42+
* performance cost. In practice backward scan is never useful for a
43+
* worktable plan node, since it cannot appear high enough in the plan
44+
* tree of a scrollable cursor to be exposed to a backward-scan
45+
* requirement. So it's not worth expending effort to support it.
3946
*/
4047
estate=node->ss.ps.state;
41-
direction=estate->es_direction;
48+
Assert(ScanDirectionIsForward(estate->es_direction));
4249

4350
tuplestorestate=node->rustate->working_table;
4451

4552
/*
4653
* Get the next tuple from tuplestore. Return NULL if no more tuples.
4754
*/
4855
slot=node->ss.ss_ScanTupleSlot;
49-
(void)tuplestore_gettupleslot(tuplestorestate,
50-
ScanDirectionIsForward(direction),
51-
slot);
56+
(void)tuplestore_gettupleslot(tuplestorestate, true,slot);
5257
returnslot;
5358
}
5459

@@ -114,7 +119,7 @@ ExecInitWorkTableScan(WorkTableScan *node, EState *estate, int eflags)
114119
WorkTableScanState*scanstate;
115120

116121
/* check for unsupported flags */
117-
Assert(!(eflags&EXEC_FLAG_MARK));
122+
Assert(!(eflags&(EXEC_FLAG_BACKWARD |EXEC_FLAG_MARK)));
118123

119124
/*
120125
* WorkTableScan should not have any children.

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