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Commit61dd418

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Keep rs_startblock the same during heap_rescan, so that a rescan of a SeqScan
node starts from the same place as the first scan did. This avoids surprisingbehavior of scrollable and WITH HOLD cursors, as seen in Mark Kirkwood's bugreport of yesterday.It's not entirely clear whether a rescan should be forced to drop out of thesyncscan mode, but for the moment I left the code behaving the same on thatpoint. Any change there would only be a performance and not a correctnessissue, anyway.Back-patch to 8.3, since the unstable behavior was created by the syncscanpatch.
1 parent2ef8c1a commit61dd418

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‎src/backend/access/heap/heapam.c

Lines changed: 14 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -8,7 +8,7 @@
88
*
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*
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* IDENTIFICATION
11-
* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.275 2009/05/12 16:43:32 tgl Exp $
11+
* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.276 2009/06/10 18:54:16 tgl Exp $
1212
*
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*
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* INTERFACE ROUTINES
@@ -93,7 +93,7 @@ static bool HeapSatisfiesHOTUpdate(Relation relation, Bitmapset *hot_attrs,
9393
* ----------------
9494
*/
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staticvoid
96-
initscan(HeapScanDescscan,ScanKeykey)
96+
initscan(HeapScanDescscan,ScanKeykey,boolis_rescan)
9797
{
9898
boolallow_strat;
9999
boolallow_sync;
@@ -143,7 +143,16 @@ initscan(HeapScanDesc scan, ScanKey key)
143143
scan->rs_strategy=NULL;
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}
145145

146-
if (allow_sync&&synchronize_seqscans)
146+
if (is_rescan)
147+
{
148+
/*
149+
* If rescan, keep the previous startblock setting so that rewinding
150+
* a cursor doesn't generate surprising results. Reset the syncscan
151+
* setting, though.
152+
*/
153+
scan->rs_syncscan= (allow_sync&&synchronize_seqscans);
154+
}
155+
elseif (allow_sync&&synchronize_seqscans)
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{
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scan->rs_syncscan= true;
149158
scan->rs_startblock=ss_get_location(scan->rs_rd,scan->rs_nblocks);
@@ -1218,7 +1227,7 @@ heap_beginscan_internal(Relation relation, Snapshot snapshot,
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else
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scan->rs_key=NULL;
12201229

1221-
initscan(scan,key);
1230+
initscan(scan,key, false);
12221231

12231232
returnscan;
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}
@@ -1240,7 +1249,7 @@ heap_rescan(HeapScanDesc scan,
12401249
/*
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* reinitialize scan descriptor
12421251
*/
1243-
initscan(scan,key);
1252+
initscan(scan,key, true);
12441253
}
12451254

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/* ----------------

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