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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 surprising
behavior of scrollable and WITH HOLD cursors, as seen in Mark Kirkwood's bug
report of yesterday.

It's not entirely clear whether a rescan should be forced to drop out of the
syncscan mode, but for the moment I left the code behaving the same on that
point.  Any change there would only be a performance and not a correctness
issue, anyway.

Back-patch to 8.3, since the unstable behavior was created by the syncscan
patch.
This commit is contained in:
Tom Lane 2009-06-10 18:54:16 +00:00
parent 2ef8c1acfd
commit 61dd4185ff
1 changed files with 14 additions and 5 deletions
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19
src/backend/access/heap/heapam.c
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@ -8,7 +8,7 @@
*
*
* IDENTIFICATION
* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.275 2009/05/12 16:43:32 tgl Exp $
* $PostgreSQL: pgsql/src/backend/access/heap/heapam.c,v 1.276 2009/06/10 18:54:16 tgl Exp $
*
*
* INTERFACE ROUTINES
@ -93,7 +93,7 @@ static bool HeapSatisfiesHOTUpdate(Relation relation, Bitmapset *hot_attrs,
* ----------------
*/
static void
initscan(HeapScanDesc scan, ScanKey key)
initscan(HeapScanDesc scan, ScanKey key, bool is_rescan)
{
bool allow_strat;
bool allow_sync;
@ -143,7 +143,16 @@ initscan(HeapScanDesc scan, ScanKey key)
scan->rs_strategy = NULL;
}
if (allow_sync && synchronize_seqscans)
if (is_rescan)
{
/*
* If rescan, keep the previous startblock setting so that rewinding
* a cursor doesn't generate surprising results. Reset the syncscan
* setting, though.
*/
scan->rs_syncscan = (allow_sync && synchronize_seqscans);
}
else if (allow_sync && synchronize_seqscans)
{
scan->rs_syncscan = true;
scan->rs_startblock = ss_get_location(scan->rs_rd, scan->rs_nblocks);
@ -1218,7 +1227,7 @@ heap_beginscan_internal(Relation relation, Snapshot snapshot,
else
scan->rs_key = NULL;
initscan(scan, key);
initscan(scan, key, false);
return scan;
}
@ -1240,7 +1249,7 @@ heap_rescan(HeapScanDesc scan,
/*
* reinitialize scan descriptor
*/
initscan(scan, key);
initscan(scan, key, true);
}
/* ----------------