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.

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 272fb32c58f232456878e20b0a53ed6779b95a21..1f6edf6cafa1f0078b17f357b5eb7ec10a2ab2e1 100644 (file)
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -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);
 }
 
 /* ----------------