Tweak indexscan and seqscan code to arrange that steps from one page to

the next are handled by ReleaseAndReadBuffer rather than separate
ReleaseBuffer and ReadBuffer calls.  This cuts the number of acquisitions
of the BufMgrLock by a factor of 2 (possibly more, if an indexscan happens
to pull successive rows from the same heap page).  Unfortunately this
doesn't seem enough to get us out of the recently discussed context-switch
storm problem, but it's surely worth doing anyway.

1 files changed, 20 insertions, 14 deletions

diff --git a/src/backend/access/index/indexam.c b/src/backend/access/index/indexam.c
index 8960220ef09..ede95a275ec 100644
--- a/src/backend/access/index/indexam.c
+++ b/src/backend/access/index/indexam.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/index/indexam.c,v 1.72 2003/11/29 19:51:40 pgsql Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/index/indexam.c,v 1.73 2004/04/21 18:24:24 tgl Exp $
  *
  * INTERFACE ROUTINES
  *		index_open		- open an index relation by relation OID
@@ -316,6 +316,13 @@ index_rescan(IndexScanDesc scan, ScanKey key)
 	SCAN_CHECKS;
 	GET_SCAN_PROCEDURE(rescan, amrescan);
 
+	/* Release any held pin on a heap page */
+	if (BufferIsValid(scan->xs_cbuf))
+	{
+		ReleaseBuffer(scan->xs_cbuf);
+		scan->xs_cbuf = InvalidBuffer;
+	}
+
 	scan->kill_prior_tuple = false;		/* for safety */
 	scan->keys_are_unique = false;		/* may be set by index AM */
 	scan->got_tuple = false;
@@ -419,13 +426,6 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
 
 	SCAN_CHECKS;
 
-	/* Release any previously held pin */
-	if (BufferIsValid(scan->xs_cbuf))
-	{
-		ReleaseBuffer(scan->xs_cbuf);
-		scan->xs_cbuf = InvalidBuffer;
-	}
-
 	/*
 	 * If we already got a tuple and it must be unique, there's no need to
 	 * make the index AM look through any additional tuples.  (This can
@@ -508,14 +508,22 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
 		scan->kill_prior_tuple = false;
 
 		if (!found)
+		{
+			/* Release any held pin on a heap page */
+			if (BufferIsValid(scan->xs_cbuf))
+			{
+				ReleaseBuffer(scan->xs_cbuf);
+				scan->xs_cbuf = InvalidBuffer;
+			}
 			return NULL;		/* failure exit */
+		}
 
 		/*
 		 * Fetch the heap tuple and see if it matches the snapshot.
 		 */
-		if (heap_fetch(scan->heapRelation, scan->xs_snapshot,
-					   heapTuple, &scan->xs_cbuf, true,
-					   &scan->xs_pgstat_info))
+		if (heap_release_fetch(scan->heapRelation, scan->xs_snapshot,
+							   heapTuple, &scan->xs_cbuf, true,
+							   &scan->xs_pgstat_info))
 			break;
 
 		/* Skip if no tuple at this location */
@@ -527,7 +535,7 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
 		 * if the tuple is dead to all transactions.  If so, signal the
 		 * index AM to not return it on future indexscans.
 		 *
-		 * We told heap_fetch to keep a pin on the buffer, so we can
+		 * We told heap_release_fetch to keep a pin on the buffer, so we can
 		 * re-access the tuple here.  But we must re-lock the buffer
 		 * first. Also, it's just barely possible for an update of hint
 		 * bits to occur here.
@@ -542,8 +550,6 @@ index_getnext(IndexScanDesc scan, ScanDirection direction)
 		if (sv_infomask != heapTuple->t_data->t_infomask)
 			SetBufferCommitInfoNeedsSave(scan->xs_cbuf);
 		LockBuffer(scan->xs_cbuf, BUFFER_LOCK_UNLOCK);
-		ReleaseBuffer(scan->xs_cbuf);
-		scan->xs_cbuf = InvalidBuffer;
 	}
 
 	/* Success exit */