Avoid unnecessary page-level SSI lock check in heap_insert().

As observed by Heikki, we need not conflict on heap page locks during an
insert; heap page locks are only aggregated tuple locks, they don't imply
locking "gaps" as index page locks do.  So we can avoid some unnecessary
conflicts, and also do the SSI check while not holding exclusive lock on
the target buffer.

Kevin Grittner, reviewed by Jeff Davis.  Back-patch to 9.1.

diff --git a/src/backend/access/heap/heapam.c b/src/backend/access/heap/heapam.c
index 450bb15ff8d7a525e1d5b0737f6f9149b19fa324..cc8b45fd489410e2b1e6e16b5235bd7378949eb0 100644 (file)
--- a/src/backend/access/heap/heapam.c
+++ b/src/backend/access/heap/heapam.c
@@ -1921,17 +1921,22 @@ heap_insert(Relation relation, HeapTuple tup, CommandId cid,
        else
                heaptup = tup;
 
+       /*
+        * We're about to do the actual insert -- but check for conflict first,
+        * to avoid possibly having to roll back work we've just done.
+        *
+        * For a heap insert, we only need to check for table-level SSI locks.
+        * Our new tuple can't possibly conflict with existing tuple locks, and
+        * heap page locks are only consolidated versions of tuple locks; they do
+        * not lock "gaps" as index page locks do.  So we don't need to identify
+        * a buffer before making the call.
+        */
+       CheckForSerializableConflictIn(relation, NULL, InvalidBuffer);
+
        /* Find buffer to insert this tuple into */
        buffer = RelationGetBufferForTuple(relation, heaptup->t_len,
                                                                           InvalidBuffer, options, bistate);
 
-       /*
-        * We're about to do the actual insert -- check for conflict at the
-        * relation or buffer level first, to avoid possibly having to roll back
-        * work we've just done.
-        */
-       CheckForSerializableConflictIn(relation, NULL, buffer);
-
        /* NO EREPORT(ERROR) from here till changes are logged */
        START_CRIT_SECTION();