Fix longstanding error in _bt_search(): should moveright at top of loop not

bottom.  Otherwise we fail to moveright when the root page was split while
we were "in flight" to it.  This is not a significant problem when the root
is above the leaf level, but if the root was also a leaf (ie, a single-page
index just got split) we may return the wrong leaf page to the caller,
resulting in failure to find a key that is in fact present.  Bug has existed
at least since 7.1, probably forever.

diff --git a/src/backend/access/nbtree/nbtsearch.c b/src/backend/access/nbtree/nbtsearch.c
index 56367d2ab35ae270f5f91f622e20f098aa0c135f..457914adf73ffb2a1dceda51f8d4dd7f505fb277 100644 (file)
--- a/src/backend/access/nbtree/nbtsearch.c
+++ b/src/backend/access/nbtree/nbtsearch.c
@@ -8,7 +8,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *       $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.76 2003/07/28 00:09:14 tgl Exp $
+ *       $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtsearch.c,v 1.77 2003/07/29 22:18:38 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -62,6 +62,13 @@ _bt_search(Relation rel, int keysz, ScanKey scankey,
                BlockNumber par_blkno;
                BTStack         new_stack;
 
+               /*
+                * Race -- the page we just grabbed may have split since we read
+                * its pointer in the parent (or metapage).  If it has, we may need
+                * to move right to its new sibling.  Do that.
+                */
+               *bufP = _bt_moveright(rel, *bufP, keysz, scankey, BT_READ);
+
                /* if this is a leaf page, we're done */
                page = BufferGetPage(*bufP);
                opaque = (BTPageOpaque) PageGetSpecialPointer(page);
@@ -99,13 +106,6 @@ _bt_search(Relation rel, int keysz, ScanKey scankey,
                _bt_relbuf(rel, *bufP);
                *bufP = _bt_getbuf(rel, blkno, BT_READ);
 
-               /*
-                * Race -- the page we just grabbed may have split since we read
-                * its pointer in the parent.  If it has, we may need to move
-                * right to its new sibling.  Do that.
-                */
-               *bufP = _bt_moveright(rel, *bufP, keysz, scankey, BT_READ);
-
                /* okay, all set to move down a level */
                stack_in = new_stack;
        }
@@ -599,8 +599,8 @@ _bt_first(IndexScanDesc scan, ScanDirection dir)
        /*
         * At this point we are positioned at the first item >= scan key, or
         * possibly at the end of a page on which all the existing items are
-        * greater than the scan key and we know that everything on later
-        * pages is less than or equal to scan key.
+        * less than the scan key and we know that everything on later
+        * pages is greater than or equal to scan key.
         *
         * We could step forward in the latter case, but that'd be a waste of
         * time if we want to scan backwards.  So, it's now time to examine