Tweak btree insertion to avoid O(N^2) slowdown with large numbers of

equal keys.  See discussion of today's date in pghackers list.

diff --git a/src/backend/access/nbtree/nbtinsert.c b/src/backend/access/nbtree/nbtinsert.c
index f16168eadd22eed3668d62bf9578aeeca619ddef..1dc945d9d962fed04fea5d237d86927244bbb46f 100644 (file)
--- a/src/backend/access/nbtree/nbtinsert.c
+++ b/src/backend/access/nbtree/nbtinsert.c
@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *   $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtinsert.c,v 1.61 2000/07/21 19:21:00 tgl Exp $
+ *   $Header: /cvsroot/pgsql/src/backend/access/nbtree/nbtinsert.c,v 1.62 2000/08/25 23:13:33 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -289,9 +289,11 @@ _bt_check_unique(Relation rel, BTItem btitem, Relation heapRel,
  *     if the new key is equal to the page's "high key" we can place it on
  *     the next page.  If it is equal to the high key, and there's not room
  *     to insert the new tuple on the current page without splitting, then
- *     we move right hoping to find more free space and avoid a split.
- *     Ordinarily, though, we'll insert it before the existing equal keys
- *     because of the way _bt_binsrch() works.
+ *     we can move right hoping to find more free space and avoid a split.
+ *     (We should not move right indefinitely, however, since that leads to
+ *     O(N^2) insertion behavior in the presence of many equal keys.)
+ *     Once we have chosen the page to put the key on, we'll insert it before
+ *     any existing equal keys because of the way _bt_binsrch() works.
  *
  *     The locking interactions in this code are critical.  You should
  *     grok Lehman and Yao's paper before making any changes.  In addition,
@@ -351,15 +353,30 @@ _bt_insertonpg(Relation rel,
    }
    else
    {
-       /*
+       /*----------
         * If we will need to split the page to put the item here,
         * check whether we can put the tuple somewhere to the right,
-        * instead.  Keep scanning until we find enough free space or
-        * reach the last page where the tuple can legally go.
+        * instead.  Keep scanning right until we
+        *      (a) find a page with enough free space,
+        *      (b) reach the last page where the tuple can legally go, or
+        *      (c) get tired of searching.
+        * (c) is not flippant; it is important because if there are many
+        * pages' worth of equal keys, it's better to split one of the early
+        * pages than to scan all the way to the end of the run of equal keys
+        * on every insert.  We implement "get tired" as a random choice,
+        * since stopping after scanning a fixed number of pages wouldn't work
+        * well (we'd never reach the right-hand side of previously split
+        * pages).  Currently the probability of moving right is set at 0.99,
+        * which may seem too high to change the behavior much, but it does an
+        * excellent job of preventing O(N^2) behavior with many equal keys.
+        *----------
         */
+       bool    movedright = false;
+
        while (PageGetFreeSpace(page) < itemsz &&
               !P_RIGHTMOST(lpageop) &&
-              _bt_compare(rel, keysz, scankey, page, P_HIKEY) == 0)
+              _bt_compare(rel, keysz, scankey, page, P_HIKEY) == 0 &&
+              random() > (MAX_RANDOM_VALUE / 100))
        {
            /* step right one page */
            BlockNumber     rblkno = lpageop->btpo_next;
@@ -368,11 +385,17 @@ _bt_insertonpg(Relation rel,
            buf = _bt_getbuf(rel, rblkno, BT_WRITE);
            page = BufferGetPage(buf);
            lpageop = (BTPageOpaque) PageGetSpecialPointer(page);
+           movedright = true;
        }
        /*
-        * This is it, so find the position...
+        * Now we are on the right page, so find the insert position.
+        * If we moved right at all, we know we should insert at the
+        * start of the page, else must find the position by searching.
         */
-       newitemoff = _bt_binsrch(rel, buf, keysz, scankey);
+       if (movedright)
+           newitemoff = P_FIRSTDATAKEY(lpageop);
+       else
+           newitemoff = _bt_binsrch(rel, buf, keysz, scankey);
    }
 
    /*