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

equal keys.  See discussion of today's date in pghackers list.
This commit is contained in:
Tom Lane 2000-08-25 23:13:33 +00:00
parent 3d3ca010aa
commit 40549e9cb5
1 changed files with 33 additions and 10 deletions
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43
src/backend/access/nbtree/nbtinsert.c
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@ -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);
}
/*