SQLite

/*
** 2004 April 6
**
** The author disclaims copyright to this source code.  In place of
** a legal notice, here is a blessing:
**
**    May you do good and not evil.
**    May you find forgiveness for yourself and forgive others.
**    May you share freely, never taking more than you give.
**
*************************************************************************
** $Id: btree.c,v 1.233 2005/01/14 22:55:49 drh Exp $
**
** This file implements a external (disk-based) database using BTrees.
** For a detailed discussion of BTrees, refer to
**
**     Donald E. Knuth, THE ART OF COMPUTER PROGRAMMING, Volume 3:
**     "Sorting And Searching", pages 473-480. Addison-Wesley
**     Publishing Company, Reading, Massachusetts.

*/
#define NN 1             /* Number of neighbors on either side of pPage */
#define NB (NN*2+1)      /* Total pages involved in the balance */

/* Forward reference */
static int balance(MemPage*, int);

/*
** This version of balance() handles the common special case where
** a new entry is being inserted on the extreme right-end of the
** tree, in other words, when the new entry will become the largest
** entry in the tree.
**
** Instead of trying balance the 3 right-most leaf pages, just add
** a new page to the right-hand side and put the one new entry in
** that page.  This leaves the right side of the tree somewhat
** unbalanced.  But odds are that we will be inserting new entries
** at the end soon afterwards so the nearly empty page will quickly
** fill up.  On average.
**
** pPage is the leaf page which is the right-most page in the tree.
** pParent is its parent.  pPage must have a single overflow entry
** which is also the right-most entry on the page.
*/
static int balance_quick(MemPage *pPage, MemPage *pParent){
  int rc;
  MemPage *pNew;
  Pgno pgnoNew;
  u8 *pCell;
  int szCell;
  CellInfo info;

  pBt = pPage->pBt;
  pParent = pPage->pParent;
  sqlite3pager_write(pParent->aData);
  assert( pParent );
  TRACE(("BALANCE: begin page %d child of %d\n", pPage->pgno, pParent->pgno));

#ifdef SQLITE_BALANCE_QUICK
  /*
  ** A special case:  If a new entry has just been inserted into a
  ** table (that is, a btree with integer keys and all data at the leaves)
  ** an the new entry is the right-most entry in the tree (it has the
  ** largest key) then use the special balance_quick() routine for
  ** balancing.  balance_quick() is much faster and results in a tighter
  ** packing of data in the common case.
  */
  if( pPage->leaf &&
      pPage->intKey &&
      pPage->leafData &&
      pPage->nOverflow==1 &&
      pPage->aOvfl[0].idx==pPage->nCell &&
      get4byte(&pParent->aData[pParent->hdrOffset+8])==pPage->pgno
  ){