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 1.641 2009/06/23 16:40:18 danielk1977 Exp $
**
** This file implements a external (disk-based) database using BTrees.
** See the header comment on "btreeInt.h" for additional information.
** Including a description of file format and an overview of operation.
*/
#include "btreeInt.h"

  ** for any b-tree or overflow pages that pTo now contains the pointers to. */
  if( ISAUTOVACUUM ){
    rc = setChildPtrmaps(pTo);
  }
  return rc;
}

/*
** This routine is called on the root page of a btree when the root
** page contains no cells. This is an opportunity to make the tree
** shallower by one level.
*/
static int balance_shallower(MemPage *pRoot, MemPage *pChild){
  /* The root page is empty but has one child.  Transfer the
  ** information from that one child into the root page if it 
  ** will fit.  This reduces the depth of the tree by one.
  **
  ** If the root page is page 1, it has less space available than
  ** its child (due to the 100 byte header that occurs at the beginning
  ** of the database fle), so it might not be able to hold all of the 
  ** information currently contained in the child.  If this is the 
  ** case, then do not do the transfer.  Leave page 1 empty except
  ** for the right-pointer to the child page.  The child page becomes
  ** the virtual root of the tree.
  */
  int rc = SQLITE_OK;                        /* Return code */
  int const hdr = pRoot->hdrOffset;          /* Offset of root page header */

  assert( sqlite3_mutex_held(pRoot->pBt->mutex) );
  assert( pRoot->nCell==0 );
  assert( pChild->pgno==get4byte(&pRoot->aData[pRoot->hdrOffset+8]) );
  assert( hdr==0 || pRoot->pgno==1 );

  if( pChild->nFree>=hdr ){
    if( hdr ){
      rc = defragmentPage(pChild);
    }
    if( rc==SQLITE_OK ){
      rc = copyNodeContent(pChild, pRoot);
    }
    if( rc==SQLITE_OK ){
      rc = freePage(pChild);
    }
  }

  return rc;
}

/*
** This routine redistributes cells on the iParentIdx'th child of pParent
** (hereafter "the page") and up to 2 siblings so that all pages have about the
** same amount of free space. Usually a single sibling on either side of the
** page are used in the balancing, though both siblings might come from one
** side if the page is the first or last child of its parent. If the page 
** has fewer than 2 siblings (something which can only happen if the page

  assert( nOld>0 );
  assert( nNew>0 );
  if( (pageFlags & PTF_LEAF)==0 ){
    u8 *zChild = &apCopy[nOld-1]->aData[8];
    memcpy(&apNew[nNew-1]->aData[8], zChild, 4);
  }
























  /* Fix the pointer-map entries for all the cells that were shifted around. 
  ** There are several different types of pointer-map entries that need to
  ** be dealt with by this routine. Some of these have been set already, but
  ** many have not. The following is a summary:
  **
  **   1) The entries associated with new sibling pages that were not
  **      siblings when this function was called. These have already
  **      been set. We don't need to worry about old siblings that were
  **      moved to the free-list - the freePage() code has taken care
  **      of those.
  **
  **   2) The pointer-map entries associated with the first overflow
  **      page in any overflow chains used by new divider cells. These 
  **      have also already been taken care of by the insertCell() code.
  **
  **   3) If the sibling pages are not leaves, then the child pages of
  **      cells stored on the sibling pages may need to be updated.
  **
  **   4) If the sibling pages are not internal intkey nodes, then any
  **      overflow pages used by these cells may need to be updated
  **      (internal intkey nodes never contain pointers to overflow pages).
  **
  **   5) If the sibling pages are not leaves, then the pointer-map
  **      entries for the right-child pages of each sibling may need
  **      to be updated.
  **
  ** Cases 1 and 2 are dealt with above by other code. The following
  ** block deals with cases 3 and 4. Since setting a pointer map entry
  ** is a relatively expensive operation, this code only sets pointer
  ** map entries for child or overflow pages that have actually moved
  ** between pages.  */
  if( ISAUTOVACUUM ){
    MemPage *pNew = apNew[0];
    MemPage *pOld = apCopy[0];
    int nOverflow = pOld->nOverflow;
    int iNextOld = pOld->nCell + nOverflow;
    int iOverflow = (nOverflow ? pOld->aOvfl[0].idx : -1);
    j = 0;                             /* Current 'old' sibling page */
    k = 0;                             /* Current 'new' sibling page */

#endif
  }

  assert( pParent->isInit );
  TRACE(("BALANCE: finished: old=%d new=%d cells=%d\n",
          nOld, nNew, nCell));

  if( rc==SQLITE_OK && pParent->nCell==0 && isRoot ){
    /* The root page of the b-tree now contains no cells. If the root-page 
    ** is not also a leaf page, it will have a single child page. Call 
    ** balance_shallower to attempt to copy the contents of the single
    ** child-page into the root page (this may not be possible if the
    ** root page is page 1).  */ 
    assert( nNew==1 );
    rc = balance_shallower(pParent, apNew[0]);
  }
 
  /*
  ** Cleanup before returning.
  */
balance_cleanup:
  sqlite3ScratchFree(apCell);
  for(i=0; i<nOld; i++){
    releasePage(apOld[i]);

** some way. This function figures out if this modification means the
** tree needs to be balanced, and if so calls the appropriate balancing 
** routine. Balancing routines are:
**
**   balance_quick()
**   balance_deeper()
**   balance_nonroot()
**
** If built with SQLITE_DEBUG, pCur->pagesShuffled is set to true if 
** balance_shallower(), balance_deeper() or balance_nonroot() is called.
** If none of these functions are invoked, pCur->pagesShuffled is left
** unmodified.
*/
static int balance(BtCursor *pCur){
  int rc = SQLITE_OK;
  const int nMin = pCur->pBt->usableSize * 2 / 3;
  u8 aBalanceQuickSpace[13];
  u8 *pFree = 0;

        rc = balance_deeper(pPage, &pCur->apPage[1]);
        if( rc==SQLITE_OK ){
          pCur->iPage = 1;
          pCur->aiIdx[0] = 0;
          pCur->aiIdx[1] = 0;
          assert( pCur->apPage[1]->nOverflow );
        }
        VVA_ONLY( pCur->pagesShuffled = 1 );
      }else{
        break;
      }
    }else if( pPage->nOverflow==0 && pPage->nFree<=nMin ){
      break;
    }else{
      MemPage * const pParent = pCur->apPage[iPage-1];

            sqlite3PageFree(pFree);
          }

          /* The pSpace buffer will be freed after the next call to
          ** balance_nonroot(), or just before this function returns, whichever
          ** comes first. */
          pFree = pSpace;
          VVA_ONLY( pCur->pagesShuffled = 1 );
        }
      }

      pPage->nOverflow = 0;

      /* The next iteration of the do-loop balances the parent page. */
      releasePage(pPage);

/*
** 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: btreeInt.h,v 1.48 2009/06/22 12:05:10 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.

  void *pKey;      /* Saved key that was cursor's last known position */
  i64 nKey;        /* Size of pKey, or last integer key */
  int skip;        /* (skip<0) -> Prev() is a no-op. (skip>0) -> Next() is */
#ifndef SQLITE_OMIT_INCRBLOB
  u8 isIncrblobHandle;      /* True if this cursor is an incr. io handle */
  Pgno *aOverflow;          /* Cache of overflow page locations */
#endif
#ifndef NDEBUG
  u8 pagesShuffled;         /* True if Btree pages are rearranged by balance()*/
#endif
  i16 iPage;                            /* Index of current page in apPage */
  MemPage *apPage[BTCURSOR_MAX_DEPTH];  /* Pages from root to current page */
  u16 aiIdx[BTCURSOR_MAX_DEPTH];        /* Current index in apPage[i] */
};

/*
** Potential values for BtCursor.eState.