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 1.639 2009/06/23 11:22:29 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"

      assert( n==pPage->pgno && e==PTRMAP_BTREE );
    }
  }
  return 1;
}
#endif




























































































/*
** 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 

**
** If aOvflSpace is set to a null pointer, this function returns 
** SQLITE_NOMEM.
*/
static int balance_nonroot(
  MemPage *pParent,               /* Parent page of siblings being balanced */
  int iParentIdx,                 /* Index of "the page" in pParent */
  u8 *aOvflSpace                  /* page-size bytes of space for parent ovfl */

){
  BtShared *pBt;               /* The whole database */
  int nCell = 0;               /* Number of cells in apCell[] */
  int nMaxCells = 0;           /* Allocated size of apCell, szCell, aFrom. */
  int nNew = 0;                /* Number of pages in apNew[] */
  int nOld;                    /* Number of pages in apOld[] */
  int i, j, k;                 /* Loop counters */

    ptrmapCheckPages(&pParent, 1);
#endif
  }

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










 
  /*
  ** Cleanup before returning.
  */
balance_cleanup:
  sqlite3ScratchFree(apCell);
  for(i=0; i<nOld; i++){
    releasePage(apOld[i]);
  }
  for(i=0; i<nNew; i++){
    releasePage(apNew[i]);
  }

  return rc;
}

/*
** This function is used to copy the contents of the b-tree node stored 
** on page pFrom to page pTo. If page pFrom was not a leaf page, then
** the pointer-map entries for each child page are updated so that the
** parent page stored in the pointer map is page pTo. If pFrom contained
** any cells with overflow page pointers, then the corresponding pointer
** map entries are also updated so that the parent page is page pTo.
**
** If pFrom is currently carrying any overflow cells (entries in the
** MemPage.aOvfl[] array), they are not copied to pTo. 
**
** Before returning, page pTo is reinitialized using sqlite3BtreeInitPage().
**
** The performance of this function is not critical. It is only used by 
** the balance_shallower() and balance_deeper() procedures, neither of
** which are called often under normal circumstances.
*/
static int copyNodeContent(MemPage *pFrom, MemPage *pTo){
  BtShared * const pBt = pFrom->pBt;
  u8 * const aFrom = pFrom->aData;
  u8 * const aTo = pTo->aData;
  int const iFromHdr = pFrom->hdrOffset;
  int const iToHdr = ((pTo->pgno==1) ? 100 : 0);
  int rc = SQLITE_OK;
  int iData;

  assert( pFrom->isInit );
  assert( pFrom->nFree>=iToHdr );
  assert( get2byte(&aFrom[iFromHdr+5])<=pBt->usableSize );

  /* Copy the b-tree node content from page pFrom to page pTo. */
  iData = get2byte(&aFrom[iFromHdr+5]);
  memcpy(&aTo[iData], &aFrom[iData], pBt->usableSize-iData);
  memcpy(&aTo[iToHdr], &aFrom[iFromHdr], pFrom->cellOffset + 2*pFrom->nCell);

  /* Reinitialize page pTo so that the contents of the MemPage structure
  ** match the new data. The initialization of pTo "cannot" fail, as the
  ** data copied from pFrom is known to be valid.  */
  pTo->isInit = 0;
  TESTONLY(rc = ) sqlite3BtreeInitPage(pTo);
  assert( rc==SQLITE_OK );

  /* If this is an auto-vacuum database, update the pointer-map entries
  ** 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){
  /* 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 */
  MemPage *pChild;                           /* Only child of pRoot */
  Pgno const pgnoChild = get4byte(&pRoot->aData[pRoot->hdrOffset+8]);
  
  assert( pRoot->nCell==0 );
  assert( sqlite3_mutex_held(pRoot->pBt->mutex) );
  assert( !pRoot->leaf );
  assert( pgnoChild>0 );
  assert( pgnoChild<=pagerPagecount(pRoot->pBt) );
  assert( hdr==0 || pRoot->pgno==1 );
  
  rc = sqlite3BtreeGetPage(pRoot->pBt, pgnoChild, &pChild, 0);
  if( rc==SQLITE_OK ){
    if( pChild->nFree>=hdr ){
      if( hdr ){
        rc = defragmentPage(pChild);
      }
      if( rc==SQLITE_OK ){
        rc = copyNodeContent(pChild, pRoot);
      }
      if( rc==SQLITE_OK ){
        rc = freePage(pChild);
      }
    }else{
      /* The child has more information that will fit on the root.
      ** The tree is already balanced.  Do nothing. */
      TRACE(("BALANCE: child %d will not fit on page 1\n", pChild->pgno));
    }
    releasePage(pChild);
  }

  return rc;
}


/*
** This function is called when the root page of a b-tree structure is
** overfull (has one or more overflow pages).

/*
** The page that pCur currently points to has just been modified in
** 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_shallower()
**   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.

          pCur->iPage = 1;
          pCur->aiIdx[0] = 0;
          pCur->aiIdx[1] = 0;
          assert( pCur->apPage[1]->nOverflow );
        }
        VVA_ONLY( pCur->pagesShuffled = 1 );
      }else{
        /* The root page of the b-tree is now empty. 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).
        **
        ** Whether or not this is possible , the tree is now balanced. 
        ** Therefore is no next iteration of the do-loop.
        */ 
        if( pPage->nCell==0 && !pPage->leaf ){
          rc = balance_shallower(pPage);
          VVA_ONLY( pCur->pagesShuffled = 1 );
        }
        break;
      }
    }else if( pPage->nOverflow==0 && pPage->nFree<=nMin ){
      break;
    }else{
      MemPage * const pParent = pCur->apPage[iPage-1];
      int const iIdx = pCur->aiIdx[iPage-1];

          ** different page). Once this subsequent call to balance_nonroot() 
          ** has completed, it is safe to release the pSpace buffer used by
          ** the previous call, as the overflow cell data will have been 
          ** copied either into the body of a database page or into the new
          ** pSpace buffer passed to the latter call to balance_nonroot().
          */
          u8 *pSpace = sqlite3PageMalloc(pCur->pBt->pageSize);
          rc = balance_nonroot(pParent, iIdx, pSpace);
          if( pFree ){
            /* If pFree is not NULL, it points to the pSpace buffer used 
            ** by a previous call to balance_nonroot(). Its contents are
            ** now stored either on real database pages or within the 
            ** new pSpace buffer, so it may be safely freed here. */
            sqlite3PageFree(pFree);
          }