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Overview
Comment:In the balance_nonroot() routine, protect the values in aPgno[] array from change during the page sort, so that aPgno[] can be used to avoid unnecessary pointer-map updates for auto_vacuum databases.
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SHA1:69c3924fe834a78d4a8d86833626bf5f68e33a3a
User & Date: drh 2014-10-31 00:05:23
Context
2014-10-31
02:51
Remove an unnecessary branch from balance_nonroot(). check-in: 9fc7c88e user: drh tags: trunk
00:05
In the balance_nonroot() routine, protect the values in aPgno[] array from change during the page sort, so that aPgno[] can be used to avoid unnecessary pointer-map updates for auto_vacuum databases. check-in: 69c3924f user: drh tags: trunk
2014-10-30
23:14
Improvements to the wording of some comments. Reinstate an assert() that is only true for non-corrupt database files by adding an "|| CORRUPT_DB" term. check-in: 67adb448 user: drh tags: trunk
Changes
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Changes to src/btree.c.

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  int szNew[NB+2];             /* Combined size of cells placed on i-th page */
  u8 **apCell = 0;             /* All cells begin balanced */
  u16 *szCell;                 /* Local size of all cells in apCell[] */
  u8 *aSpace1;                 /* Space for copies of dividers cells */
  Pgno pgno;                   /* Temp var to store a page number in */
  u8 abDone[NB+2];             /* True after i'th new page is populated */
  Pgno aPgno[NB+2];            /* Page numbers of new pages before shuffling */

  u16 aPgFlags[NB+2];          /* flags field of new pages before shuffling */

  memset(abDone, 0, sizeof(abDone));
  pBt = pParent->pBt;
  assert( sqlite3_mutex_held(pBt->mutex) );
  assert( sqlite3PagerIswriteable(pParent->pDbPage) );

................................................................................
  ** An O(n^2) insertion sort algorithm is used, but since n is never more 
  ** than (NB+2) (a small constant), that should not be a problem.
  **
  ** When NB==3, this one optimization makes the database about 25% faster 
  ** for large insertions and deletions.
  */
  for(i=0; i<nNew; i++){
    aPgno[i] = apNew[i]->pgno;
    aPgFlags[i] = apNew[i]->pDbPage->flags;
    for(j=0; j<i; j++){
      if( aPgno[j]==aPgno[i] ){
        /* This branch is taken if the set of sibling pages somehow contains
        ** duplicate entries. This can happen if the database is corrupt. 
        ** It would be simpler to detect this as part of the loop below, but
        ** we do the detection here in order to avoid populating the pager
................................................................................
      }
    }
  }
  for(i=0; i<nNew; i++){
    int iBest = 0;                /* aPgno[] index of page number to use */
    Pgno pgno;                    /* Page number to use */
    for(j=1; j<nNew; j++){
      if( aPgno[j]<aPgno[iBest] ) iBest = j;
    }
    pgno = aPgno[iBest];
    aPgno[iBest] = 0xffffffff;
    if( iBest!=i ){
      if( iBest>i ){
        sqlite3PagerRekey(apNew[iBest]->pDbPage, pBt->nPage+iBest+1, 0);
      }
      sqlite3PagerRekey(apNew[i]->pDbPage, pgno, aPgFlags[iBest]);
      apNew[i]->pgno = pgno;
    }







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  int szNew[NB+2];             /* Combined size of cells placed on i-th page */
  u8 **apCell = 0;             /* All cells begin balanced */
  u16 *szCell;                 /* Local size of all cells in apCell[] */
  u8 *aSpace1;                 /* Space for copies of dividers cells */
  Pgno pgno;                   /* Temp var to store a page number in */
  u8 abDone[NB+2];             /* True after i'th new page is populated */
  Pgno aPgno[NB+2];            /* Page numbers of new pages before shuffling */
  Pgno aPgOrder[NB+2];         /* Copy of aPgno[] used for sorting pages */
  u16 aPgFlags[NB+2];          /* flags field of new pages before shuffling */

  memset(abDone, 0, sizeof(abDone));
  pBt = pParent->pBt;
  assert( sqlite3_mutex_held(pBt->mutex) );
  assert( sqlite3PagerIswriteable(pParent->pDbPage) );

................................................................................
  ** An O(n^2) insertion sort algorithm is used, but since n is never more 
  ** than (NB+2) (a small constant), that should not be a problem.
  **
  ** When NB==3, this one optimization makes the database about 25% faster 
  ** for large insertions and deletions.
  */
  for(i=0; i<nNew; i++){
    aPgOrder[i] = aPgno[i] = apNew[i]->pgno;
    aPgFlags[i] = apNew[i]->pDbPage->flags;
    for(j=0; j<i; j++){
      if( aPgno[j]==aPgno[i] ){
        /* This branch is taken if the set of sibling pages somehow contains
        ** duplicate entries. This can happen if the database is corrupt. 
        ** It would be simpler to detect this as part of the loop below, but
        ** we do the detection here in order to avoid populating the pager
................................................................................
      }
    }
  }
  for(i=0; i<nNew; i++){
    int iBest = 0;                /* aPgno[] index of page number to use */
    Pgno pgno;                    /* Page number to use */
    for(j=1; j<nNew; j++){
      if( aPgOrder[j]<aPgOrder[iBest] ) iBest = j;
    }
    pgno = aPgOrder[iBest];
    aPgOrder[iBest] = 0xffffffff;
    if( iBest!=i ){
      if( iBest>i ){
        sqlite3PagerRekey(apNew[iBest]->pDbPage, pBt->nPage+iBest+1, 0);
      }
      sqlite3PagerRekey(apNew[i]->pDbPage, pgno, aPgFlags[iBest]);
      apNew[i]->pgno = pgno;
    }