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Overview
Comment:Remove an unnecessary conditional, replacing it with an assert(). Improvements to comments.
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SHA1: 834bf1c367e1ccd498c7f9f843be2d2aa11ffb3c
User & Date: drh 2014-04-01 01:24:34
Context
2014-04-04
18:49
Performance improvements when reading large blobs, especially if SQLITE_DIRECT_OVERFLOW_READ is defined. check-in: 2312eb6a user: drh tags: trunk
2014-04-01
01:24
Remove an unnecessary conditional, replacing it with an assert(). Improvements to comments. Closed-Leaf check-in: 834bf1c3 user: drh tags: overflow-pgno-cache
2014-03-31
23:57
Fix a compiler warning when SQLITE_DIRECT_OVERFLOW_READ is defined. Minor performance enhancement and size reduction. check-in: 96385dc4 user: drh tags: overflow-pgno-cache
Changes
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Changes to src/btree.c.

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  int bEnd;                                   /* True if reading to end of data */
#endif

  assert( pPage );
  assert( pCur->eState==CURSOR_VALID );
  assert( pCur->aiIdx[pCur->iPage]<pPage->nCell );
  assert( cursorHoldsMutex(pCur) );


  getCellInfo(pCur);
  aPayload = pCur->info.pCell + pCur->info.nHeader;
  nKey = (pPage->intKey ? 0 : (int)pCur->info.nKey);
#ifdef SQLITE_DIRECT_OVERFLOW_READ
  bEnd = (offset+amt==nKey+pCur->info.nData);
#endif
................................................................................

  if( rc==SQLITE_OK && amt>0 ){
    const u32 ovflSize = pBt->usableSize - 4;  /* Bytes content per ovfl page */
    Pgno nextPage;

    nextPage = get4byte(&aPayload[pCur->info.nLocal]);

    /* If the BTCF_Incrblob flag is set and the BtCursor.aOverflow[]
    ** has not been allocated, allocate it now. The array is sized at


    ** one entry for each overflow page in the overflow chain. The
    ** page number of the first overflow page is stored in aOverflow[0],
    ** etc. A value of 0 in the aOverflow[] array means "not yet known"
    ** (the cache is lazily populated).
    */
    if( eOp!=2 && (pCur->curFlags & BTCF_ValidOvfl)==0 ){
      int nOvfl = (pCur->info.nPayload-pCur->info.nLocal+ovflSize-1)/ovflSize;
      if( nOvfl>pCur->nOvflAlloc ){
        Pgno *aNew = (Pgno*)sqlite3DbRealloc(
            pCur->pBtree->db, pCur->aOverflow, nOvfl*2*sizeof(Pgno)
        );
................................................................................

      if( offset>=ovflSize ){
        /* The only reason to read this page is to obtain the page
        ** number for the next page in the overflow chain. The page
        ** data is not required. So first try to lookup the overflow
        ** page-list cache, if any, then fall back to the getOverflowPage()
        ** function.



        */

        if( (pCur->curFlags & BTCF_ValidOvfl)!=0 && pCur->aOverflow[iIdx+1] ){

          nextPage = pCur->aOverflow[iIdx+1];
        } else 
          rc = getOverflowPage(pBt, nextPage, 0, &nextPage);

        offset -= ovflSize;
      }else{
        /* Need to read this page properly. It contains some of the
        ** range of data that is being read (eOp==0) or written (eOp!=0).
        */
#ifdef SQLITE_DIRECT_OVERFLOW_READ
        sqlite3_file *fd;







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  int bEnd;                                   /* True if reading to end of data */
#endif

  assert( pPage );
  assert( pCur->eState==CURSOR_VALID );
  assert( pCur->aiIdx[pCur->iPage]<pPage->nCell );
  assert( cursorHoldsMutex(pCur) );
  assert( eOp!=2 || offset==0 );      /* Always start from beginning for eOp==2 */

  getCellInfo(pCur);
  aPayload = pCur->info.pCell + pCur->info.nHeader;
  nKey = (pPage->intKey ? 0 : (int)pCur->info.nKey);
#ifdef SQLITE_DIRECT_OVERFLOW_READ
  bEnd = (offset+amt==nKey+pCur->info.nData);
#endif
................................................................................

  if( rc==SQLITE_OK && amt>0 ){
    const u32 ovflSize = pBt->usableSize - 4;  /* Bytes content per ovfl page */
    Pgno nextPage;

    nextPage = get4byte(&aPayload[pCur->info.nLocal]);


    /* If the BtCursor.aOverflow[] has not been allocated, allocate it now.
    ** Except, do not allocate aOverflow[] for eOp==2.
    **
    ** The aOverflow[] array is sized at one entry for each overflow page
    ** in the overflow chain. The page number of the first overflow page is
    ** stored in aOverflow[0], etc. A value of 0 in the aOverflow[] array
    ** means "not yet known" (the cache is lazily populated).
    */
    if( eOp!=2 && (pCur->curFlags & BTCF_ValidOvfl)==0 ){
      int nOvfl = (pCur->info.nPayload-pCur->info.nLocal+ovflSize-1)/ovflSize;
      if( nOvfl>pCur->nOvflAlloc ){
        Pgno *aNew = (Pgno*)sqlite3DbRealloc(
            pCur->pBtree->db, pCur->aOverflow, nOvfl*2*sizeof(Pgno)
        );
................................................................................

      if( offset>=ovflSize ){
        /* The only reason to read this page is to obtain the page
        ** number for the next page in the overflow chain. The page
        ** data is not required. So first try to lookup the overflow
        ** page-list cache, if any, then fall back to the getOverflowPage()
        ** function.
        **
        ** Note that the aOverflow[] array must be allocated because eOp!=2
        ** here.  If eOp==2, then offset==0 and this branch is never taken.
        */
        assert( eOp!=2 );
        assert( pCur->curFlags & BTCF_ValidOvfl );
        if( pCur->aOverflow[iIdx+1] ){
          nextPage = pCur->aOverflow[iIdx+1];
        }else{
          rc = getOverflowPage(pBt, nextPage, 0, &nextPage);
        }
        offset -= ovflSize;
      }else{
        /* Need to read this page properly. It contains some of the
        ** range of data that is being read (eOp==0) or written (eOp!=0).
        */
#ifdef SQLITE_DIRECT_OVERFLOW_READ
        sqlite3_file *fd;