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Overview
Comment:The sqlite3BtreeInsert() routine tries to overwrite an existing cell with modified content if the new content is the same size. Pages are only dirtied if they change. This prototype works some, but still has issues.
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SHA3-256:489451b378819621537231c1c8a07704437e11c1f5384fd53b09f3977d2213a4
User & Date: drh 2018-05-03 03:59:02
Context
2018-05-03
12:57
Bug fixes in the overwrite optimization. check-in: 0cb6cd2a user: drh tags: cell-overwrite-prototype
03:59
The sqlite3BtreeInsert() routine tries to overwrite an existing cell with modified content if the new content is the same size. Pages are only dirtied if they change. This prototype works some, but still has issues. check-in: 489451b3 user: drh tags: cell-overwrite-prototype
2018-05-02
15:00
Prevent VACUUM from running any commands in sqlite_master.sql other than CREATE statements. That is all that should be there anyhow. This fixes a problem discovered by OSSFuzz. Test cases in TH3. check-in: ab0d99d0 user: drh tags: trunk
Changes
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Changes to src/btree.c.

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  if( pFree ){
    sqlite3PageFree(pFree);
  }
  return rc;
}















































































/*
** Insert a new record into the BTree.  The content of the new record
** is described by the pX object.  The pCur cursor is used only to
** define what table the record should be inserted into, and is left
** pointing at a random location.
**
................................................................................
    assert( (flags & BTREE_SAVEPOSITION)==0 || 
            ((pCur->curFlags&BTCF_ValidNKey)!=0 && pX->nKey==pCur->info.nKey) );

    /* If the cursor is currently on the last row and we are appending a
    ** new row onto the end, set the "loc" to avoid an unnecessary
    ** btreeMoveto() call */
    if( (pCur->curFlags&BTCF_ValidNKey)!=0 && pX->nKey==pCur->info.nKey ){





      loc = 0;
    }else if( loc==0 ){
      rc = sqlite3BtreeMovetoUnpacked(pCur, 0, pX->nKey, flags!=0, &loc);
      if( rc ) return rc;
    }
  }else if( loc==0 && (flags & BTREE_SAVEPOSITION)==0 ){
    if( pX->nMem ){







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  if( pFree ){
    sqlite3PageFree(pFree);
  }
  return rc;
}

/* Overwrite content from pX into pDest.  Only do the write if the
** content is different from what is already there.
*/
static int btreeOverwriteContent(
  MemPage *pPage,           /* MemPage on which writing will occur */
  u8 *pDest,                /* Pointer to the place to start writing */
  const BtreePayload *pX,   /* Source of data to write */
  int iOffset,              /* Offset of first byte to write */
  int iAmt                  /* Number of bytes to be written */
){
  int nData = pX->nData - iOffset;
  if( nData<=0 ){
    /* Overwritting with zeros */
    int i;
    for(i=0; i<iAmt && pDest[i]==0; i++){}
    if( i<iAmt ){
      int rc = sqlite3PagerWrite(pPage->pDbPage);
      if( rc ) return rc;
      memset(pDest + i, 0, iAmt - i);
    }
  }else{
    if( nData<iAmt ){
      /* Mixed read data and zeros at the end.  Make a recursive call
      ** to write the zeros then fall through to write the real data */
      btreeOverwriteContent(pPage, pDest+nData, pX, iOffset+nData, iAmt-nData);
      iAmt = nData;
    }
    if( memcmp(pDest, ((u8*)pX->pData) + iOffset, iAmt)!=0 ){
      int rc = sqlite3PagerWrite(pPage->pDbPage);
      if( rc ) return rc;
      memcpy(pDest, ((u8*)pX->pData) + iOffset, iAmt);
    }
  }
  return SQLITE_OK;
}

/*
** Overwrite the cell that cursor pCur is pointing to with fresh content
** contained in pX.
*/
static int btreeOverwriteCell(BtCursor *pCur, const BtreePayload *pX){
  int iOffset;                        /* Next byte of pX->pData to write */
  int nTotal = pX->nData + pX->nZero; /* Total bytes of to write */
  int rc;                             /* Return code */
  MemPage *pPage = pCur->pPage;       /* Page being written */
  BtShared *pBt;                      /* Btree */
  Pgno ovflPgno;                      /* Next overflow page to write */
  u32 ovflPageSize;                   /* Size to write on overflow page */

  /* Overwrite the local portion first */
  rc = btreeOverwriteContent(pPage, pCur->info.pPayload, pX,
                             0, pCur->info.nLocal);
  if( rc ) return rc;
  if( pCur->info.nLocal==nTotal ) return SQLITE_OK;

  /* Now overwrite the overflow pages */
  iOffset = pCur->info.nLocal;
  ovflPgno = get4byte(pCur->info.pPayload + iOffset);
  pBt = pPage->pBt;
  ovflPageSize = pBt->usableSize - 4;
  do{
    rc = btreeGetPage(pBt, ovflPgno, &pPage, 0);
    if( rc ) return rc;
    if( iOffset+ovflPageSize>nTotal ){
      ovflPgno = get4byte(pPage->aData + ovflPageSize);
    }else{
      ovflPageSize = nTotal - iOffset;
    }
    rc = btreeOverwriteContent(pPage, pPage->aData, pX,
                               iOffset, ovflPageSize);
    if( rc ) return rc;
    iOffset += ovflPageSize;
    sqlite3PagerUnref(pPage->pDbPage);
  }while( iOffset<nTotal );
  return SQLITE_OK;    
}


/*
** Insert a new record into the BTree.  The content of the new record
** is described by the pX object.  The pCur cursor is used only to
** define what table the record should be inserted into, and is left
** pointing at a random location.
**
................................................................................
    assert( (flags & BTREE_SAVEPOSITION)==0 || 
            ((pCur->curFlags&BTCF_ValidNKey)!=0 && pX->nKey==pCur->info.nKey) );

    /* If the cursor is currently on the last row and we are appending a
    ** new row onto the end, set the "loc" to avoid an unnecessary
    ** btreeMoveto() call */
    if( (pCur->curFlags&BTCF_ValidNKey)!=0 && pX->nKey==pCur->info.nKey ){
      /* The current is currently pointing to the entry that is to be
      ** overwritten */
      if( pCur->info.nPayload==pX->nData+pX->nZero ){
        return btreeOverwriteCell(pCur, pX);
      }
      loc = 0;
    }else if( loc==0 ){
      rc = sqlite3BtreeMovetoUnpacked(pCur, 0, pX->nKey, flags!=0, &loc);
      if( rc ) return rc;
    }
  }else if( loc==0 && (flags & BTREE_SAVEPOSITION)==0 ){
    if( pX->nMem ){