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Overview
Comment:Small performance optimization in sqlite3WhereBegin().
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SHA3-256:39f708d1e286931365a8992dbe7f900108ff1dad146032a284ad1dec09b11e06
User & Date: drh 2017-08-25 13:34:18
Context
2017-08-25
15:43
Size and performance optimization for sqlite3Init(). check-in: 776d9128 user: drh tags: trunk
13:34
Small performance optimization in sqlite3WhereBegin(). check-in: 39f708d1 user: drh tags: trunk
13:02
Convert a branch made unreachable by [59560d07] into an assert(). check-in: 2738b8db user: drh tags: trunk
Changes
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Changes to src/where.c.

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  /* Special case: No FROM clause
  */
  if( nTabList==0 ){
    if( pOrderBy ) pWInfo->nOBSat = pOrderBy->nExpr;
    if( wctrlFlags & WHERE_WANT_DISTINCT ){
      pWInfo->eDistinct = WHERE_DISTINCT_UNIQUE;
    }
  }

  /* Assign a bit from the bitmask to every term in the FROM clause.
  **
  ** The N-th term of the FROM clause is assigned a bitmask of 1<<N.
  **
  ** The rule of the previous sentence ensures thta if X is the bitmask for
  ** a table T, then X-1 is the bitmask for all other tables to the left of T.
  ** Knowing the bitmask for all tables to the left of a left join is
  ** important.  Ticket #3015.
  **
  ** Note that bitmasks are created for all pTabList->nSrc tables in
  ** pTabList, not just the first nTabList tables.  nTabList is normally
  ** equal to pTabList->nSrc but might be shortened to 1 if the
  ** WHERE_OR_SUBCLAUSE flag is set.
  */

  for(ii=0; ii<pTabList->nSrc; ii++){
    createMask(pMaskSet, pTabList->a[ii].iCursor);
    sqlite3WhereTabFuncArgs(pParse, &pTabList->a[ii], &pWInfo->sWC);
  }
#ifdef SQLITE_DEBUG
  {
    Bitmask mx = 0;
    for(ii=0; ii<pTabList->nSrc; ii++){
      Bitmask m = sqlite3WhereGetMask(pMaskSet, pTabList->a[ii].iCursor);
      assert( m>=mx );
      mx = m;
    }
  }
#endif


  /* Analyze all of the subexpressions. */
  sqlite3WhereExprAnalyze(pTabList, &pWInfo->sWC);
  if( db->mallocFailed ) goto whereBeginError;

  /* Special case: WHERE terms that do not refer to any tables in the join
  ** (constant expressions). Evaluate each such term, and jump over all the
  ** generated code if the result is not true.  







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  /* Special case: No FROM clause
  */
  if( nTabList==0 ){
    if( pOrderBy ) pWInfo->nOBSat = pOrderBy->nExpr;
    if( wctrlFlags & WHERE_WANT_DISTINCT ){
      pWInfo->eDistinct = WHERE_DISTINCT_UNIQUE;
    }
  }else{

    /* Assign a bit from the bitmask to every term in the FROM clause.
    **
    ** The N-th term of the FROM clause is assigned a bitmask of 1<<N.
    **
    ** The rule of the previous sentence ensures thta if X is the bitmask for
    ** a table T, then X-1 is the bitmask for all other tables to the left of T.
    ** Knowing the bitmask for all tables to the left of a left join is
    ** important.  Ticket #3015.
    **
    ** Note that bitmasks are created for all pTabList->nSrc tables in
    ** pTabList, not just the first nTabList tables.  nTabList is normally
    ** equal to pTabList->nSrc but might be shortened to 1 if the
    ** WHERE_OR_SUBCLAUSE flag is set.
    */
    ii = 0;
    do{
      createMask(pMaskSet, pTabList->a[ii].iCursor);
      sqlite3WhereTabFuncArgs(pParse, &pTabList->a[ii], &pWInfo->sWC);
    }while( (++ii)<pTabList->nSrc );
  #ifdef SQLITE_DEBUG
    {
      Bitmask mx = 0;
      for(ii=0; ii<pTabList->nSrc; ii++){
        Bitmask m = sqlite3WhereGetMask(pMaskSet, pTabList->a[ii].iCursor);
        assert( m>=mx );
        mx = m;
      }
    }
  #endif
  }
  
  /* Analyze all of the subexpressions. */
  sqlite3WhereExprAnalyze(pTabList, &pWInfo->sWC);
  if( db->mallocFailed ) goto whereBeginError;

  /* Special case: WHERE terms that do not refer to any tables in the join
  ** (constant expressions). Evaluate each such term, and jump over all the
  ** generated code if the result is not true.