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Overview
Comment:Improvements to query planning for joins: Avoid unnecessary calls to "optimal scan" checks in cases where table reordering is not possible. Make sure optimal scan checks are carried out for CROSS JOINs and LEFT JOINs.
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SHA1:d5ebb7877885839e93eee3b322624d4c4215c1c4
User & Date: drh 2013-01-16 00:46:09
Context
2013-01-16
20:33
Fix the activate_extensions pragma so that it is a no-op when the required argument is omitted. check-in: 6195ebd8 user: drh tags: trunk
17:08
Enhance the query planner to exploit transitivity of join constraints in a multi-way join. check-in: 13171eb5 user: drh tags: transitive-constraints
00:46
Improvements to query planning for joins: Avoid unnecessary calls to "optimal scan" checks in cases where table reordering is not possible. Make sure optimal scan checks are carried out for CROSS JOINs and LEFT JOINs. check-in: d5ebb787 user: drh tags: trunk
2013-01-15
18:49
Fix a missing word in a comment. Enhance the "wheretrace" debugging output to show the estimated cost of each table option while planning the join order. check-in: ac4e119a user: drh tags: trunk
Changes
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Changes to src/where.c.

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  for(sWBI.i=iFrom=0, pLevel=pWInfo->a; sWBI.i<nTabList; sWBI.i++, pLevel++){
    WhereCost bestPlan;         /* Most efficient plan seen so far */
    Index *pIdx;                /* Index for FROM table at pTabItem */
    int j;                      /* For looping over FROM tables */
    int bestJ = -1;             /* The value of j */
    Bitmask m;                  /* Bitmask value for j or bestJ */
    int isOptimal;              /* Iterator for optimal/non-optimal search */

    int nUnconstrained;         /* Number tables without INDEXED BY */
    Bitmask notIndexed;         /* Mask of tables that cannot use an index */

    memset(&bestPlan, 0, sizeof(bestPlan));
    bestPlan.rCost = SQLITE_BIG_DBL;
    WHERETRACE(("*** Begin search for loop %d ***\n", sWBI.i));

................................................................................
    ** Since the cost of a linear scan through table t2 is the same 
    ** as the cost of a linear scan through table t1, a simple greedy 
    ** algorithm may choose to use t2 for the outer loop, which is a much
    ** costlier approach.
    */
    nUnconstrained = 0;
    notIndexed = 0;
    for(isOptimal=(iFrom<nTabList-1); isOptimal>=0 && bestJ<0; isOptimal--){
      for(j=iFrom, sWBI.pSrc=&pTabList->a[j]; j<nTabList; j++, sWBI.pSrc++){
        int doNotReorder;    /* True if this table should not be reordered */
  
        doNotReorder =  (sWBI.pSrc->jointype & (JT_LEFT|JT_CROSS))!=0;
        if( j!=iFrom && doNotReorder ) break;


        m = getMask(pMaskSet, sWBI.pSrc->iCursor);
        if( (m & sWBI.notValid)==0 ){
          if( j==iFrom ) iFrom++;
          continue;






















        }
        sWBI.notReady = (isOptimal ? m : sWBI.notValid);
        if( sWBI.pSrc->pIndex==0 ) nUnconstrained++;
  
        WHERETRACE(("   === trying table %d (%s) with isOptimal=%d ===\n",
                    j, sWBI.pSrc->pTab->zName, isOptimal));
        assert( sWBI.pSrc->pTab );
................................................................................
                  || sWBI.cost.plan.u.pIdx==sWBI.pSrc->pIndex );

        if( isOptimal && (sWBI.cost.plan.wsFlags & WHERE_NOT_FULLSCAN)==0 ){
          notIndexed |= m;
        }
        if( isOptimal ){
          pWInfo->a[j].rOptCost = sWBI.cost.rCost;
        }else if( iFrom<nTabList-1 ){
          /* If two or more tables have nearly the same outer loop cost, but
          ** very different inner loop (optimal) cost, we want to choose
          ** for the outer loop that table which benefits the least from
          ** being in the inner loop.  The following code scales the 
          ** outer loop cost estimate to accomplish that. */
          WHERETRACE(("   scaling cost from %.1f to %.1f\n",
                      sWBI.cost.rCost,
................................................................................
                      "       cost=%.1f, nRow=%.1f, nOBSat=%d, wsFlags=%08x\n",
                      j, sWBI.pSrc->pTab->zName,
                      sWBI.cost.rCost, sWBI.cost.plan.nRow,
                      sWBI.cost.plan.nOBSat, sWBI.cost.plan.wsFlags));
          bestPlan = sWBI.cost;
          bestJ = j;
        }
        if( doNotReorder ) break;





      }
    }
    assert( bestJ>=0 );
    assert( sWBI.notValid & getMask(pMaskSet, pTabList->a[bestJ].iCursor) );




    WHERETRACE(("*** Optimizer selects table %d (%s) for loop %d with:\n"
                "    cost=%.1f, nRow=%.1f, nOBSat=%d, wsFlags=0x%08x\n",
                bestJ, pTabList->a[bestJ].pTab->zName,
                pLevel-pWInfo->a, bestPlan.rCost, bestPlan.plan.nRow,
                bestPlan.plan.nOBSat, bestPlan.plan.wsFlags));
    if( (bestPlan.plan.wsFlags & WHERE_DISTINCT)!=0 ){
      assert( pWInfo->eDistinct==0 );







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  for(sWBI.i=iFrom=0, pLevel=pWInfo->a; sWBI.i<nTabList; sWBI.i++, pLevel++){
    WhereCost bestPlan;         /* Most efficient plan seen so far */
    Index *pIdx;                /* Index for FROM table at pTabItem */
    int j;                      /* For looping over FROM tables */
    int bestJ = -1;             /* The value of j */
    Bitmask m;                  /* Bitmask value for j or bestJ */
    int isOptimal;              /* Iterator for optimal/non-optimal search */
    int ckOptimal;              /* Do the optimal scan check */
    int nUnconstrained;         /* Number tables without INDEXED BY */
    Bitmask notIndexed;         /* Mask of tables that cannot use an index */

    memset(&bestPlan, 0, sizeof(bestPlan));
    bestPlan.rCost = SQLITE_BIG_DBL;
    WHERETRACE(("*** Begin search for loop %d ***\n", sWBI.i));

................................................................................
    ** Since the cost of a linear scan through table t2 is the same 
    ** as the cost of a linear scan through table t1, a simple greedy 
    ** algorithm may choose to use t2 for the outer loop, which is a much
    ** costlier approach.
    */
    nUnconstrained = 0;
    notIndexed = 0;

    /* The optimal scan check only occurs if there are two or more tables
    ** available to be reordered */
    if( iFrom==nTabList-1 ){
      ckOptimal = 0;  /* Common case of just one table in the FROM clause */
    }else{
      ckOptimal = -1;
      for(j=iFrom, sWBI.pSrc=&pTabList->a[j]; j<nTabList; j++, sWBI.pSrc++){
        m = getMask(pMaskSet, sWBI.pSrc->iCursor);
        if( (m & sWBI.notValid)==0 ){
          if( j==iFrom ) iFrom++;
          continue;
        }
        if( j>iFrom && (sWBI.pSrc->jointype & (JT_LEFT|JT_CROSS))!=0 ) break;
        if( ++ckOptimal ) break;
        if( (sWBI.pSrc->jointype & JT_LEFT)!=0 ) break;
      }
    }
    assert( ckOptimal==0 || ckOptimal==1 );

    for(isOptimal=ckOptimal; isOptimal>=0 && bestJ<0; isOptimal--){
      for(j=iFrom, sWBI.pSrc=&pTabList->a[j]; j<nTabList; j++, sWBI.pSrc++){
        if( j>iFrom && (sWBI.pSrc->jointype & (JT_LEFT|JT_CROSS))!=0 ){
          /* This break and one like it in the ckOptimal computation loop
          ** above prevent table reordering across LEFT and CROSS JOINs.
          ** The LEFT JOIN case is necessary for correctness.  The prohibition
          ** against reordering across a CROSS JOIN is an SQLite feature that
          ** allows the developer to control table reordering */
          break;
        }
        m = getMask(pMaskSet, sWBI.pSrc->iCursor);
        if( (m & sWBI.notValid)==0 ){
          assert( j>iFrom );
          continue;
        }
        sWBI.notReady = (isOptimal ? m : sWBI.notValid);
        if( sWBI.pSrc->pIndex==0 ) nUnconstrained++;
  
        WHERETRACE(("   === trying table %d (%s) with isOptimal=%d ===\n",
                    j, sWBI.pSrc->pTab->zName, isOptimal));
        assert( sWBI.pSrc->pTab );
................................................................................
                  || sWBI.cost.plan.u.pIdx==sWBI.pSrc->pIndex );

        if( isOptimal && (sWBI.cost.plan.wsFlags & WHERE_NOT_FULLSCAN)==0 ){
          notIndexed |= m;
        }
        if( isOptimal ){
          pWInfo->a[j].rOptCost = sWBI.cost.rCost;
        }else if( ckOptimal ){
          /* If two or more tables have nearly the same outer loop cost, but
          ** very different inner loop (optimal) cost, we want to choose
          ** for the outer loop that table which benefits the least from
          ** being in the inner loop.  The following code scales the 
          ** outer loop cost estimate to accomplish that. */
          WHERETRACE(("   scaling cost from %.1f to %.1f\n",
                      sWBI.cost.rCost,
................................................................................
                      "       cost=%.1f, nRow=%.1f, nOBSat=%d, wsFlags=%08x\n",
                      j, sWBI.pSrc->pTab->zName,
                      sWBI.cost.rCost, sWBI.cost.plan.nRow,
                      sWBI.cost.plan.nOBSat, sWBI.cost.plan.wsFlags));
          bestPlan = sWBI.cost;
          bestJ = j;
        }


        /* In a join like "w JOIN x LEFT JOIN y JOIN z"  make sure that
        ** table y (and not table z) is always the next inner loop inside
        ** of table x. */
        if( (sWBI.pSrc->jointype & JT_LEFT)!=0 ) break;
      }
    }
    assert( bestJ>=0 );
    assert( sWBI.notValid & getMask(pMaskSet, pTabList->a[bestJ].iCursor) );
    assert( bestJ==iFrom || (pTabList->a[iFrom].jointype & JT_LEFT)==0 );
    testcase( bestJ>iFrom && (pTabList->a[iFrom].jointype & JT_CROSS)!=0 );
    testcase( bestJ>iFrom && bestJ<nTabList-1
                          && (pTabList->a[bestJ+1].jointype & JT_LEFT)!=0 );
    WHERETRACE(("*** Optimizer selects table %d (%s) for loop %d with:\n"
                "    cost=%.1f, nRow=%.1f, nOBSat=%d, wsFlags=0x%08x\n",
                bestJ, pTabList->a[bestJ].pTab->zName,
                pLevel-pWInfo->a, bestPlan.rCost, bestPlan.plan.nRow,
                bestPlan.plan.nOBSat, bestPlan.plan.wsFlags));
    if( (bestPlan.plan.wsFlags & WHERE_DISTINCT)!=0 ){
      assert( pWInfo->eDistinct==0 );