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Overview
Comment:Only eliminate inner loops of a JOIN if they are the RHS of a LEFT JOIN and if they give no more than a single result. This appears to give correct answers in all cases.
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SHA1: d7a25cc79794817504ca1a4262008a68b2a4dece
User & Date: drh 2013-06-21 02:15:48
Context
2013-06-22
15:44
Add the ability to disable the omit-join-table optimization for testing purposes. Closed-Leaf check-in: d929df9b user: drh tags: omit-join-table-opt
2013-06-21
02:15
Only eliminate inner loops of a JOIN if they are the RHS of a LEFT JOIN and if they give no more than a single result. This appears to give correct answers in all cases. check-in: d7a25cc7 user: drh tags: omit-join-table-opt
02:05
Attempt to disable inner loops of a join that do not generate output. This does not work, since the inner loops might run zero times and thus inhibit all output. Needs to be enhanced to work only for LEFT JOINs or when we know that the inner loop will always run at least once. check-in: ca839723 user: drh tags: omit-join-table-opt
Changes
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Changes to src/where.c.

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#endif
  /* Attempt to omit tables from the join that do not effect the result */
  if( pResultSet!=0 && pWInfo->nLevel>=2 ){
    Bitmask tabUsed = exprListTableUsage(pMaskSet, pResultSet);
    if( pOrderBy ) tabUsed |= exprListTableUsage(pMaskSet, pOrderBy);
    while( pWInfo->nLevel>=2 ){
      pLoop = pWInfo->a[pWInfo->nLevel-1].pWLoop;

      if( ((wctrlFlags & WHERE_WANT_DISTINCT)!=0
           || (pLoop->wsFlags & WHERE_ONEROW)!=0)
       && (tabUsed & pLoop->maskSelf)==0
      ){



        WHERETRACE(0xffff, ("-> drop loop %c not used\n", pLoop->cId));
        pWInfo->nLevel--;
        nTabList--;
      }else{
        break;
      }
    }
  }
  WHERETRACE(0xffff,("*** Optimizer Finished ***\n"));
  pWInfo->pParse->nQueryLoop += pWInfo->nRowOut;

  /* If the caller is an UPDATE or DELETE statement that is requesting
  ** to use a one-pass algorithm, determine if this is appropriate.







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#endif
  /* Attempt to omit tables from the join that do not effect the result */
  if( pResultSet!=0 && pWInfo->nLevel>=2 ){
    Bitmask tabUsed = exprListTableUsage(pMaskSet, pResultSet);
    if( pOrderBy ) tabUsed |= exprListTableUsage(pMaskSet, pOrderBy);
    while( pWInfo->nLevel>=2 ){
      pLoop = pWInfo->a[pWInfo->nLevel-1].pWLoop;
      if( (pWInfo->pTabList->a[pLoop->iTab].jointype & JT_LEFT)==0 ) break;
      if( (wctrlFlags & WHERE_WANT_DISTINCT)==0
       && (pLoop->wsFlags & WHERE_ONEROW)==0

      ){
        break;
      }
      if( (tabUsed & pLoop->maskSelf)!=0 ) break;
      WHERETRACE(0xffff, ("-> drop loop %c not used\n", pLoop->cId));
      pWInfo->nLevel--;
      nTabList--;



    }
  }
  WHERETRACE(0xffff,("*** Optimizer Finished ***\n"));
  pWInfo->pParse->nQueryLoop += pWInfo->nRowOut;

  /* If the caller is an UPDATE or DELETE statement that is requesting
  ** to use a one-pass algorithm, determine if this is appropriate.