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Overview
Comment:Fix a couple of out-of-date comments in where.c.
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Timelines: family | ancestors | descendants | both | experimental-costs
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SHA1: eefeda32d54efbbdf7d20b719299eda48b891fae
User & Date: dan 2014-04-30 14:47:01
Context
2014-04-30
14:53
Update a couple of test cases to account for the fact that this branch prefers an index scan and partial sort over a full-table scan and full external sort. check-in: 9b975bf3 user: dan tags: experimental-costs
14:47
Fix a couple of out-of-date comments in where.c. check-in: eefeda32 user: dan tags: experimental-costs
14:22
Improved rendering of LogEst values corresponding to real values near 0.0 in the tool/logest.c utility program. check-in: 32910c8c user: drh tags: experimental-costs
Changes
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Changes to src/where.c.

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    if( j<0 ){
      pLoop->nOut += (pTerm->truthProb<=0 ? pTerm->truthProb : -1);
    }
  }
}

/*
** We have so far matched pBuilder->pNew->u.btree.nEq terms of the index pIndex.
** Try to match one more.





**
** If pProbe->tnum==0, that means pIndex is a fake index used for the
** INTEGER PRIMARY KEY.
*/
static int whereLoopAddBtreeIndex(
  WhereLoopBuilder *pBuilder,     /* The WhereLoop factory */
  struct SrcList_item *pSrc,      /* FROM clause term being analyzed */
................................................................................
  saved_prereq = pNew->prereq;
  saved_nOut = pNew->nOut;
  pNew->rSetup = 0;
  rLogSize = estLog(pProbe->aiRowLogEst[0]);

  /* Consider using a skip-scan if there are no WHERE clause constraints
  ** available for the left-most terms of the index, and if the average
  ** number of repeats in the left-most terms is at least 18.  The magic
  ** number 18 was found by experimentation to be the payoff point where
  ** skip-scan become faster than a full-scan.
  */






  assert( 42==sqlite3LogEst(18) );
  if( pTerm==0
   && saved_nEq==saved_nSkip
   && saved_nEq+1<pProbe->nKeyCol
   && pProbe->aiRowLogEst[saved_nEq+1]>=42  /* TUNING: Minimum for skip-scan */
   && (rc = whereLoopResize(db, pNew, pNew->nLTerm+1))==SQLITE_OK
  ){
................................................................................
        nOut = pFrom->nRow + pWLoop->nOut;
        maskNew = pFrom->maskLoop | pWLoop->maskSelf;
        if( isOrdered<0 ){
          isOrdered = wherePathSatisfiesOrderBy(pWInfo,
                       pWInfo->pOrderBy, pFrom, pWInfo->wctrlFlags,
                       iLoop, pWLoop, &revMask);
          if( isOrdered>=0 && isOrdered<nOrderBy ){
            /* TUNING: Estimated cost of sorting is N*log(N).
            ** If the order-by clause has X terms but only the last Y terms
            ** are out of order, then block-sorting will reduce the sorting
            ** cost to N*log(N)*log(Y/X).  The log(Y/X) term is computed
            ** by rScale.
            ** TODO: Should the sorting cost get a small multiplier to help
            ** discourage the use of sorting and encourage the use of index
            ** scans instead?
            */




            LogEst rScale, rSortCost;
            assert( nOrderBy>0 && 66==sqlite3LogEst(100) );
            rScale = sqlite3LogEst((nOrderBy-isOrdered)*100/nOrderBy) - 66;
            rSortCost = nRowEst + estLog(nRowEst) + rScale + 16;

            /* TUNING: The cost of implementing DISTINCT using a B-TREE is
            ** also N*log(N) but it has a larger constant of proportionality.
            ** Multiply by 3.0. */
            if( pWInfo->wctrlFlags & WHERE_WANT_DISTINCT ){
              rSortCost += 16;
            }
            WHERETRACE(0x002,
               ("---- sort cost=%-3d (%d/%d) increases cost %3d to %-3d\n",
                rSortCost, (nOrderBy-isOrdered), nOrderBy, rCost,
                sqlite3LogEstAdd(rCost,rSortCost)));







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    if( j<0 ){
      pLoop->nOut += (pTerm->truthProb<=0 ? pTerm->truthProb : -1);
    }
  }
}

/*
** We have so far matched pBuilder->pNew->u.btree.nEq terms of the 
** index pIndex. Try to match one more.
**
** When this function is called, pBuilder->pNew->nOut contains the 
** number of rows expected to be visited by filtering using the nEq 
** terms only. If it is modified, this value is restored before this 
** function returns.
**
** If pProbe->tnum==0, that means pIndex is a fake index used for the
** INTEGER PRIMARY KEY.
*/
static int whereLoopAddBtreeIndex(
  WhereLoopBuilder *pBuilder,     /* The WhereLoop factory */
  struct SrcList_item *pSrc,      /* FROM clause term being analyzed */
................................................................................
  saved_prereq = pNew->prereq;
  saved_nOut = pNew->nOut;
  pNew->rSetup = 0;
  rLogSize = estLog(pProbe->aiRowLogEst[0]);

  /* Consider using a skip-scan if there are no WHERE clause constraints
  ** available for the left-most terms of the index, and if the average
  ** number of repeats in the left-most terms is at least 18. 


  **
  ** The magic number 18 is selected on the basis that scanning 17 rows
  ** is almost always quicker than an index seek (even though if the index
  ** contains fewer than 2^17 rows we assume otherwise in other parts of
  ** the code). And, even if it is not, it should not be too much slower. 
  ** On the other hand, the extra seeks could end up being significantly
  ** more expensive.  */
  assert( 42==sqlite3LogEst(18) );
  if( pTerm==0
   && saved_nEq==saved_nSkip
   && saved_nEq+1<pProbe->nKeyCol
   && pProbe->aiRowLogEst[saved_nEq+1]>=42  /* TUNING: Minimum for skip-scan */
   && (rc = whereLoopResize(db, pNew, pNew->nLTerm+1))==SQLITE_OK
  ){
................................................................................
        nOut = pFrom->nRow + pWLoop->nOut;
        maskNew = pFrom->maskLoop | pWLoop->maskSelf;
        if( isOrdered<0 ){
          isOrdered = wherePathSatisfiesOrderBy(pWInfo,
                       pWInfo->pOrderBy, pFrom, pWInfo->wctrlFlags,
                       iLoop, pWLoop, &revMask);
          if( isOrdered>=0 && isOrdered<nOrderBy ){
            /* TUNING: Estimated cost of a full external sort, where N is 
            ** the number of rows to sort is:
            **
            **   cost = (3.0 * N * log(N)).
            ** 
            ** Or, if the order-by clause has X terms but only the last Y 
            ** terms are out of order, then block-sorting will reduce the 
            ** sorting cost to:
            **
            **   cost = (3.0 * N * log(N)) * (Y/X)
            **
            ** The (Y/X) term is implemented using stack variable rScale
            ** below.  */
            LogEst rScale, rSortCost;
            assert( nOrderBy>0 && 66==sqlite3LogEst(100) );
            rScale = sqlite3LogEst((nOrderBy-isOrdered)*100/nOrderBy) - 66;
            rSortCost = nRowEst + estLog(nRowEst) + rScale + 16;

            /* TUNING: The cost of implementing DISTINCT using a B-TREE is
            ** similar but with a larger constant of proportionality. 
            ** Multiply by an additional factor of 3.0.  */
            if( pWInfo->wctrlFlags & WHERE_WANT_DISTINCT ){
              rSortCost += 16;
            }
            WHERETRACE(0x002,
               ("---- sort cost=%-3d (%d/%d) increases cost %3d to %-3d\n",
                rSortCost, (nOrderBy-isOrdered), nOrderBy, rCost,
                sqlite3LogEstAdd(rCost,rSortCost)));