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Overview
Comment:Lower the threshold for using skip-scan from 50 to 18, based on experiments that show that 18 is the approximate break-even point for a variety of schemas.
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SHA1:83c0bb9913838d18ba355033afde6e38b4690842
User & Date: drh 2013-11-27 04:22:27
Context
2013-11-27
13:24
Make sure the colWidth array is correctly initialized in the ".explain" command of the shell. check-in: ceebcdca user: drh tags: trunk
04:22
Lower the threshold for using skip-scan from 50 to 18, based on experiments that show that 18 is the approximate break-even point for a variety of schemas. check-in: 83c0bb99 user: drh tags: trunk
04:00
Avoid using the GetVersionEx functions if they are considered deprecated. check-in: afdca299 user: mistachkin tags: trunk
Changes
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Changes to src/where.c.

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  saved_prereq = pNew->prereq;
  saved_nOut = pNew->nOut;
  pNew->rSetup = 0;
  rLogSize = estLog(sqlite3LogEst(pProbe->aiRowEst[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 50.


  */
  if( pTerm==0
   && saved_nEq==saved_nSkip
   && saved_nEq+1<pProbe->nKeyCol
   && pProbe->aiRowEst[saved_nEq+1]>50  /* TUNING: Minimum for skip-scan */
  ){
    LogEst nIter;
    pNew->u.btree.nEq++;
    pNew->u.btree.nSkip++;
    pNew->aLTerm[pNew->nLTerm++] = 0;
    pNew->wsFlags |= WHERE_SKIPSCAN;
    nIter = sqlite3LogEst(pProbe->aiRowEst[0]/pProbe->aiRowEst[saved_nEq+1]);







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  saved_prereq = pNew->prereq;
  saved_nOut = pNew->nOut;
  pNew->rSetup = 0;
  rLogSize = estLog(sqlite3LogEst(pProbe->aiRowEst[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.
  */
  if( pTerm==0
   && saved_nEq==saved_nSkip
   && saved_nEq+1<pProbe->nKeyCol
   && pProbe->aiRowEst[saved_nEq+1]>=18  /* TUNING: Minimum for skip-scan */
  ){
    LogEst nIter;
    pNew->u.btree.nEq++;
    pNew->u.btree.nSkip++;
    pNew->aLTerm[pNew->nLTerm++] = 0;
    pNew->wsFlags |= WHERE_SKIPSCAN;
    nIter = sqlite3LogEst(pProbe->aiRowEst[0]/pProbe->aiRowEst[saved_nEq+1]);