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Comment:Increase the estimated cost of sorting when sorting wide results sets, to account for the extra storage space and I/O required for the external sort.
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SHA1:38298ef923c8dad6860385ac4a20849eeac2d174
User & Date: drh 2017-02-17 21:23:25
Context
2017-02-17
21:23
Increase the estimated cost of sorting when sorting wide results sets, to account for the extra storage space and I/O required for the external sort. Leaf check-in: 38298ef9 user: drh tags: apple-increased-sorting-cost
2017-02-16
21:29
Increase the estimated cost of sorting when sorting wide results sets, to account for the extra storage space and I/O required for the external sort. check-in: aa0703e5 user: drh tags: increased-sorting-cost
2016-11-17
14:02
When opening the *-shm file for a readonly database, try to open it in read-write mode before falling back to readonly. This is in case some other read/write connection within the same process uses the same file descriptor. check-in: a07c581e user: dan tags: apple-osx
Changes
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Changes to src/where.c.

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  **
  ** The (Y/X) term is implemented using stack variable rScale
  ** below.  */
  LogEst rScale, rSortCost;
  assert( nOrderBy>0 && 66==sqlite3LogEst(100) );
  rScale = sqlite3LogEst((nOrderBy-nSorted)*100/nOrderBy) - 66;
  rSortCost = nRow + rScale + 16;











  /* Multiple by log(M) where M is the number of output rows.
  ** Use the LIMIT for M if it is smaller */
  if( (pWInfo->wctrlFlags & WHERE_USE_LIMIT)!=0 && pWInfo->iLimit<nRow ){
    nRow = pWInfo->iLimit;
  }
  rSortCost += estLog(nRow);







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  **
  ** The (Y/X) term is implemented using stack variable rScale
  ** below.  */
  LogEst rScale, rSortCost;
  assert( nOrderBy>0 && 66==sqlite3LogEst(100) );
  rScale = sqlite3LogEst((nOrderBy-nSorted)*100/nOrderBy) - 66;
  rSortCost = nRow + rScale + 16;

  /* For wide sorts (many payload columns) increase the sorting cost
  ** to account for the additional I/O used by the external sorting
  ** algorithm when it flushes PMAs to disk.
  */
  if( pWInfo->pDistinctSet
   && pWInfo->pDistinctSet->nExpr>6
  ){
    rSortCost += sqlite3LogEst(pWInfo->pDistinctSet->nExpr) - 26;
  }

  /* Multiple by log(M) where M is the number of output rows.
  ** Use the LIMIT for M if it is smaller */
  if( (pWInfo->wctrlFlags & WHERE_USE_LIMIT)!=0 && pWInfo->iLimit<nRow ){
    nRow = pWInfo->iLimit;
  }
  rSortCost += estLog(nRow);