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Comment:Deploy heuristics (well-commented) to better estimate how much unindexed terms in the WHERE clause filter the number of output rows from a single table.
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SHA1:221659945c3f78d3b6789bfe8fdeb8d3ee1fa038
User & Date: drh 2014-11-22 18:50:44
Context
2014-11-22
19:52
Fix an error in the comments from the previous check-in. check-in: 9660ce54 user: drh tags: trunk
18:50
Deploy heuristics (well-commented) to better estimate how much unindexed terms in the WHERE clause filter the number of output rows from a single table. check-in: 22165994 user: drh tags: trunk
12:22
Remove a redundant test case (probably a copy/paste error). Add an assert() to where.c to ensure that automatic indexes do not have there output row counts adjusted downward by supplementary constraints. check-in: eea47933 user: drh tags: trunk
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Changes to src/where.c.

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  return SQLITE_OK;
}

/*
** Adjust the WhereLoop.nOut value downward to account for terms of the
** WHERE clause that reference the loop but which are not used by an
** index.





**
** In the current implementation, the first extra WHERE clause term reduces
** the number of output rows by a factor of 10 and each additional term
** reduces the number of output rows by sqrt(2).


















*/
static void whereLoopOutputAdjust(
  WhereClause *pWC,      /* The WHERE clause */
  WhereLoop *pLoop,      /* The loop to adjust downward */
  LogEst nRow            /* Number of rows in the entire table */
){
  WhereTerm *pTerm, *pX;
  Bitmask notAllowed = ~(pLoop->prereq|pLoop->maskSelf);
  int i, j;
  int nEq = 0;    /* Number of = constraints not within likely()/unlikely() */

  assert( (pLoop->wsFlags & WHERE_AUTO_INDEX)==0 );
  for(i=pWC->nTerm, pTerm=pWC->a; i>0; i--, pTerm++){
    if( (pTerm->wtFlags & TERM_VIRTUAL)!=0 ) break;
    if( (pTerm->prereqAll & pLoop->maskSelf)==0 ) continue;
    if( (pTerm->prereqAll & notAllowed)!=0 ) continue;
    for(j=pLoop->nLTerm-1; j>=0; j--){
................................................................................
      pX = pLoop->aLTerm[j];
      if( pX==0 ) continue;
      if( pX==pTerm ) break;
      if( pX->iParent>=0 && (&pWC->a[pX->iParent])==pTerm ) break;
    }
    if( j<0 ){
      if( pTerm->truthProb<=0 ){


        pLoop->nOut += pTerm->truthProb;
      }else{


        pLoop->nOut--;
        if( pTerm->eOperator&WO_EQ ) nEq++;








      }
    }
  }
  /* TUNING:  If there is at least one equality constraint in the WHERE
  ** clause that does not have a likelihood() explicitly assigned to it
  ** then do not let the estimated number of output rows exceed half 
  ** the number of rows in the table. */
  if( nEq && pLoop->nOut>nRow-10 ){
    pLoop->nOut = nRow - 10;
  }
}

/*
** Adjust the cost C by the costMult facter T.  This only occurs if
** compiled with -DSQLITE_ENABLE_COSTMULT
*/
#ifdef SQLITE_ENABLE_COSTMULT







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  return SQLITE_OK;
}

/*
** Adjust the WhereLoop.nOut value downward to account for terms of the
** WHERE clause that reference the loop but which are not used by an
** index.
*
** For every WHERE clause term that is not used by the index
** and which has a truth probability assigned by one of the likelihood(),
** likely(), or unlikely() SQL functions, reduce the estimated number
** of output rows by the probability specified.
**



** TUNING:  For every WHERE clause term that is not used by the index
** and which does not have an assigned truth probability, heuristics
** described below are used to try to estimate the truth probability.
** TODO --> Perhaps this is something that could be improved by better
** table statistics.
**
** Heuristic 1:  Estimate the truth probability as 6.25%.  The 6.25%
** value corresponds to 1 in LogEst notation, so this means decrement
** the WhereLoop.nOut field for every such WHERE clause term.
**
** Heuristic 2:  If there exists one or more WHERE clause terms of the
** form "x==EXPR" and EXPR is not a constant 0 or 1, then make sure the
** final output row estimate is no greater than 1/4 of the total number
** of rows in the table.  In other words, assume that x==EXPR will filter
** out at least 3 out of 4 rows.  If EXPR is -1 or 0 or 1, then maybe the
** "x" column is boolean or else -1 or 0 or 1 is a common default value
** on the "x" column and so in that case only cap the output row estimate
** at 1/2 instead of 1/4.
*/
static void whereLoopOutputAdjust(
  WhereClause *pWC,      /* The WHERE clause */
  WhereLoop *pLoop,      /* The loop to adjust downward */
  LogEst nRow            /* Number of rows in the entire table */
){
  WhereTerm *pTerm, *pX;
  Bitmask notAllowed = ~(pLoop->prereq|pLoop->maskSelf);
  int i, j, k;
  LogEst iReduce = 0;    /* pLoop->nOut should not exceed nRow-iReduce */

  assert( (pLoop->wsFlags & WHERE_AUTO_INDEX)==0 );
  for(i=pWC->nTerm, pTerm=pWC->a; i>0; i--, pTerm++){
    if( (pTerm->wtFlags & TERM_VIRTUAL)!=0 ) break;
    if( (pTerm->prereqAll & pLoop->maskSelf)==0 ) continue;
    if( (pTerm->prereqAll & notAllowed)!=0 ) continue;
    for(j=pLoop->nLTerm-1; j>=0; j--){
................................................................................
      pX = pLoop->aLTerm[j];
      if( pX==0 ) continue;
      if( pX==pTerm ) break;
      if( pX->iParent>=0 && (&pWC->a[pX->iParent])==pTerm ) break;
    }
    if( j<0 ){
      if( pTerm->truthProb<=0 ){
        /* If a truth probability is specified using the likelihood() hints,
        ** then use the probability provided by the application. */
        pLoop->nOut += pTerm->truthProb;
      }else{
        /* In the absence of explicit truth probabilities, use heuristics to
        ** guess a reasonable truth probability. */
        pLoop->nOut--;
        if( pTerm->eOperator&WO_EQ ){
          Expr *pRight = pTerm->pExpr->pRight;
          if( sqlite3ExprIsInteger(pRight, &k) && k>=(-1) && k<=1 ){
            k = 10;
          }else{
            k = 20;
          }
          if( iReduce<k ) iReduce = k;
        }
      }
    }
  }
  if( pLoop->nOut > nRow-iReduce )  pLoop->nOut = nRow - iReduce;






}

/*
** Adjust the cost C by the costMult facter T.  This only occurs if
** compiled with -DSQLITE_ENABLE_COSTMULT
*/
#ifdef SQLITE_ENABLE_COSTMULT

Changes to test/scanstatus.test.

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  PRAGMA foreign_keys=on;
}
do_execsql_test    4.2.1 { DELETE FROM p1 WHERE x=4 }
do_scanstatus_test 4.2.2 { 
  nLoop 1 nVisit 1 nEst 1.0 zName sqlite_autoindex_p1_1 
  zExplain {SEARCH TABLE p1 USING INDEX sqlite_autoindex_p1_1 (x=?)}

  nLoop 1 nVisit 3 nEst 524288.0 zName c1 zExplain {SCAN TABLE c1}
}

#-------------------------------------------------------------------------
# Further tests of different scan types.
#
reset_db
proc tochar {i} {







|







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  PRAGMA foreign_keys=on;
}
do_execsql_test    4.2.1 { DELETE FROM p1 WHERE x=4 }
do_scanstatus_test 4.2.2 { 
  nLoop 1 nVisit 1 nEst 1.0 zName sqlite_autoindex_p1_1 
  zExplain {SEARCH TABLE p1 USING INDEX sqlite_autoindex_p1_1 (x=?)}

  nLoop 1 nVisit 3 nEst 262144.0 zName c1 zExplain {SCAN TABLE c1}
}

#-------------------------------------------------------------------------
# Further tests of different scan types.
#
reset_db
proc tochar {i} {