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Change the cost estimator in the query planner to take into account the

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logN rowid lookup cost when going from an index to a table.

FossilOrigin-Name: b442525b0ba642bb8d57b87b7b9e373b6046454a
This commit is contained in:
drh 2011-01-24 15:11:23 +00:00
parent 534230cf2e
commit b966194a63
3 changed files with 24 additions and 26 deletions
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18
manifest
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@ -1,8 +1,8 @@
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D 2011-01-22T00:10:45.721
C Change\sthe\scost\sestimator\sin\sthe\squery\splanner\sto\stake\sinto\saccount\sthe\nlogN\srowid\slookup\scost\swhen\sgoing\sfrom\san\sindex\sto\sa\stable.
D 2011-01-24T15:11:23.443
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@ -243,7 +243,7 @@ F src/vtab.c b297e8fa656ab5e66244ab15680d68db0adbec30
F src/wal.c dbca424f71678f663a286ab2a98f947af1d412a7
F src/wal.h c1aac6593a0b02b15dc625987e619edeab39292e
F src/walker.c 3112bb3afe1d85dc52317cb1d752055e9a781f8f
F src/where.c 99a9ea77114b649d68d01127331119f6785a80f1
F src/where.c 87de2616150606fd1b61d7c88afdb90089ddd53d
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5d5bddd290e71a7b03bcc23ff29881c23233cbff
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30
src/where.c
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@ -2919,31 +2919,29 @@ static void bestBtreeIndex(
}
#endif /* SQLITE_ENABLE_STAT2 */
/* Assume constant cost to access a row and logarithmic cost to
** do a binary search. Hence, the initial cost is the number of output
** rows plus log2(table-size) times the number of binary searches.
*/
cost = nRow + nInMul*estLog(aiRowEst[0]);
/* Adjust the number of rows and the cost downward to reflect rows
** that are excluded by range constraints.
*/
nRow = (nRow * (double)estBound) / (double)100;
cost = (cost * (double)estBound) / (double)100;
/* Add in the estimated cost of sorting the result
/* Assume constant cost to access a row and logarithmic cost to
** do a binary search. Hence, the initial cost is the number of output
** rows plus log2(table-size) times the number of binary searches.
*/
if( pIdx && bLookup ){
cost = nRow + (nInMul+nRow)*estLog(aiRowEst[0]);
}else{
cost = nRow + nInMul*estLog(aiRowEst[0]);
}
/* Add in the estimated cost of sorting the result. This cost is expanded
** by a fudge factor of 3.0 to account for the fact that a sorting step
** involves a write and is thus more expensive than a lookup step.
*/
if( bSort ){
cost += cost*estLog(cost);
cost += nRow*estLog(nRow)*(double)3;
}
/* If all information can be taken directly from the index, we avoid
** doing table lookups. This reduces the cost by half. (Not really -
** this needs to be fixed.)
*/
if( pIdx && bLookup==0 ){
cost /= (double)2;
}
/**** Cost of using this index has now been computed ****/
/* If there are additional constraints on this table that cannot