diff options
Diffstat (limited to 'src/where.c')
| -rw-r--r-- | src/where.c | 17 |
1 files changed, 16 insertions, 1 deletions
diff --git a/src/where.c b/src/where.c index ec00482e9..9f35dce02 100644 --- a/src/where.c +++ b/src/where.c @@ -3042,8 +3042,23 @@ static int whereLoopAddBtree( /* Full table scan */ pNew->iSortIdx = b ? iSortIdx : 0; - /* TUNING: Cost of full table scan is (N*3.0). */ + /* TUNING: Cost of full table scan is 3.0*N. The 3.0 factor is an + ** extra cost designed to discourage the use of full table scans, + ** since index lookups have better worst-case performance if our + ** stat guesses are wrong. Reduce the 3.0 penalty slightly + ** (to 2.75) if we have valid STAT4 information for the table. + ** At 2.75, a full table scan is preferred over using an index on + ** a column with just two distinct values where each value has about + ** an equal number of appearances. Without STAT4 data, we still want + ** to use an index in that case, since the constraint might be for + ** the scarcer of the two values, and in that case an index lookup is + ** better. + */ +#ifdef SQLITE_ENABLE_STAT4 + pNew->rRun = rSize + 16 - 2*((pTab->tabFlags & TF_HasStat4)!=0); +#else pNew->rRun = rSize + 16; +#endif ApplyCostMultiplier(pNew->rRun, pTab->costMult); whereLoopOutputAdjust(pWC, pNew, rSize); rc = whereLoopInsert(pBuilder, pNew); |