diff options
Diffstat (limited to 'sql/sql_statistics.cc')
| -rw-r--r-- | sql/sql_statistics.cc | 13 |
1 files changed, 8 insertions, 5 deletions
diff --git a/sql/sql_statistics.cc b/sql/sql_statistics.cc index be011adb60c..5aca25d7a1b 100644 --- a/sql/sql_statistics.cc +++ b/sql/sql_statistics.cc @@ -3932,7 +3932,8 @@ double Histogram::point_selectivity(double pos, double avg_sel) /* So: - each bucket has the same #rows - - values are unformly distributed across the [min_value,max_value] domain. + - We assume that values are unformly distributed across the + [min_value,max_value] domain. If a bucket has value range that's N times bigger then average, than each value will have to have N times fewer rows than average. @@ -3940,11 +3941,13 @@ double Histogram::point_selectivity(double pos, double avg_sel) sel= avg_sel * avg_bucket_width / current_bucket_width; /* - (Q: if we just follow this proportion we may end up in a situation - where number of different values we expect to find in this bucket - exceeds the number of rows that this histogram has in a bucket. Are - we ok with this or we would want to have certain caps?) + Note that this adjustment is just a (brave?) heuristic. What we know for + certain is that the searched value fits into one histogram bucket. Do not + return an estimate larger than that. */ + double bucket_sel= 1.0/(get_width() + 1); + if (sel >= bucket_sel) + sel= bucket_sel; } return sel; } |