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MDEV-31067: selectivity_from_histogram >1.0 for a DOUBLE_PREC_HB histogram

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Variant #2.

When Histogram::point_selectivity() sees that the point value of interest
falls into one bucket, it tries to guess whether the bucket has many
different (unpopular) values or a few popular values. (The number of
rows is fixed, as it's a Height-balanced histogram).
The basis for this guess is the "width" of the value range the bucket
covers. Buckets covering wider value ranges are assumed to contain
values with proportionally lower frequencies.

This is just a [brave] guesswork. For a very narrow bucket, it may
produce an estimate that's larger than total #rows in the bucket
or even in the whole table.

Remove the guesswork and replace it with basic logic: return
either the per-table average selectivity of col=const, or selectivity
of one bucket, whichever is lower.
This commit is contained in:
Sergei Petrunia
2023-04-19 15:15:27 +03:00
parent bc970573b3
commit 85cc831880
5 changed files with 234 additions and 53 deletions
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6
mysql-test/main/selectivity_no_engine.result
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@ -36,12 +36,12 @@ test.t2 analyze status OK
# The following two must have the same in 'Extra' column:
explain extended select * from t2 where col1 IN (20, 180);
id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE t2 ALL NULL NULL NULL NULL 1100 1.35 Using where
1 SIMPLE t2 ALL NULL NULL NULL NULL 1100 1.00 Using where
Warnings:
Note 1003 select `test`.`t2`.`col1` AS `col1` from `test`.`t2` where `test`.`t2`.`col1` in (20,180)
explain extended select * from t2 where col1 IN (180, 20);
id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE t2 ALL NULL NULL NULL NULL 1100 1.35 Using where
1 SIMPLE t2 ALL NULL NULL NULL NULL 1100 1.00 Using where
Warnings:
Note 1003 select `test`.`t2`.`col1` AS `col1` from `test`.`t2` where `test`.`t2`.`col1` in (180,20)
drop table t1, t2;
@ -102,7 +102,7 @@ test.t1 analyze status Engine-independent statistics collected
test.t1 analyze status OK
explain extended select * from t1 where col1 in (1,2,3);
id select_type table type possible_keys key key_len ref rows filtered Extra
1 SIMPLE t1 ALL NULL NULL NULL NULL 10000 3.37 Using where
1 SIMPLE t1 ALL NULL NULL NULL NULL 10000 2.97 Using where
Warnings:
Note 1003 select `test`.`t1`.`col1` AS `col1` from `test`.`t1` where `test`.`t1`.`col1` in (1,2,3)
# Must not cause fp division by zero, or produce nonsense numbers: