MySQL Partition Pruning PARTITION BY LIST

Question

I have created a MySQL table which is partitioned by the months of a date column like this:

CREATE TABLE `my_big_table` (
  `some_id` varchar(60) COLLATE utf8_unicode_ci NOT NULL,
  `some_value` varchar(256) COLLATE utf8_unicode_ci NOT NULL,
  `some_other_value` varchar(256) COLLATE utf8_unicode_ci NOT NULL,
  `time` datetime NOT NULL
) ENGINE=MyISAM DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci
PARTITION BY LIST(MONTH(time))
(PARTITION p1 VALUES IN (1) ENGINE = MyISAM,
 PARTITION p2 VALUES IN (2) ENGINE = MyISAM,
 PARTITION p3 VALUES IN (3) ENGINE = MyISAM,
 PARTITION p4 VALUES IN (4) ENGINE = MyISAM,
 PARTITION p5 VALUES IN (5) ENGINE = MyISAM,
 PARTITION p6 VALUES IN (6) ENGINE = MyISAM,
 PARTITION p7 VALUES IN (7) ENGINE = MyISAM,
 PARTITION p8 VALUES IN (8) ENGINE = MyISAM,
 PARTITION p9 VALUES IN (9) ENGINE = MyISAM,
 PARTITION p10 VALUES IN (10) ENGINE = MyISAM,
 PARTITION p11 VALUES IN (11) ENGINE = MyISAM,
 PARTITION p12 VALUES IN (12) ENGINE = MyISAM)

Now, when I run

explain partitions select * from my_big_table where month(time) = 2 limit 10;

I get as output:

id  select_type table       partitions              type    possible_keys   key key_len ref rows        Extra
1   SIMPLE      my_big_table    p1,p2,p3,p4,p5,p6,p7,p8,p9,p10,p11,p12  ALL NULL        NULL    NULL    NULL    10762868    "Using where"

So apparently all partitions are being read, and I wonder why. Shouldn't the engine be clever enough to figure out that matching values can only be found in partition p2? I get the same output for a date based query, like

explain partitions select * from my_big_table where date(time) = '2013-02-01' limit 10;

Any clues?

Cheers, Alex

Answer 1

Ok, after some more googling and some more trial and error:
The issue appears to be with the month(...) or date(...) statement in the where clause of my query. Querying

explain partitions select * from my_big_table where time = '2013-02-01 00:00:00' limit 10;

only scans partition p2 as expected. So it seems like partition pruning only works if the raw value of a partition column is part of the where clause.

Cheers, Alex

Answer 2

All flavors except PARTITION BY RANGE are useless. This is especially the case when you have more than a single value (as you found out).

Instead, consider PARTITION BY RANGE(TO_DAYS(time)) and use WHERE time >= '2014-02-01' AND time < '2014-02-01' + INTERVAL 7 MONTH (as an example of scanning one month). Do not use MONTH(time).

More discussion

Answer 3

You should never use a function on the table value month(time) = 2, MySQL will always investigate all values (ie full table scan) on such queries.

Normally you find this kind of "problem" related to indexes not being used but the same is true for partition pruning.