Allow partition pruning when using functions.

> mysql [localhost] {msandbox} (test) > explain partitions select * FROM t2 WHERE time_segment = '2016-04-29 09:30:00';

> +------+-------------+-------+------------+------+---------------+---------+---------+-------+------+-------------+

> | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | Extra |

> +------+-------------+-------+------------+------+---------------+---------+---------+-------+------+-------------+

> | 1 | SIMPLE | t2 | p4 | ref | PRIMARY | PRIMARY | 5 | const | 1 | Using index |

> +------+-------------+-------+------------+------+---------------+---------+---------+-------+------+-------------+

> 1 row in set (0.00 sec)

> mysql [localhost] {msandbox} (test) > explain partitions select * FROM t2 WHERE date(time_segment) = '2016-04-29'; <<<< ***************** You see another pruning problem when using a function (date). *******

> +------+-------------+-------+----------------+-------+---------------+---------+---------+------+------+--------------------------+

> | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | Extra |

> +------+-------------+-------+----------------+-------+---------------+---------+---------+------+------+--------------------------+

> | 1 | SIMPLE | t2 | p0,p1,p2,p3,p4 | index | NULL | PRIMARY | 9 | NULL | 6 | Using where; Using index |

> +------+-------------+-------+----------------+-------+---------------+---------+---------+------+------+--------------------------+

> 1 row in set (0.00 sec)