MySQL inconsistent table scan results

Posted by user148207 on Stack Overflow See other posts from Stack Overflow or by user148207
Published on 2010-03-26T21:24:43Z Indexed on 2010/03/26 21:53 UTC
Read the original article Hit count: 272

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What's going on here?

mysql> select count(*) from notes where date(updated_at) > date('2010-03-25');
+----------+
| count(*) |
+----------+
|        0 | 
+----------+
1 row in set (0.59 sec)

mysql> select count(*) from notes where message like'%***%' and date(updated_at) > date('2010-03-25');
+----------+
| count(*) |
+----------+
|       26 | 
+----------+
1 row in set (1.30 sec)

mysql> explain select count(*) from notes where date(updated_at) > date('2010-03-25');
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | notes | ALL  | NULL          | NULL | NULL    | NULL | 588106 | Using where | 
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.07 sec)

mysql> explain select updated_at from notes where message like'%***%' and date(updated_at) > date('2010-03-25');
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
| id | select_type | table | type | possible_keys | key  | key_len | ref  | rows   | Extra       |
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
|  1 | SIMPLE      | notes | ALL  | NULL          | NULL | NULL    | NULL | 588106 | Using where | 
+----+-------------+-------+------+---------------+------+---------+------+--------+-------------+
1 row in set (0.09 sec)

mysql> 

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