MySQL 5.1 using filesort event when an index is present

问题

Probably I'm missing some silly thing... Apparently MySQL 5.1 keeps doing a Filesort even when there is an index that matches exactly the column in the ORDER BY clause. To post it here, I've oversimplified the data model, but the issue is still happening:

Table definition:

CREATE TABLE `event` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `name` varchar(255) DEFAULT NULL,
  `owner_id` int(11) DEFAULT NULL,
  `date_created` datetime DEFAULT NULL,
  PRIMARY KEY (`id`),
  KEY `owner_id` (`owner_id`),
  KEY `date_created` (`date_created`),
  CONSTRAINT `event_ibfk_1` FOREIGN KEY (`owner_id`) REFERENCES `user_profile` (`id`)
) ENGINE=InnoDB AUTO_INCREMENT=7 DEFAULT CHARSET=utf8;

My problem is that event a simple SELECT is showing "Using filesort":

explain select * from event order by date_created desc;

And the result for the query explain:

id  select_type table   type    possible_keys   key key_len ref rows    Extra   
1   SIMPLE      event   ALL NULL        NULL    NULL    NULL    6       Using filesort

Is there any way for this type of queries to use the index insteas of doing a filesort?

Thanks in advance to everybody.

回答1:

Since your CREATE TABLE statement indicates that you have less than 10 rows (AUTO_INCREMENT=7) and using FORCE INDEX on my installation will make MySQL use the index, I'm guessing the optimizer thinks a table scan is faster (less random I/O) than an index scan (since you're selecting all columns, not just date_created). This is confirmed by the following:

mysql> explain select date_created from event order by date_created;
+----+-------------+-------+-------+---------------+--------------+---------+------+------+-------------+
| id | select_type | table | type  | possible_keys | key          | key_len | ref  | rows | Extra       |
+----+-------------+-------+-------+---------------+--------------+---------+------+------+-------------+
|  1 | SIMPLE      | event | index | NULL          | date_created | 9       | NULL |    1 | Using index |
+----+-------------+-------+-------+---------------+--------------+---------+------+------+-------------+
1 row in set (0.00 sec)

In the above case, the index scan is faster because only the indexed column needs to be returned.

The MySQL documentation has some cases where using an index is considered slower: http://dev.mysql.com/doc/refman/5.1/en/how-to-avoid-table-scan.html

回答2:

Q: Is there any way for this type of queries to use the index instead of doing a filesort?

A: To have MySQL use the index if at all possible, try:

EXPLAIN SELECT * FROM event FORCE INDEX (date_created) ORDER BY date_created DESC;

By using the FORCE INDEX (index_name), this tells MySQL to make use of the index if it's at all possible. Absent that directive, MySQL will choose the most efficient way to return the result set. A filesort may be more efficient than using the index.

来源：https://stackoverflow.com/questions/6114880/mysql-5-1-using-filesort-event-when-an-index-is-present

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