【翻译】查找和调优MySQL慢查询_MySQL

翻译自：http://parand.com/say/index.php/2009/09/01/finding-and-fixing-slow-mysql-queries/bar.html

编辑 my.cnf或者my.ini文件，去除下面这几行代码的注释：

log_slow_queries        = /var/log/mysql/mysql-slow.loglong_query_time = 2log-queries-not-using-indexes

这将使得慢查询和没有使用索引的查询被记录下来。

这样做之后，对mysql-slow.log文件执行tail -f命令，将能看到其中记录的慢查询和未使用索引的查询。

随便提取一个慢查询，执行explain：

explain low_query

你将看到下面的结果：

+----+-------------+---------------------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table               | type | possible_keys | key  | key_len | ref  | rows | Extra       |
+----+-------------+---------------------+------+---------------+------+---------+------+------+-------------+
|  1 | SIMPLE      | some_table          | ALL  | NULL          | NULL | NULL    | NULL |  166 | Using where |
+----+-------------+---------------------+------+---------------+------+---------+------+------+-------------+

注意上面的rows和key列。rows显示该查询影响了多少行记录，我们不想让这个值太大。key显示用了哪个索引，为NULL时表示查询未用任何索引。

如果想让查询更快，你或许需要为某些列增加索引：

CREATE INDEX myapp_mytable_myfield_idx on myapp_mytable(myfield);

除了配置mysql配置文件来实现记录慢查询外，还有下面的方法可以记录慢查询：

SELECT t.TABLE_SCHEMA AS `db`,t.TABLE_NAME AS `table`,s.INDEX_NAME AS `index name`,s.COLUMN_NAME AS `FIELD name`,s.SEQ_IN_INDEX `seq IN index`,s2.max_columns AS `# cols`,s.CARDINALITY AS `card`,t.TABLE_ROWS AS `est rows`,ROUND(((s.CARDINALITY / IFNULL(t.TABLE_ROWS, 0.01)) * 100), 2) AS `sel %`FROM INFORMATION_SCHEMA.STATISTICS sINNER JOIN INFORMATION_SCHEMA.TABLES t ON s.TABLE_SCHEMA = t.TABLE_SCHEMA AND s.TABLE_NAME = t.TABLE_NAMEINNER JOIN (SELECT TABLE_SCHEMA, TABLE_NAME, INDEX_NAME, MAX(SEQ_IN_INDEX) AS max_columnsFROM INFORMATION_SCHEMA.STATISTICSWHERE TABLE_SCHEMA != 'mysql' GROUP BY TABLE_SCHEMA, TABLE_NAME, INDEX_NAME ) AS s2 ON s.TABLE_SCHEMA = s2.TABLE_SCHEMA AND s.TABLE_NAME = s2.TABLE_NAME AND s.INDEX_NAME = s2.INDEX_NAMEWHERE t.TABLE_SCHEMA != 'mysql' /* Filter out the mysql system DB */AND t.TABLE_ROWS > 10 /* Only tables with some rows */AND s.CARDINALITY IS NOT NULL /* Need at least one non-NULL value in the field */AND (s.CARDINALITY / IFNULL(t.TABLE_ROWS, 0.01)) < 1.00 /* unique indexes are perfect anyway */ORDER BY `sel %`, s.TABLE_SCHEMA, s.TABLE_NAME /* DESC for best non-unique indexes */LIMIT 10;