【翻译】查找和调优MySQL慢查询
2013-07-04 11:51
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【翻译】查找和调优MySQL慢查询
2013-05-01 23:12 57人阅读 评论(0) 收藏 举报 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.log long_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 `indexname`,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_NAME WHERE 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;
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