【翻譯】尋找和調優MySQL慢查詢

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

編輯 my.cnf或者my.ini檔案，去除下面這幾行代碼的注釋：

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

這將使得慢查詢和沒有使用索引的查詢被記錄下來。

這樣做之後，對mysql-slow.log檔案執行tail -f命令，將能看到其中記錄的慢查詢和未使用索引的查詢。

隨便提取一個慢查詢，執行explain：

explain low_query

你將看到下面的結果：

+----+-------------+---------------------+------+---------------+------+---------+------+------+-------------+

| 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顯示該查詢影響了多少行記錄，我們不想讓這個值太大。key顯示用了哪個索引，為NULL時表示查詢未用任何索引。

如果想讓查詢更快，你或許需要為某些列增加索引：

CREATE INDEX myapp_mytable_myfield_idx on myapp_mytable(myfield);

除了配置mysql設定檔來實現記錄慢查詢外，還有下面的方法可以記錄慢查詢：

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;