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MySQL慢查詢查找和調(diào)優(yōu)測試

更新時間：2013年01月02日 11:08:58 作者：

MySQL慢查詢查找和調(diào)優(yōu)測試,接下來詳細介紹，需要了解的朋友可以參考下

編輯 my.cnf或者my.ini文件，去除下面這幾行代碼的注釋：

 
log_slow_queries = /var/log/mysql/mysql-slow.log 
long_query_time = 2 
log-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。
這樣做之后，對mysql-slow.log文件執(zhí)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(zhí)行explain：

復(fù)制代碼代碼如下:

explain low_query

復(fù)制代碼代碼如下:

 
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 s 
INNER JOIN INFORMATION_SCHEMA.TABLES t ON s.TABLE_SCHEMA = t.TABLE_SCHEMA AND s.TABLE_NAME = t.TABLE_NAME 
INNER JOIN ( 
SELECT TABLE_SCHEMA, TABLE_NAME, INDEX_NAME, MAX(SEQ_IN_INDEX) AS max_columns 
FROM INFORMATION_SCHEMA.STATISTICS 
WHERE 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; 