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：
explain low_query
你將看到下面的結(jié)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配置文件來實現(xiàn)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 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;
