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 = 2
log-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 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;
