mysql查询优化相关技巧 用EXPLAIN语句检查优化器操作

  次阅读 作者:智能小宝 来源:互联网 2016-01-27 15:49 我要评论(0)

使用EXPLAIN语句检查优化器操作 +----+-------------+----------+-------+---------------+------+---------+------+------+----------------- | id | select_type | table | type | possible_keys | key | key_len| ref | rows | Extra +----+-------------+----------+-------+---------------+------+---------+------+------+----------------- | 1 |SIMPLE | car_info | range | name | name | 768 | NULL | 9 | Using where; Using index | +----+-------------+----------+-------+---------------+------+---------+------+------+----------------

EXPLAIN输出解释

select_type 有如下几种类型: SIMPLE:未使用连接查询或者子查询的简单select语句 explain select * from car_info;

PRIMARY:最外层的select语句 explain select * from (select name from car_info where name like '凯迪拉克%') as a;

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

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

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

| 1 | PRIMARY | | ALL | NULL | NULL | NULL | NULL | 9 | |

| 2 | DERIVED | car_info | range | name | name | 768 | NULL | 9 | Using where; Using index |

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

UNION:union中的第二个,或后面的select语句 explain select name from car_info where id =100 union select name from web_car_brands where id =5; +------+--------------+----------------+-------+---------------+---------+---------+-------+------+-- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+--------------+----------------+-------+---------------+---------+---------+-------+------+--- | 1 | PRIMARY | car_info | const | PRIMARY | PRIMARY | 8 | const | 1 | | | 2 | UNION | web_car_brands | const | PRIMARY,id | PRIMARY | 4 | const | 1 | | | NULL | UNION RESULT |

DEPENDENT UNION:union中的第二个或后面的色了传统语句,取决于外面的查询

mysql> explain select * from t3 where id in (select id from t3 where id=3952602 union all select id from t3) ;

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

| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |

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

| 1 | PRIMARY | t3 | ALL | NULL | NULL | NULL | NULL | 1000 | Using where

| 2 | DEPENDENT SUBQUERY | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | const | 1 | Using index |

| 3 | DEPENDENT UNION | t3 | eq_ref | PRIMARY,idx_t3_id | PRIMARY | 4 | func | 1 | Using where; Using index |

|NULL | UNION RESULT |

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

UNION RESULT:union的结果 explain select name from car_info where id =100 union select name from web_car_brands where id =5; +------+--------------+----------------+-------+---------------+---------+---------+-------+------+----- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +------+--------------+----------------+-------+---------------+---------+---------+-------+------+-- | 1 | PRIMARY | car_info | const | PRIMARY | PRIMARY | 8 | const | 1 | | | 2 | UNION | web_car_brands | const | PRIMARY,id | PRIMARY | 4 | const | 1 | | | NULL | UNION RESULT |

SUBQUERY:子查询中的第一个SELECT语句 explain select name from car_info where id = (select id from web_car_series where id = 5); +----+-------------+----------------+-------+---------------+---------+---------+-------+------+----- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+----------------+-------+---------------+---------+---------+-------+------+------ | 1 | PRIMARY | car_info | const | PRIMARY | PRIMARY | 8 | const | 1 | | | 2 | SUBQUERY | web_car_series | const | PRIMARY | PRIMARY | 4 | | 1 | Using index | +----+-------------+----------------+-------+---------------+---------+---------+-------+------+-----

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