mysql查询优化相关技巧 - 数据库编程

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 | | ALL | NULL | NULL | NULL | NULL | NULL | | +------+--------------+----------------+-------+---------------+---------+---------+-------+------+---

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 | | ALL | NULL | NULL | NULL | NULL | NULL | |
+----+--------------------+------------+--------+-------------------+---------+---------+-------+------+-

U NION 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

mysql查询优化相关技巧(一)