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 | +----+-------------+----------------+-------+---------------+---------+---------+-------+------+-----
DEPENDENT SUBQUERY：子查询中的第一个SELECT，取决于外面的查询
explain select name from car_info where id in (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 | index | NULL | name | 768 | NULL | 145 | Using where; Using index | | 2 | DEPENDENT SUBQUERY | web_car_series | const | PRIMARY | PRIMARY | 4 | const | 1 | Using index | +----+--------------------+----------------+-------+---------------+---------+---------+-------+-----
DERIVED：在from列表中包含子查询，mysql会递归的执行该子查询，并把结果放在临时表中
explain select * from (select name from car_info where id = 100) a; +----+-------------+------------+--------+---------------+---------+---------+------+------+- | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+--------+---------------+---------+---------+------+------+- | 1 | PRIMARY | | system | NULL | NULL | NULL | NULL | 1 | | | 2 | DERIVED | car_info | const | PRIMARY | PRIMARY | 8 | | 1 | | +----+-------------+------------+--------+---------------+---------+---------+------+------+-
type列： MySQL 在表里找到所需行的方式包括如下几张(由左至右，由最差到最好)： All-->index-->range-->ref -->eq_ref-->const,system -->null
ALL：进行全数据表扫描 index：按照索引的次序扫描表，先读索引，然后读取具体的数据行，其实还是全表扫描，好处在于不用排序，按照索引的顺序 range：按照某个范围读取数据行 ref：非唯一性索引访问 eq_ref：使用唯一性索引访问(主键或者唯一性索引) const：最多只有一个匹配行，const常用于数值比较如 primary key
null：在优化过程中已经得到结果，不需要访问表或者索引 如：explain select min(id) from car_info;
possible_keys列： possible_keys列指出MySQL能使用哪个索引在该表中找到行。注意，该列完全独立于EXPLAIN输出所示的表的次序。这意味着在possible_keys中的某些键实际上不能按生成的表次序使用。
如果该列是NULL，则没有相关的索引。在这种情况下，可以通过检查WHERE子句看是否它引用某些列或适合索引的列来提高你的查询性能。如果是这样，创造一个适当的索引并且再次用EXPLAIN检查查询
key列 key列显示MySQL实际决定使用的键(索引)。要想强制MySQL使用或忽视possible_keys列中的索引，在查询中使用FORCE INDEX、USE INDEX或者IGNORE INDEX。
key_len列 key_len列显示MySQL决定使用的键长度。使用的索引的长度，在不损失精确性的情况下，长度越短越好
rows列 rows列显示MySQL认为它执行查询时必须检查的行数
mysql相关优化技巧
尽量使用数据类型相同的数据列进行比较
使带索引的数据列在比较表达式中单独出现
不要在like模式的开始位置使用通配符，此时索引无效
尽量使用数值操作，少使用字符串操作
数据类型合理选用，尽量"小"，选择适用于存储引擎的数据格式
尽量将数据列声明为NOT NULL ，因为MYSQL不需要在查询处理期间检查数据列值是否为NULL
考虑使用ENUM数据列，ENUM在MYSQL内部被表示为一系列数值，处理速度快
利用Procedure analyse()语句 该语句可以将数据列中可以采用ENUM方式字段列出，procedure analyse(16,256)语句表示数据列中不同取值超过16个的或者长度超过256个字节的，不提出ENUM类型的建议
对容易产生碎片化的数据表进行整理，对于可变长度的数据列，随着数据的大量修改或者删除极易产生碎片，因此需要定期optimize table
尽量避免对BLOB或TEXT值进行索引