文章目录
- 生猛干货
- 官方文档
- Explain介绍
- 测试数据
- explain 使用
- explain重要列说明
-
- id
- select_type
-
- simple
- primary
- subquery
- derived
- union
- table
- type
-
- NULL
- const, system
- eq_ref
- ref
- range
- index
- ALL
- possible_keys
- key
- key_len
-
- key_len计算规则
- ref
- rows
- Extra
-
- Using index
- Using where
- Using index condition
- Using temporary
- Using filesort
- Select tables optimized away
- 搞定MySQL实战
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官方文档
https://dev.mysql.com/doc/refman/5.7/en/explain-output.html
Explain介绍
使用EXPLAIN关键字可以模拟优化器执行SQL语句,分查询语句或是结构的性能瓶颈
在select 语句之前增加 explain 关键字,MySQL 会在查询上设置一个标记,执行查询会返回执行计划的信息,而不是执行这条SQL。
如果from 中包含子查询,仍会执行该子查询,将结果放入临时表中 。
测试数据
DBVersion
mysql> select version();
+------------+
| version() |
+------------+
| 5.7.29-log |
+------------+
1 row in set (0.00 sec)
mysql>
DROP TABLE IF EXISTS actor;
CREATE TABLE actor (
id int(11) NOT NULL,
name varchar(45) DEFAULT NULL,
update_time datetime DEFAULT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO actor (id, name, update_time) VALUES (1,'a','2017-12-22 15:27:18'), (2,'b','2017-12-22 15:27:18'), (3,'c','2017-12-22 15:27:18');
###############################
DROP TABLE IF EXISTS film;
CREATE TABLE film (
id int(11) NOT NULL AUTO_INCREMENT,
name varchar(10) DEFAULT NULL,
PRIMARY KEY (id),
KEY idx_name (name)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO film (id, name) VALUES (3,'film0'),(1,'film1'),(2,'film2');
###############################
DROP TABLE IF EXISTS film_actor;
CREATE TABLE film_actor (
id int(11) NOT NULL,
film_id int(11) NOT NULL,
actor_id int(11) NOT NULL,
remark varchar(255) DEFAULT NULL,
PRIMARY KEY (id),
KEY idx_film_actor_id (film_id,actor_id)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;
INSERT INTO film_actor (id, film_id, actor_id) VALUES (1,1,1),(2,1,2),(3,2,1);
explain 使用
explain 两个扩展的使用
explain extended: 提供: 额外一些查询优化的信息 (‘EXTENDED’ is deprecated and will be removed in a future release.)
mysql> explain extended select * from film where id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | film | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 2 warnings (0.00 sec)
# 2 warnings
# 可以通过 show warnings 命令查看
mysql> show warnings;
+---------+------+----------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------------------------------------------------+
| Warning | 1681 | 'EXTENDED' is deprecated and will be removed in a future release. |
| Note | 1003 | /* select#1 */ select '1' AS id,'film1' AS name from dbtest.film where 1 |
+---------+------+----------------------------------------------------------------------------------+
2 rows in set (0.00 sec)
mysql>
filtered 列: 百分比,计算公式 rows * filtered/100 可以估算出将要和 explain 中前一个表进行连接的行数(前一个表指 explain 中的id值比当前表id值小的表) , 供参考
第二个**‘PARTITIONS’ is deprecated and will be removed in a future**
mysql> explain partitions select * from film where id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | film | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 2 warnings (0.00 sec)
mysql> show warnings;
+---------+------+----------------------------------------------------------------------------------+
| Level | Code | Message |
+---------+------+----------------------------------------------------------------------------------+
| Warning | 1681 | 'PARTITIONS' is deprecated and will be removed in a future release. |
| Note | 1003 | /* select#1 */ select '1' AS id,'film1' AS name from dbtest.film where 1 |
+---------+------+----------------------------------------------------------------------------------+
2 rows in set (0.00 sec)
mysql>
所以只使用explain就足够了 。
explain重要列说明
mysql> explain select * from film_actor a where a.actor_id = (select id from actor where name = 'a');
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | a | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
| 2 | SUBQUERY | actor | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
id
id列的编号是 select 的序列号,有几个 select 就有几个id,并且id的顺序是按 select 出现的顺序增长的。
执行顺序:
id越大执行优先级越高,id相同则从上往下执行,id为NULL最后执行
select_type
表示的对应行是简单还是复杂的查询
simple
简单查询,查询不包含子查询和union
mysql> explain select * from film where id=1;
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 1 | SIMPLE | film | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+-------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql>
primary
复杂查询中最外层的 select
mysql> explain select * from film_actor a where a.actor_id = (select id from actor where name = 'a');
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | a | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
| 2 | SUBQUERY | actor | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
subquery
包含在select 中的子查询(不在 from 子句中)
mysql> explain select * from film_actor a where a.actor_id = (select id from actor where name = 'a');
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | PRIMARY | a | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
| 2 | SUBQUERY | actor | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
derived
包含在 from 子句中的子查询。
MySQL会将结果存放在一个临时表中,也称为派生表
derived: 衍生的
mysql> set session optimizer_switch='derived_merge=off'; #关闭mysql5.7新特性对衍生表的合并优化
Query OK, 0 rows affected (0.00 sec)
mysql> explain select (select 1 from actor where id = 1 ) from (select * from film where id =1 ) t ;
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
| 1 | PRIMARY | <derived3> | NULL | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL |
| 3 | DERIVED | film | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
| 2 | SUBQUERY | actor | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | Using index |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)
mysql>
union
在union 中的第二个和随后的 select
mysql> EXPLAIN select 1 union select 1 ;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
| 2 | UNION | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
mysql>
table
表示explain 的一行正在访问哪个表
mysql> explain select (select 1 from actor where id = 1 ) from (select * from film where id =1 ) t ;
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
| 1 | PRIMARY | <derived3> | NULL | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL |
| 3 | DERIVED | film | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
| 2 | SUBQUERY | actor | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | Using index |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+
3 rows in set, 1 warning (0.00 sec)
当from 子句中有子查询时,table列是 <derivenN>
格式,表示当前查询依赖 id=N 的查询,于是先执行 id=N 的查询。
mysql> EXPLAIN select 1 union select 1 ;
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | PRIMARY | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
| 2 | UNION | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | No tables used |
| NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------------+------+---------------+------+---------+------+------+----------+-----------------+
3 rows in set, 1 warning (0.00 sec)
mysql>
当有 union 时,UNION RESULT 的 table 列的值为<union1,2>,1和2表示参与 union 的 select 行id。
type
表示关联类型或访问类型,即MySQL决定如何查找表中的行,查找数据行记录的大概范围。
依次从最优到最差分别为:system > const > eq_ref > ref > range > index > ALL
一般来说,得保证查询达到range级别,最好达到ref
NULL
mysql能够在优化阶段分解查询语句,在执行阶段用不着再访问表或索引.
例如:在索引列中选取最小值,可以单独查找索引来完成,不需要在执行时访问表
mysql> explain select min(id) from actor;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
| 1 | SIMPLE | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | NULL | Select tables optimized away |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+------------------------------+
1 row in set, 1 warning (0.00 sec)
const, system
mysql能对查询的某部分进行优化并将其转化成一个常量(可以看show warnings 的结果)。用于primary key 或 unique key 的所有列与常数比较时,所以表最多有一个匹配行,读取1次,速度比较快。
system是const的特例,表里只有一条元组匹配时为system.
mysql> EXPLAIN select * from (select * from film where id=1) t ;
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+
| 1 | PRIMARY | <derived2> | NULL | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL |
| 2 | DERIVED | film | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL |
+----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
mysql> show warnings;
+-------+------+---------------------------------------------------------------+
| Level | Code | Message |
+-------+------+---------------------------------------------------------------+
| Note | 1003 | /* select#1 */ select '1' AS id,'film1' AS name from dual |
+-------+------+---------------------------------------------------------------+
1 row in set (0.00 sec)
mysql>
eq_ref
primary key 或 unique key 索引的所有部分被连接使用 ,最多只会返回一条符合条件的记录。这可能是在const 之外最好的联接类型了,简单的 select 查询不会出现这种 type。
mysql> EXPLAIN select * from film_actor a left join film b on a.film_id = b.id ;
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------+
| 1 | SIMPLE | a | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100.00 | NULL |
| 1 | SIMPLE | b | NULL | eq_ref | PRIMARY | PRIMARY | 4 | dbtest.a.film_id | 1 | 100.00 | NULL |
+----+-------------+-------+------------+--------+---------------+---------+---------+------------------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
mysql> show warnings;
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note | 1003 | /* select#1 */ select dbtest.a.id AS id,dbtest.a.film_id AS film_id,dbtest.a.actor_id AS actor_id,dbtest.a.remark AS remark,dbtest.b.id AS id,dbtest.b.name AS name from dbtest.film_actor a left join dbtest.film b on((dbtest.b.id = dbtest.a.film_id)) where 1 |
+-------+------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)
mysql>
ref
相比 eq_ref,不使用唯一索引,而是使用普通索引或者唯一性索引的部分前缀,索引要和某个值相比较,可能会找到多个符合条件的行.
【简单 select 查询,name是普通索引(非唯一索引)】
mysql> show INDEX from film ;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| film | 0 | PRIMARY | 1 | id | A | 3 | NULL | NULL | | BTREE | | |
| film | 1 | idx_name | 1 | name | A | 3 | NULL | NULL | YES | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
2 rows in set (0.00 sec)
mysql>
mysql> EXPLAIN select * from film a where a.name = 'film0';
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | a | NULL | ref | idx_name | idx_name | 33 | const | 1 | 100.00 | Using index |
+----+-------------+-------+------------+------+---------------+----------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql>
【关联表查询,idx_film_actor_id是film_id和actor_id的联合索引,这里使用到了film_actor的左边前缀film_id部分。】
mysql> show index from film_actor;
+------------+------------+-------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+------------+------------+-------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| film_actor | 0 | PRIMARY | 1 | id | A | 3 | NULL | NULL | | BTREE | | |
| film_actor | 1 | idx_film_actor_id | 1 | film_id | A | 2 | NULL | NULL | | BTREE | | |
| film_actor | 1 | idx_film_actor_id | 2 | actor_id | A | 3 | NULL | NULL | | BTREE | | |
+------------+------------+-------------------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
3 rows in set (0.00 sec)
mysql>
mysql> EXPLAIN select film_id from film left join film_actor on film.id =film_actor.film_id ;
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+----------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+----------------+------+----------+-------------+
| 1 | SIMPLE | film | NULL | index | NULL | idx_name | 33 | NULL | 3 | 100.00 | Using index |
| 1 | SIMPLE | film_actor | NULL | ref | idx_film_actor_id | idx_film_actor_id | 4 | dbtest.film.id | 1 | 100.00 | Using index |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+----------------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
mysql>
range
范围扫描通常出现在 in(), between ,> ,<, >= 等操作中。使用一个索引来检索给定范围的行。
mysql> explain select * from actor where id > 1 ;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | actor | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 2 | 100.00 | Using where |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
index
扫描全索引就能拿到结果,一般是扫描某个二级索引,这种扫描不会从索引树根节点开始快速查找,而是直接对二级索引的叶子节点遍历和扫描,速度还是比较慢的,这种查询一般为使用覆盖索引,二级索引一般比较小,所以这种通常比ALL快一些
mysql> explain select * from film;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | film | NULL | index | NULL | idx_name | 33 | NULL | 3 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
ALL
全表扫描,扫描你的聚簇索引的所有叶子节点。通常情况下这需要增加索引来进行优化了
mysql> explain select * from actor ;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | actor | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
possible_keys
显示查询可能使用哪些索引来查找
explain 时可能出现 possible_keys 有列,而 key 显示 NULL 的情况,这种情况是因为表中数据不多,mysql认为索引对此查询帮助不大,选择了全表查询。
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 子句看是否可以创造一个适当的索引来提高查询性能,然后用 explain 查看效果。
key
mysql实际采用哪个索引来优化对该表的访问。
如果没有使用索引,则该列是 NULL。如果想强制mysql使用或忽视possible_keys列中的索引,在查询中使用 force index、ignore index。
key_len
显示了mysql在索引里使用的字节数,通过这个值可以算出具体使用了索引中的哪些列。
举个例子 :
film_actor的联合索引 idx_film_actor_id 由 film_id 和 actor_id 两个int列组成,并且每个int是4字节。
mysql> explain select * from film_actor where film_id=1;
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | film_actor | NULL | ref | idx_film_actor_id | idx_film_actor_id | 4 | const | 2 | 100.00 | NULL |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql>
通过结果中的key_len=4可推断出查询使用了第一个列:film_id列来执行索引查找。
key_len计算规则
【字符串】
- char(n):n字节长度
- varchar(n):如果是utf-8,则长度 3n + 2 字节,加的2字节用来存储字符串长度
【数值类型】
- tinyint:1字节
- smallint:2字节
- int:4字节
- bigint:8字节
【时间类型】
- date:3字节
- timestamp:4字节
- datetime:8字节
如果字段允许为 NULL,需要1字节记录是否为 NULL
索引最大长度是768字节,当字符串过长时,mysql会做一个类似左前缀索引的处理,将前半部分的字符提取出来做索引
ref
显示了在key列记录的索引中,表查找值所用到的列或常量,常见的有:const(常量),字段名(例:film.id)
mysql> explain select * from film_actor where film_id=1;
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | film_actor | NULL | ref | idx_film_actor_id | idx_film_actor_id | 4 | const | 2 | 100.00 | NULL |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN select film_id from film left join film_actor on film.id =film_actor.film_id ;
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+----------------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+----------------+------+----------+-------------+
| 1 | SIMPLE | film | NULL | index | NULL | idx_name | 33 | NULL | 3 | 100.00 | Using index |
| 1 | SIMPLE | film_actor | NULL | ref | idx_film_actor_id | idx_film_actor_id | 4 | dbtest.film.id | 1 | 100.00 | Using index |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+----------------+------+----------+-------------+
2 rows in set, 1 warning (0.00 sec)
rows
mysql估计要读取并检测的行数,注意这个不是结果集里的行数。
Extra
展示的是额外信息。
列举几个常见的值
Using index
使用覆盖索引 : 无需回表
mysql执行计划explain结果里的key有使用索引,如果select后面查询的字段都可以从这个索引的树中获取,这种情况一般可以说是用到了覆盖索引,extra里一般都有using index;
覆盖索引一般针对的是辅助索引,整个查询结果只通过辅助索引就能拿到结果,不需要通过辅助索引树找到主键,再通过主键去主键索引树里获取其它字段值
mysql> explain select film_id from film_actor where film_id=1;
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------------+
| 1 | SIMPLE | film_actor | NULL | ref | idx_film_actor_id | idx_film_actor_id | 4 | const | 2 | 100.00 | Using index |
+----+-------------+------------+------------+------+-------------------+-------------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql>
Using where
使用 where 语句来处理结果,并且查询的列未被索引覆盖
mysql> show index from actor;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| actor | 0 | PRIMARY | 1 | id | A | 3 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)
mysql> explain select * from actor where name = 'a';
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
| 1 | SIMPLE | actor | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 33.33 | Using where |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql>
Using index condition
查询的列不完全被索引覆盖,where条件中是一个前导列的范围;
mysql> explain select * from film_actor where film_id > 1 ;
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+------+------+----------+-----------------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+------+------+----------+-----------------------+
| 1 | SIMPLE | film_actor | NULL | range | idx_film_actor_id | idx_film_actor_id | 4 | NULL | 1 | 100.00 | Using index condition |
+----+-------------+------------+------------+-------+-------------------+-------------------+---------+------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)
mysql>
Using temporary
mysql需要创建一张临时表来处理查询。出现这种情况一般是要进行优化的,首先是想到用索引来优化。
【actor.name没有索引,此时创建了张临时表来distinct】
mysql> explain select distinct name from actor;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
| 1 | SIMPLE | actor | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100.00 | Using temporary |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-----------------+
1 row in set, 1 warning (0.00 sec)
【film.name建立了idx_name索引,此时查询时extra是using index,没有用临时表】
mysql> explain select distinct name from film;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | film | NULL | index | idx_name | idx_name | 33 | NULL | 3 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql>
Using filesort
将用外部排序而不是索引排序,数据较小时从内存排序,否则需要在磁盘完成排序。这种情况下一般也是要考虑使用索引来优化的
【actor.name未创建索引,会浏览actor整个表,保存排序关键字name和对应的id,然后排序name并检索行记录】
mysql> explain select * from actor order by name;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
| 1 | SIMPLE | actor | NULL | ALL | NULL | NULL | NULL | NULL | 3 | 100.00 | Using filesort |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+----------------+
1 row in set, 1 warning (0.00 sec)
mysql>
【film.name建立了idx_name索引,此时查询时extra是using index】
mysql> explain select * from film order by name ;
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
| 1 | SIMPLE | film | NULL | index | NULL | idx_name | 33 | NULL | 3 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+----------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql>
Select tables optimized away
使用某些聚合函数(比如 max、min)来访问存在索引的某个字段
搞定MySQL实战
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