MySQL进阶
01、MySQL进阶：剖析MySQL索引底层数据结构
02、MySQL进阶：MySQL不同存储引擎下索引的实现
03、MySQL进阶：Explain深度剖析
04、MySQL进阶：践行索引优化
05、MySQL进阶：锁等待及死锁初探
06、MySQL进阶：无索引行锁升级为表锁
07、MySQL进阶：共享锁和排它锁初探
08、MySQL进阶：索引优化案例实操
09、MySQL进阶：索引下推IndexConditionPushdown初探
10、MySQL进阶：使用trace工具来窥探MySQL是如何选择执行计划的
11、MySQL进阶：orderby和groupby优化初探
12、MySQL进阶：分页查询优化的两个案例解析
13、MySQL进阶：Join关联查询优化
14、MySQL进阶：In和Exists的优化案例讲解
15、MySQL进阶：存储引擎初探
16、MySQL进阶：体系结构初探
17、MySQL进阶：解读MySQL事务与锁机制
18、MySQL进阶：多版本控制MVCC机制初探
19、MySQL进阶：并发事务问题及解决方案
20、MySQL进阶：锁机制初探
21、MySQL进阶：高效的设计MySQL库表
22、MySQL进阶：库表设计之IP和TIMESTAMP的处理
23、MySQL进阶：orderby出现usingfilesort根因分析及优化
24、MySQL进阶：canal实现mysql数据同步到redis｜实现自定义canal客户端
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03、MySQL进阶：Explain深度剖析
### **官方文档** [https://dev.mysql.com/doc/refman/5.7/en/explain-output.html](https://dev.mysql.com/doc/refman/5.7/en/explain-output.html "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** ```python mysql> select version(); +------------+ | version() | +------------+ | 5.7.29-log | +------------+ 1 row in set (0.00 sec) mysql> ``` ```python 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**.） ```python 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** ```python 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** ```python 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 ```python 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 ```python 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） ```python 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 ```python 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 ```python 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 ```python 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。** ```python 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 ```python 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. ```python 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。 ```python 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）】** ```python 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。】 ```python 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。 ```python 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 ```python 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 ```python 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。 ```python 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） ```python 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) ``` ```python 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。 ```python 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 ```python 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； ```python 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】 ```python 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】 ```python 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】 ```python 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】 ```python 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 
李智
2025年3月17日 13:31
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