说到MySQL语句优化，必然就会想到explain，那么，今天我们来详细讲解下explain。
语法
mysql> explain select id from user;
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
| 1 | SIMPLE | user | index | NULL | PRIMARY | 4 | NULL | 8 | Using index |
+----+-------------+-------+-------+---------------+---------+---------+------+------+-------------+
他的用法很简单，直接在select语句前面加上explain就可以了。
参数解释
id
id是SELECT 查询的标识符，每个 SELECT 都会自动分配一个唯一的标识符。该值可能为NULL，如果这一行用来说明的是其他行的联合结果。select_type
SELECT 查询的类型，取值可以有一下几种：
(1) SIMPLE 简单查询，不包含子查询、联合查询
(2) PRIMARY 复杂查询中最外层的查询
(3) DERIVED 包含在from子句中的子查询，eg：explain select id from (select ...)
mysql> explain select uid from (select * from user) b;
+----+-------------+------------+------+---------------+------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+------+---------------+------+---------+------+------+-------+
| 1 | PRIMARY | | ALL | NULL | NULL | NULL | NULL | 8 | NULL |
| 2 | DERIVED | user | ALL | NULL | NULL | NULL | NULL | 8 | NULL |
+----+-------------+------------+------+---------------+------+---------+------+------+-------+
(4) UNION 在UNION查询中第二个和随后的SELECT查询
(5) UNION RESULT UNION查询的结果，id列为NULL
mysql> explain select uid from user union select uid from score;
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
| 1 | PRIMARY | user | ALL | NULL | NULL | NULL | NULL | 8 | NULL |
| 2 | UNION | score | ALL | NULL | NULL | NULL | NULL | 8 | NULL |
| NULL | UNION RESULT | | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------+------------+------+---------------+------+---------+------+------+-----------------+
(6) SUBQUERY 子查询中的第一个SELECT，不在from子句中
mysql> explain select uid from user where uid = (select uid from score);
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
| 1 | PRIMARY | user | ALL | NULL | NULL | NULL | NULL | 8 | Using where |
| 2 | SUBQUERY | score | ALL | NULL | NULL | NULL | NULL | 8 | NULL |
+----+-------------+-------+------+---------------+------+---------+------+------+-------------+
(7) DEPENDENT UNION UNION中第二个或者后面的查询语句，取决于外面的查询
(8) DEPENDENT SUBQUERY 子查询中的第一个SELECT，取决于外面的查询，即子查询依赖外面的查询结果
mysql> explain select uid from user where uid in(select uid from score union select uid from user);
+----+--------------------+------------+------+---------------+------+---------+------+------+-----------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+--------------------+------------+------+---------------+------+---------+------+------+-----------------+
| 1 | PRIMARY | user | ALL | NULL | NULL | NULL | NULL | 8 | Using where |
| 2 | DEPENDENT SUBQUERY | score | ALL | NULL | NULL | NULL | NULL | 8 | Using where |
| 3 | DEPENDENT UNION | user | ALL | NULL | NULL | NULL | NULL | 8 | Using where |
| NULL | UNION RESULT | | ALL | NULL | NULL | NULL | NULL | NULL | Using temporary |
+----+--------------------+------------+------+---------------+------+---------+------+------+-----------------+
此SELECT语句可写成，select uid from user a where exists(select uid from score where score.uid = a.uid union select uid from user where user.uid = a.uid)
table
涉及到的表或派生表
N为id值，指该id值对应的那一步操作的结果
union的结果 type
用来说明表与表之间是如何关联操作的，有没有使用索引。
(1) system 表仅有一行且满足条件
(2) const 针对主键或唯一索引的等值查询，最多只返回1条数据
(3) eq_ref 主键或唯一索引的所有部分都被使用
(4) ref 使用普通索引或唯一索引的部分前缀，可用于=，<=>操作符的带索引的列
(5) fulltext 按全文索引进行搜索
(6) range 使用索引进行范围查询，ref字段为NULL(MySQL无法使用范围列后面的其他索引)
(7) index 全索引扫描
(8) all 全表扫描
性能比较：system > const > eq_ref > ref> range> index > all
possible_keys
能够用到的索引key
真正使用的索引
possible_keys 揭示了哪个索引能有助于高效查找
key 揭示了哪个索引可以最小化查询成本
key_len
索引字节数
char(n) n字节
varchar(n) utf8 (3n+2)字节
utf8mb4 (4n+2)字节
tinyint 1字节
smallint 2字节
mediumint 3字节
int 4字节
bigint 8字节
date 3字节
timestamp 4字节
datetime 8字节rows
检查的行数，是一个预估值(取第1页和最后一页的数据，以及往右再取8个页的数据，计算这10页的平均值就是所要检查的行数。可以查看MySQL里面InnoDB的数据结构：https://blog.csdn.net/bohu83/article/details/81086474)ref
使用了哪列或常数与key列一起从表中选择行Extra
额外信息
using filesort MySQL需要额外的排序，不能通过索引顺序达到排序
using index 覆盖索引扫描，查询可在索引树中查找所需的数据，不需要扫描表数据文件
using temporary 查询使用到临时表
using where MySQL在存储引擎检索行后再进行过滤，先读数据，再按where条件检查，符合留下，不符丢弃
explain限制
(1) 不会告诉你存储过程，触发器会如何影响查询
(2) 不会告诉你MySQL在执行计划中所做的特定的优化
(3) 不支持存储过程