MySQL EXPLAIN关键字是执行查询还是只是解释查询？

EXPLAIN关键字告诉MySQL如何执行查询。让我们首先创建一个表-

create table DemoTable1375
   -> (
   -> Id int NOT NULL AUTO_INCREMENT PRIMARY KEY,
   -> FirstName varchar(20),
   -> INDEX FIRST_INDEX(FirstName)
   -> );

使用插入命令在表中插入一些记录-

insert into DemoTable1375(FirstName) values('Chris');
insert into DemoTable1375(FirstName) values('Bob');
insert into DemoTable1375(FirstName) values('Sam');
insert into DemoTable1375(FirstName) values('David');

使用select语句显示表中的所有记录-

select * from DemoTable1375;

这将产生以下输出-

+----+-----------+
| Id | FirstName |
+----+-----------+
|  2 | Bob       |
|  1 | Chris     |
|  4 | David     |
|  3 | Sam       |
+----+-----------+
4 rows in set (0.00 sec)

这是使用MySQL EXPLAIN的查询-

explain  select * from DemoTable1375;

这将产生以下输出-

+----+-------------+---------------+------------+-------+---------------+-------------+---------+------+------+----------+-------------+
| id | select_type | table         | partitions | type | possible_keys  | key         | key_len | ref | rows | filtered  | Extra       |
+----+-------------+---------------+------------+-------+---------------+-------------+---------+------+------+----------+-------------+
|  1 | SIMPLE      | DemoTable1375 | NULL       | index | NULL          | FIRST_INDEX | 63      | NULL | 4    | 100.00   | Using index |
+----+-------------+---------------+------------+-------+---------------+-------------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.03 sec)