mysql中explain怎么用

这个主要用来查看查询sql的执行过程的，也是检验所用的查询sql是否按照理想的情况在执行，比如是否使用指定的索引，执行时大概过滤的条数，也是用来优化sql语句的一种重要工具

您好，我们常常用到explain这个命令来查看一个这些SQL语句的执行计划，推荐查看官网

https://dev.mysql.com/doc/refman/8.0/en/explain-output.html

mysql> explain select * from num;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key  | key_len | ref  | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
|  1 | SIMPLE      | num   | NULL       | ALL  | NULL          | NULL | NULL    | NULL |    4 |   100.00 | NULL  |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.03 sec)