Mysql 执行计划

转移：
https://gitee.com/chenlushun12/ytx-api/blob/master/blog/mysql/Mysql-执行计划.md

参考文档

https://dev.mysql.com/doc/refman/5.7/en/explain-output.html
https://dev.mysql.com/doc/refman/5.7/en/explain.html
https://blog.csdn.net/baiducheng/article/details/79725535

语法

{EXPLAIN | DESCRIBE | DESC}
    tbl_name [col_name | wild]

{EXPLAIN | DESCRIBE | DESC}
    [explain_type]
    {explainable_stmt | FOR CONNECTION connection_id}

explain_type: {
    EXTENDED
  | PARTITIONS
  | FORMAT = format_name
}

format_name: {
    TRADITIONAL
  | JSON
}

explainable_stmt: {
    SELECT statement
  | DELETE statement
  | INSERT statement
  | REPLACE statement
  | UPDATE statement
}

EXPLAIN 、DESCRIBE 、DESC效果是一样的，DESCRIBE、DESC多用于查看表结构，类似于SHOW COLUMNS，例如，

mysql> desc select * from purchase_order;
+----+-------------+----------------+------+---------------+------+---------+------+------+-------+
| id | select_type | table          | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+----------------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | purchase_order | ALL  | NULL          | NULL | NULL    | NULL |  158 | NULL  |
+----+-------------+----------------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)
mysql> explain  select * from purchase_order;
+----+-------------+----------------+------+---------------+------+---------+------+------+-------+
| id | select_type | table          | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+----------------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | purchase_order | ALL  | NULL          | NULL | NULL    | NULL |  158 | NULL  |
+----+-------------+----------------+------+---------------+------+---------+------+------+-------+
1 row in set (0.01 sec)

mysql> show columns in  purchase_order;
+-------------------+---------------+------+-----+---------+----------------+
| Field             | Type          | Null | Key | Default | Extra          |
+-------------------+---------------+------+-----+---------+----------------+
| id                | bigint(20)    | NO   | PRI | NULL    | auto_increment |
| serial_number     | char(19)      | YES  |     | NULL    |                |
+-------------------+---------------+------+-----+---------+----------------+
20 rows in set (0.00 sec)

mysql> desc purchase_order;
+-------------------+---------------+------+-----+---------+----------------+
| Field             | Type          | Null | Key | Default | Extra          |
+-------------------+---------------+------+-----+---------+----------------+
| id                | bigint(20)    | NO   | PRI | NULL    | auto_increment |
| serial_number     | char(19)      | YES  |     | NULL    |                |
+-------------------+---------------+------+-----+---------+----------------+
20 rows in set (0.00 sec)

SHOW COLUMNS Syntax

一个简单的执行计划

在sql前面加EXPLAIN查看执行假话

mysql> EXPLAIN  SELECT * from purchase_order;
+----+-------------+----------------+------+---------------+------+---------+------+------+-------+
| id | select_type | table          | type | possible_keys | key  | key_len | ref  | rows | Extra |
+----+-------------+----------------+------+---------------+------+---------+------+------+-------+
|  1 | SIMPLE      | purchase_order | ALL  | NULL          | NULL | NULL    | NULL |   11 | NULL  |
+----+-------------+----------------+------+---------------+------+---------+------+------+-------+
1 row in set (0.00 sec)

先概括的看一下执行计划返回列的含义

列	说明
select_type	select类型
table	输出结果集的表
type	访问类型

下面详细介绍以下每个列的含义

• select_type

这个参数表示select类型，每个取值代表的含义如下

取值	说明
SIMPLE	简单表，即不使用关联查询、自查询
PRIMARY	主查询，即最外层的查询
SUBQUERY	子查询的第一个select
UNION	UNION中的第二个或者后面的查询语句

• type

从上之下，性能由差变强

取值	说明
ALL	全表扫描,mysql遍历全表来找到匹配的行
index	索引全扫描,mysql遍历全部索引来查询匹配的行
range	索引范围扫描，常见的操作符，<,<=,>,>=,between
ref	使用非唯一索引扫描、唯一索引前缀扫描
eq_ref	唯一索引
const,system
NULL	mysql不访问索引或者表，就能返回结果