标签：mysql explain

参考：http://www.2cto.com/database/201307/230048.html

http://blog.csdn.net/zhuxineli/article/details/14455029

explain是用来分析mysql如何使用索引来处理select语句以及连接表。可以帮助选择更好的索引和写出更优化的查询语句。

1. id 

语句的执行顺序标识。是指select语句执行的顺序

2、select_type:

  2.1、simple 

    简单类型，只要 语句中没有子查询或union。

EXPLAIN SELECT * FROM `fm_company`

   2.2 primary 

最外面的select,在有子查询的语句中，最外面的select查询就是primary。查询关键字和主键无关

EXPLAIN SELECT * FROM `fm_company` AS t1 WHERE orgId IN(SELECT orgId FROM fm_company AS t2 WHERE t2.`id` = 1)

2.3  union

 union语句的后面那个 select 执行执行语句。

EXPLAIN SELECT * FROM `fm_company` AS t1 
UNION
SELECT * FROM `fm_company` AS t2

 2.4 dependent subquery 

  子查询中内层中第一个select语句。

EXPLAIN SELECT * FROM `fm_company` AS t1 WHERE orgId IN(SELECT orgId FROM fm_company AS sub_t1 WHERE sub_t1.`id` = 148)

   2.5、devived 

   派生表(中间表)的查询语句

EXPLAIN  SELECT  * FROM (

SELECT * FROM `fm_company` AS t1 
UNION
SELECT * FROM `fm_company` AS t2
)  subQuery

2.6、dependent  union 

  子查询中union且为union中第二个select开始的后面所有select，依赖于外部的结果集。

EXPLAIN SELECT * 
FROM `fm_company` as t1 
WHERE orgId IN(
SELECT orgId FROM fm_company as sub_t1 WHERE sub_t1.`id` = 148 
union 
SELECT orgId FROM fm_company AS sub_t2 WHERE sub_t2.`id` = 149
)

2.7 union result  

顾名思义就是 union中合并的结果

3. table 

4. type 

 EXPLAIN SELECT * FROM fm_company WHERE id=148

 EXPLAIN SELECT * FROM fm_company t1  ,fm_company t2  WHERE t1.id=t2.id

mysql 中 sql语句 explain(一)

标签：mysql explain

原文地址：http://blog.csdn.net/wei_ya_wen/article/details/42463705