mysql explain index和ref的区别，MySQL EXPLAIN 语句的 SELECT TYPE有哪些值...
mysql explain index和ref的区别
onst联接类型的一个特例。表仅有一行满足条件.如下(t3表上的id是 primary key)
mysql> explain select * from (select * from t3 where id=3952602) a ;
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
| 1 | PRIMARY | | system | NULL | NULL | NULL | NULL | 1 | |
| 2 | DERIVED | t3 | const | PRIMARY,idx_t3_id | PRIMARY | 4 | | 1 | |
+----+-------------+------------+--------+-------------------+---------+---------+------+------+-------+
MySQL EXPLAIN 语句的 SELECT TYPE有哪些值
explain每输行提供表相关信息并且每行包括面列：
1id select识别符select查询序列号
2select_type 任何种类型
simple 简单select(使用union或查询)
primary 外面select
union union第二或面select语句
dependent union union第二或面select语句取决于外面查询
union result union结
subquery 查询第select
dependent subquery 查询第select取决于外面查询
derived 导表select(from句查询)
3table 输行所引用表
4type 联接类型面给各种联接类型按照佳类型坏类型进行排序：
system 表仅行(=系统表)const联接类型特例
const 表匹配行查询始读取仅行行列值优化器剩余部认数const表快读取
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