mysql left join快速转inner join的过程

在日常优化过程中,发现一个怪事情,同一个SQL出现两个完全不一样执行计划,left join 连驱动表都可以变成不一样。

对于left join,如果where条件里有被关联表过滤,left join有可能被转成inner join ,本案例中shopInfo有ShopCategory = 'LOC'过滤条件; 保证shopInfo的记录非NULL,因此left join在优化过程中可以转为inner join。 那么O和S的JOIN顺序就是可以交换的。

验证结论:

创建表:

--班级表

CREATE TABLE T_CLASS(

class_id int not null,

class_name VARCHAR2(100)

);

添加索引

alter table T_CLASS add index inx_class_id(class_id);

--学生表

CREATE TABLE T_STUDENT(

student_id int not null,

class_id int not null,

student_name VARCHAR(100),

age int,

sex int

)

添加索引

alter table T_STUDENT add index index_age(AGE);

--班级数据

insert into T_CLASS (CLASS_ID, CLASS_NAME)

values (1, '一班');

insert into T_CLASS (CLASS_ID, CLASS_NAME)

values (2, '二班');

insert into T_CLASS (CLASS_ID, CLASS_NAME)

values (3, '三班');

insert into T_CLASS (CLASS_ID, CLASS_NAME)

values (4, '四班');

insert into T_CLASS (CLASS_ID, CLASS_NAME)

values (5, '五班');

--学生数据

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (1, 1, '李1', 3, '1');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (2, 1, '李2', 2, '1');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (3, 1, '李3', 3, '1');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (4, 2, '李4', 4, '1');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (5, 2, '李5', 3, '2');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (6, 2, '李6', 3, '1');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (7, 3, '李7', 6, '2');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (8, 3, '李8', 4, '2');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (9, 2, '李9', 2, '2');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (10, 2, '李10', 3, '1');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (11, 3, '李11', 3, '2');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (12, 2, '李12', 8, '2');

insert into T_STUDENT (STUDENT_ID, CLASS_ID, STUDENT_NAME, AGE, SEX)

values (13, 1, '李13', 6, '2');

案例1:B表有where条件且不为null

案例2: A表和B表均有where条件且不为null

案例3:A表和B表均有where条件且不为null,删除B表索引

结论:

left join 只有被关联表有where条件,且其过滤条件优于关联表的情况下,mysql优化器才转成inner join.

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