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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