mysql left join轉inner join

在日常優化過程中，發現一個怪事情，同一個SQL出現兩個完全不一樣執行計划，left join 連驅動表都可以變成不一樣。

對於left join，如果where條件里有被關聯表過濾，left join有可能被轉成inner join ,本案例中shopInfo有ShopCategory = 'LOC'過濾條件； 保證shopInfo的記錄非NULL，因此left join在優化過程中可以轉為inner join。 那么O和S的JOIN順序就是可以交換的。

驗證結論：

創建表：

--班級表
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);

--學生表
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);

--班級數據
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');

--學生數據
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條件且不為null

案例2： A表和B表均有where條件且不為null

案例3：A表和B表均有where條件且不為null,刪除B表索引

結論：

left join 只有被關聯表有where條件,且其過濾條件優於關聯表的情況下，mysql優化器才轉成inner join.

福祿·運維 澤西