mysql left join快速轉(zhuǎn)inner join的過程
在日常優(yōu)化過程中,發(fā)現(xiàn)一個(gè)怪事情,同一個(gè)SQL出現(xiàn)兩個(gè)完全不一樣執(zhí)行計(jì)劃,left join 連驅(qū)動(dòng)表都可以變成不一樣。
對(duì)于left join,如果where條件里有被關(guān)聯(lián)表過濾,left join有可能被轉(zhuǎn)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(yōu)化過程中可以轉(zhuǎn)為inner join。 那么O和S的JOIN順序就是可以交換的。
驗(yàn)證結(jié)論:
創(chuàng)建表:
--班級(jí)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(xué)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(jí)數(shù)據(jù) 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(xué)生數(shù)據(jù) 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(jié)論:
left join 只有被關(guān)聯(lián)表有where條件,且其過濾條件優(yōu)于關(guān)聯(lián)表的情況下,mysql優(yōu)化器才轉(zhuǎn)成inner join.
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