MySQL基礎之STRAIGHT JOIN用法簡介

MySQL基礎之STRAIGHT JOIN用法簡介

引用mysql官方手冊的說法：

STRAIGHT_JOIN is similar to JOIN, except that the left table is always read before the right table. This can be used for those (few) cases for which the join optimizer processes the tables in a suboptimal order.

翻譯過來就是：STRAIGHT_JOIN與 JOIN 類似，只不過左表始終在右表之前讀取。這可用於聯接優化器以次優順序處理表的那些（少數）情況。

注意：總的來說STRAIGHT_JOIN只適用於內連接，因為left join、right join已經知道了哪個表作為驅動表，哪個表作為被驅動表，比如left join就是以左表為驅動表，right join反之，而STRAIGHT_JOIN就是在內連接中使用，而強制使用左表來當驅動表，所以這個特性可以用於一些調優，強制改變mysql的優化器選擇的執行計划

ok，寫個例子進行實驗

創建部門表dept

/*Table structure for table `dept` */

DROP TABLE IF EXISTS `dept`;

CREATE TABLE `dept` (
  `DEPTNO` int(2) NOT NULL,
  `DNAME` varchar(14) DEFAULT NULL,
  `LOC` varchar(13) DEFAULT NULL,
  PRIMARY KEY (`DEPTNO`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

/*Data for the table `dept` */
# 往dept表寫數據
insert  into `dept`(`DEPTNO`,`DNAME`,`LOC`) values (10,'ACCOUNTING','NEW YORK'),(20,'RESEARCH','DALLAS'),(30,'SALES','CHICAGO'),(40,'OPERATIONS','BOSTON');

創建員工emp表

DROP TABLE IF EXISTS `emp`;

CREATE TABLE `emp` (
  `EMPNO` int(4) NOT NULL,
  `ENAME` varchar(10) DEFAULT NULL,
  `JOB` varchar(9) DEFAULT NULL,
  `MGR` int(4) DEFAULT NULL,
  `HIREDATE` date DEFAULT NULL,
  `SAL` int(7) DEFAULT NULL,
  `COMM` int(7) DEFAULT NULL,
  `DEPTNO` int(2) DEFAULT NULL,
  PRIMARY KEY (`EMPNO`),
  KEY `FK_DEPTNO` (`DEPTNO`),
  CONSTRAINT `FK_DEPTNO` FOREIGN KEY (`DEPTNO`) REFERENCES `dept` (`DEPTNO`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

/*Data for the table `emp` */
# 往員工emp表寫數據
insert  into `emp`(`EMPNO`,`ENAME`,`JOB`,`MGR`,`HIREDATE`,`SAL`,`COMM`,`DEPTNO`) values (7369,'SMITH','CLERK',7902,'1980-12-17',800,NULL,20),(7499,'ALLEN','SALESMAN',7698,'1981-02-20',1600,300,30),(7521,'WARD','SALESMAN',7698,'1981-02-22',1250,500,30),(7566,'JONES','MANAGER',7839,'1981-04-02',2975,NULL,20),(7654,'MARTIN','SALESMAN',7698,'1981-09-28',1250,1400,30),(7698,'BLAKE','MANAGER',7839,'1981-05-01',2850,NULL,30),(7782,'CLARK','MANAGER',7839,'1981-06-09',2450,NULL,10),(7788,'SCOTT','ANALYST',7566,'1987-04-19',3000,NULL,20),(7839,'KING','PRESIDENT',NULL,'1981-11-17',5000,NULL,10),(7844,'TURNER','SALESMAN',7698,'1981-09-08',1500,0,30),(7876,'ADAMS','CLERK',7788,'1987-05-23',1100,NULL,20),(7900,'JAMES','CLERK',7698,'1981-12-03',950,NULL,30),(7902,'FORD','ANALYST',7566,'1981-12-03',3000,NULL,20),(7934,'MILLER','CLERK',7782,'1982-01-23',1300,NULL,10);

先用inner join的看看

EXPLAIN 
SELECT 
  a.`EMPNO`,
  a.`ENAME`,
  a.`JOB`,
  a.`SAL`,
  b.`DNAME`  
FROM
  emp a 
  INNER JOIN dept b 
    ON a.`DEPTNO` = b.`DEPTNO` 
ORDER BY a.`EMPNO` ;

查看執行計划，可以看出是以dept表為驅動表的

ps：對於執行計划的知識可以參考我之前博客:MySQL調優之Explain執行計划學習筆記

用STRAIGHT_JOIN連接的寫法：

EXPLAIN 
SELECT 
  a.`EMPNO`,
  a.`ENAME`,
  a.`JOB`,
  a.`SAL`,
  b.`DNAME` 
FROM
  emp a 
  STRAIGHT_JOIN dept b 
    ON a.`DEPTNO` = b.`DEPTNO` 
ORDER BY a.`EMPNO` ;

從執行計划可以看出強制用emp表做驅動表了，而且不走索引，全表掃描了，之前的還是ref，是有走索引的