MySQL Study之--Index的强制使用和忽略
1、查看表结构
mysql> show create table emp\G
*************************** 1. row ***************************
Table: empCreate Table: CREATE TABLE `emp` ( `empno` int(4) NOT NULL DEFAULT '0', `ENAME` varchar(10) DEFAULT NULL, `JOB` varchar(9) DEFAULT NULL, `MGR` int(4) DEFAULT NULL, `HIRE` date DEFAULT NULL, `SAL` int(7) DEFAULT NULL, `COMM` int(7) DEFAULT NULL, `deptno` int(2) DEFAULT NULL, PRIMARY KEY (`empno`), KEY `deptno` (`deptno`), CONSTRAINT `emp_ibfk_1` FOREIGN KEY (`deptno`) REFERENCES `dept` (`deptNO`)) ENGINE=InnoDB DEFAULT CHARSET=latin11 row in set (0.00 sec)
---在此表中empno 为主键，在deptno上建有索引
mysql> select * from emp;
+-------+--------+-----------+------+------------+------+------+--------+| empno | ENAME | JOB | MGR | HIRE | SAL | COMM | deptno |+-------+--------+-----------+------+------------+------+------+--------+| 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 | 1987-07-13 | 3000 | NULL | 10 || 7839 | KING | PRESIDENT | NULL | 1981-11-17 | 5000 | NULL | 10 || 7844 | TURNER | SALESMAN | 7698 | 1981-09-08 | 1500 | 0 | 30 || 7876 | ADAMS | CLERK | 7788 | 1987-06-13 | 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 |+-------+--------+-----------+------+------------+------+------+--------+14 rows in set (0.00 sec)
1）全表扫描查询
mysql> explain select * from emp \G
*************************** 1. row ***************************
id: 1 select_type: SIMPLE table: emp type: ALLpossible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 14 Extra: NULL1 row in set (0.00 sec)
2、通过主键查询
mysql> explain select * from emp where empno=7788\G
*************************** 1. row ***************************
id: 1 select_type: SIMPLE table: emp type: constpossible_keys: PRIMARY key: PRIMARY key_len: 4 ref: const rows: 1 Extra: NULL1 row in set (0.02 sec)
3）忽略主键索引
mysql> explain select * from emp ignore index(pri) where empno=7788\G
*************************** 1. row ***************************
id: 1 select_type: SIMPLE table: emp type: ALLpossible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 14 Extra: Using where1 row in set (0.00 sec)
4）强制使用主键
mysql> explain select * from emp force index(pri) where empno=7788\G
*************************** 1. row ***************************
id: 1 select_type: SIMPLE table: emp type: constpossible_keys: PRIMARY key: PRIMARY key_len: 4 ref: const rows: 1 Extra: NULL1 row in set (0.00 sec)
5）通过索引deptno查询
mysql> explain select * from emp where deptno=10\G
*************************** 1. row ***************************
id: 1 select_type: SIMPLE table: emp type: refpossible_keys: deptno key: deptno key_len: 5 ref: const rows: 4 Extra: NULL1 row in set (0.00 sec)
6）忽略索引的使用
mysql> explain select * from emp ignore index(deptno) where deptno=10\G
*************************** 1. row ***************************
id: 1 select_type: SIMPLE table: emp type: ALLpossible_keys: NULL key: NULL key_len: NULL ref: NULL rows: 14 Extra: Using where1 row in set (0.00 sec)
7）强制使用索引
mysql> explain select * from emp force index (deptno) where deptno=10\G
*************************** 1. row ***************************
id: 1 select_type: SIMPLE table: emp type: refpossible_keys: deptno key: deptno key_len: 5 ref: const rows: 4 Extra: NULL1 row in set (0.00 sec)