MySQL Study之--Index的强制使用和忽略

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: emp
Create 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=latin1
1 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: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 14
Extra: NULL
1 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: const
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: const
rows: 1
Extra: NULL
1 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: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 14
Extra: Using where
1 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: const
possible_keys: PRIMARY
key: PRIMARY
key_len: 4
ref: const
rows: 1
Extra: NULL
1 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: ref
possible_keys: deptno
key: deptno
key_len: 5
ref: const
rows: 4
Extra: NULL
1 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: ALL
possible_keys: NULL
key: NULL
key_len: NULL
ref: NULL
rows: 14
Extra: Using where
1 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: ref
possible_keys: deptno
key: deptno
key_len: 5
ref: const
rows: 4
Extra: NULL
1 row in set (0.00 sec)

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