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1、查看表结构
mysql> show create table emp\G
---在此表中empno 为主键,在deptno上建有索引
mysql> select * from emp;
1)全表扫描查询
mysql> explain select * from emp \G
2、通过主键查询
mysql> explain select * from emp where empno=7788\G
3)忽略主键索引
mysql> explain select * from emp ignore index(pri) where empno=7788\G
4)强制使用主键
mysql> explain select * from emp force index(pri) where empno=7788\G
5)通过索引deptno查询
mysql> explain select * from emp where deptno=10\G
6)忽略索引的使用
mysql> explain select * from emp ignore index(deptno) where deptno=10\G
7)强制使用索引
mysql> explain select * from emp force index (deptno) where deptno=10\G