Mysql数据库性能优化二

select * from emp where ename='研发部'; 

ALTER TABLE emp ADD PRIMARY KEY(empno); 

alter table emp drop primary key; 

select * from emp where empno = 1; 

select * from emp where sex = '男' 

select * from emp where logincount = 1 

create table news(id int , title varchar(32),con varchar(1024)) engine=MyISAM; 

create fulltext index ful_inx on news (con); 

'but it often happens that they are not above supporting themselves by dishonest means.which should be more disreputable.Cultivate poverty like a garden herb'

mysql> select match(con) against('poverty') from news;
+-------------------------------+
| match(con) against('poverty') |
+-------------------------------+
| 0 |
| 0 |
| 0 |
| 0.9853024482727051 |
+------------------------------+ 

create index 索引名 on 表名(列1，列2); 

create [UNIQUE|FULLTEXT] index index_name on tbl_name (col_name [(length)] [ASC | DESC] , …..);
alter table table_name ADD INDEX [index_name] (index_col_name,...) 
添加主键(索引) ALTER TABLE 表名 ADD PRIMARY KEY(列名,..); 联合主键

DROP INDEX index_name ON tbl_name;
alter table table_name drop index index_name; 
删除主键(索引)比较特别: alter table t_b drop primary key;

show index(es) from table_name;
show keys from table_name;
desc table_Name; 

CREATE TABLE dept(
deptno MEDIUMINT UNSIGNED NOT NULL DEFAULT 0,
dname VARCHAR(20) NOT NULL DEFAULT "",
loc VARCHAR(13) NOT NULL DEFAULT ""
) ENGINE=MyISAM DEFAULT CHARSET=utf8 ; 
--放入数据，前面应该已经添加了，如果没有则需要重新添加
--测试开始.

alter table dept add primary key (deptno) 

explain select * from dept where deptno=1; 

mysql> explain select * from dept where deptno=1;
*************************** 1. row ***************************
id: 1
select_type: SIMPLE
table: dept
type: const
possible_keys: PRIMARY
key: PRIMARY
key_len: 3
ref: const
rows: 1
Extra:
1 row in set (0.00 sec) 

alter table dept add index myind (dname,loc); 

explain select * from dept where dname='研发部'; 会显示使用到了索引myind
explain select * from dept where loc='MsBDpMRX'; 不会显示使用到了索引myind

explain select * from dept where dname like '%研发部'; 不会显示使用到了索引myind
explain select * from dept where dname like '研发部%'; 会显示使用到了索引myind

alter table dept drop index myind
alter table dept add index myind (dname)
explain select * from dept where dname='研发部' or loc='aa';-- 就不会使用到dname列上的 

select * from dept from dname=1234; //不会使用到索引
select * from dept from dname='1234'; //会使用到索引

show status like 'Handler_read%';

alter table table_name disable keys;
loading data//insert语句;
alter table table_name enable keys; 

select * from 表名 where 条件1='' or 条件2='tt'
explaine select * from dept group by dname; =>这时显示 extra: using filesort 说明会进行排序
explaine select * from dept group by dname order by null =>这时不含有显示 extra: using filesort 说明不会进行排序

explain select * from emp , dept where emp.deptno=dept.deptno;

explain select * from emp left join dept on emp.deptno=dept.deptno;

create table bbs(id int ,con varchar(1024) , pub_time int);
date('Ymd',时间-3*24*60*60); 2038年-1-19

create table temp1( t1 float(10,2), t2 decimal(10,2));
insert into temp1 values(1000000.32,1000000,32); 发现 t1 成了 1000000.31 所以有问题.

create table temp2( id int) engine=MyISAM;
insert into temp2 values(1); insert into temp2 values(2); insert into temp2 values(3);
insert into temp2 select * from temp2;--复制
delete from temp2 where id=1; 发现 该表对于的数据文件没有变小