什么是延迟索引？使用索引查询出来数据，之后把查询结果和同一张表中数据进行连接查询，进而提高查询速度!

分页是一个很常见功能，select ** from tableName limit ($page - 1 ) * $n ,$n

通过一个存储过程插入10000条数据进行测试：

create table smth1 (
	id int auto_increment ,
	ver int(11) default null,
	content varchar(1000) not null,
	intro varchar(1000) not null,
	primary key(id),
	key idver(id,ver)
	
)engine = innodb default charset = utf8;






create procedure smthTest1()
begin 
	declare num int default 100001;
	while num < 1000000 do
		set num := num +1;
		insert into smth1 values (num ,num,'我是*****','我是谁');
	end while ;

end;

查询：

mysql> show profiles;
+----------+------------+----------------------------------------------+
| Query_ID | Duration   | Query                                        |
+----------+------------+----------------------------------------------+
|        1 |   0.002006 | select id ,content from smth1 limit 1000,10  |
|        2 |   0.030106 | select id ,content from smth1 limit 5000,10  |
|        3 |   0.042428 | select id ,content from smth1 limit 9000,10  |
|        4 | 0.01297225 | select id ,content from smth1 limit 10000,10 |
|        5 | 0.13077625 | select id ,content from smth1 limit 20000,10 |

可见随着查询$page 变大，时间会越来越大！

 

怎样避免这种情况？

一般我们数据库里面数据都不会直接删除，数据时很宝贵的，不舍得删除，另一方便能提高查询数据

先利用索引查询出来数据，再进行联合查询不就行了

 select C.id,C.content from smth1 C inner join 
(
	select id from smth1 where id > 1000 limit 10
) as t on C.id = t.id ;

select C.id,C.content from smth1 C inner join 
(
	select id from smth1 where id > 5000 limit 10
) as t on C.id = t.id ;

select C.id,C.content from smth1 C inner join 
(
	select id from smth1 where id > 9000 limit 10
) as t on C.id = t.id ;

select C.id,C.content from smth1 C inner join 
(
	select id from smth1 where id > 10000 limit 10
) as t on C.id = t.id ;

select C.id,C.content from smth1 C inner join 
(
	select id from smth1 where id > 20000 limit 10
) as t on C.id = t.id ;

进行执行计划分析，没有一个大于1s的

11 | 0.04538625 | select C.id,C.content from smth1 C inner join
(
	select id from smth1 where id > 5000 limit 10
) as t on C.id = t.id  |
|       12 |   0.023278 | select C.id,C.content from smth1 C inner join
(
	select id from smth1 where id > 9000 limit 10
) as t on C.id = t.id  |
|       13 | 0.02320425 | select C.id,C.content from smth1 C inner join
(
	select id from smth1 where id > 10000 limit 10
) as t on C.id = t.id |
|       14 |   0.001938 | select C.id,C.content from smth1 C inner join
(
	select id from smth1 where id > 20000 limit 10
) as t on C.id = t.id |

此外，还会想到用in来查询而不是子查询，为什么不用in，使用in会先查询出来一条id，之后再去和下面进行匹配，会进行smth1进行全表扫描！