order by 的時候 select 上的列導(dǎo)致的影響 分析

create table t1 (
a1 bigint  not null primary key auto_increment,
c10 char(10),
c100 char(100),
c200 char(200),
c30 char(255),
c300 text
)

insert into t1 (c10,c100,c200,c30,c300)values ('a','a','a','a','a');

insert into t1 (c10,c100,c200,c30,c300) select lpad(c10,10,'1'),lpad(c100,100,'1'),lpad(c200,190,'a'),lpad(c30,250,'1'),lpad(c300,2000,'1') from t1;


root@localhost [test]>select count(a1) from t1;
+-----------+
| count(a1) |
+-----------+
|     65536 |

select SQL_NO_CACHE count(1) from (
       select a1 from t1 IGNORE INDEX (primary)   order by c10 desc  limit  1000 ) a;


select SQL_NO_CACHE count(1) from (
       select a1,c100 from t1 IGNORE INDEX (primary)   order by c10 desc  limit  1000 ) a;

select SQL_NO_CACHE count(1) from (
       select a1,c100,c200 from t1 IGNORE INDEX (primary)   order by c10 desc  limit  1000 ) a;
   
   
select SQL_NO_CACHE count(1) from (
       select a1,c100,c200,c30 from t1 IGNORE INDEX (primary)   order by c10 desc  limit  1000 ) a; 

select SQL_NO_CACHE count(1) from (
       select a1,c100,c200,c30 ,c300 from t1 IGNORE INDEX (primary)   order by c10 desc  limit  1000 ) a;    

select SQL_NO_CACHE count(1) from (
       select * from t1 IGNORE INDEX (primary)   order by c10 desc  limit  1000 ) a;