order by primary key slowly

[varky chan]

There is a user table on my database with InnoDB storage engine like following:

CREATE TABLE user (
id int not null auto_increment,
name varchar(50) not null,
password char(64) not null,
email varchar(80) not null,
primary key (id),
unique key (username)
) Engine=InnoDB;

And about 7 millions user data in it. When i execute some queries as follows:

1. select id from user limit 1000000,10;
2. select id from user order by id desc limit 1000000,10;

The first query above is faster, the second query is very slow. What's happen about the second query? Why it is slow?