Why doesn't wordpress create an unique index for `wp_users`.`user_login`?

Question

In this page, you can see what I mean,

http://codex.wordpress.org/Database_Description#Table:_wp_users

It only creates a normal index for the user_login column while I think an unique index should be created for it.

Answer 1

Well since the unique index (primary key) is the ID column, the coder probably didn't see the need to define another column in the table as unique. As you point out, though, user_login column is indexed so that gives the performance advantages when querying that table.

Answer 2

不知道wp，但也许它需要允许多个具有不同user_status user_logins ？