Let's say you set this up in your search ldaps://dc1.domain.com/
That will completely enumerate every object in the directory until it matches the one you're filtering on (typically cn or uid)
So you get a little smarter and do this, ldaps://dc1.domain.com/ou=People,dc=domain,dc=com?sAMAccountName
That will recursively enumerate the sAMAccountName
attribute of every object under the People container until it finds one that matches.
You're not having to list every single goddamn attribute of every object, but if you're storing your user objects all in that OU? Fuck me that's still a lot.