Comment by rarrrrrr

Here's an example of doing that in PostgreSQL:

  create table user_email (
      email text not null 
  );
  -- create a index on the lowercase form 
  -- of the email 
  create unique index user_email_case_idx 
      on user_email (lower(email));
  -- select using the index, with the lowercase form.
  select 1 
    from user_email 
   where lower(email)=lower('[email protected]');