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Email addresses should be treated as case-insensitive because they are. If a user is trying to sign in with their email address, we shouldn’t care if they type user@example.com or User@example.com. Both of those email addresses should be treated as equal and ultimately lead us to the same User record.

With the citext extension, we can create a column that acts as a case-insensitive text type. Any comparisons on a column of that type will internally have the lower function executed on the arguments.

The following example shows this in action:

create extension if not exists citext;

create table citext_emails (
  id serial primary key,
  email citext not null unique

insert into citext_emails (email) values ('LizLemon@nbc.com');

select * from citext_emails where email = 'lizlemon@nbc.com';
--  id |      email
-- ----+------------------
--   1 | LizLemon@nbc.com

Via jbranchaud/til.