drop table if exists users;
create table users (
  id serial primary key,
  email varchar(255) not null check(strpos(email, '@') > 0),
  password varchar(255) not null check(password <> '')
  --, unique (lower(email)) не будет работать
);
create unique index email_unique_idx on users (lower(email));
insert into users (email, password) values ('test@mail.com', 'test123'), ('TEST@mail.com', 'qweasd');
-- ERROR:  duplicate key value violates unique constraint "email_unique_idx"
-- DETAIL:  Key (lower(email::text))=(test@mail.com) already exists.
-- SQL state: 23505