Filtering emails by domain in PostgreSQL

email.sql

BEGIN;
CREATE FUNCTION email_domain(email text) RETURNS text
    LANGUAGE sql IMMUTABLE
    AS $$
        SELECT reverse(split_part(lower(email), '@', 2))
      $$;

CREATE INDEX idx_users_email_domain ON users (email_domain(email));
EXPLAIN ANALYZE SELECT "users".* FROM "users" WHERE email_domain(email) = 'moc.sseldnuob'  ORDER BY "users"."proposed_changes_count" DESC;
ROLLBACK;

explain

Timing is on.
BEGIN
Time: 0.253 ms
CREATE FUNCTION
Time: 1.516 ms
CREATE INDEX
Time: 10614.353 ms
                                                                 QUERY PLAN
--------------------------------------------------------------------------------------------------------------------------------------------
 Sort  (cost=6525.17..6529.08 rows=1564 width=1994) (actual time=11.447..12.107 rows=1108 loops=1)
   Sort Key: proposed_changes_count
   Sort Method: external merge  Disk: 688kB
   ->  Bitmap Heap Scan on users  (cost=36.55..5087.69 rows=1564 width=1994) (actual time=0.907..8.351 rows=1108 loops=1)
         Recheck Cond: (reverse(split_part(lower((email)::text), '@'::text, 2)) = 'moc.sseldnuob'::text)
         ->  Bitmap Index Scan on idx_users_email_domain  (cost=0.00..36.16 rows=1564 width=0) (actual time=0.670..0.670 rows=1108 loops=1)
               Index Cond: (reverse(split_part(lower((email)::text), '@'::text, 2)) = 'moc.sseldnuob'::text)
 Total runtime: 14.187 ms
(8 rows)

Time: 17.440 ms
ROLLBACK
Time: 1.439 ms