Create user account race condition - postgresql

Question

I have the following procedure:
CREATE OR REPLACE FUNCTION create_user(p_email character varying) RETURNS integer AS
$$
DECLARE
v_user_id integer;
BEGIN
SELECT user_id FROM user WHERE email = p_email INTO v_user_id;
IF v_user_id IS NULL THEN
INSERT
INTO user (email, status)
VALUES (p_email, 'active')
RETURNING user_id INTO v_user_id;
END IF;
RETURN v_user_id;
END;
$$ LANGUAGE plpgsql;
But if there multiple parallel requests to DB, it causes race condition, the procedure gets called multiple times and since the delay is so low, all of them trying to create a user and only one of them is succeeding. Is there any good workaround for this?

Answer 1

Is there any good workaround for this?
Yes, create a unique index on the email column, then use insert on conflict:
...
BEGIN
INSERT INTO "user" (email, status)
VALUES (p_email, 'active')
ON CONFLICT (email) DO NOTHING
RETURNING user_id INTO v_user_id;
RETURN v_user_id;
END;
The above will however not return the user_id if the email already exists. If you need that, you will need something like this:
...
BEGIN
INSERT INTO "user" (email, status)
VALUES (p_email, 'active')
ON CONFLICT (email) DO NOTHING
RETURNING user_id INTO v_user_id;
if v_user_id is null then
-- no insert happened
select user_id
into v_user_id
from "user"
where email = p_email;
end if;
RETURN v_user_id;
END;