The event trigger on the login event can be useful for logging user logins, for verifying the connection and assigning roles according to current circumstances, or for session data initialization. It is very important that any event trigger using the login event checks whether or not the database is in recovery before performing any writes. Writing to a standby server will make it inaccessible.

The following example demonstrates these options.

-- create test tables and roles
CREATE TABLE user_login_log (
  "user" text,
  "session_start" timestamp with time zone
);
CREATE ROLE day_worker;
CREATE ROLE night_worker;

-- the example trigger function
CREATE OR REPLACE FUNCTION init_session()
  RETURNS event_trigger SECURITY DEFINER
  LANGUAGE plpgsql AS
$$
DECLARE
  hour integer = EXTRACT('hour' FROM current_time);
  rec boolean;
BEGIN
-- 1. Forbid logging in during nighttime
IF hour BETWEEN 2 AND 4 THEN
  RAISE EXCEPTION 'Login forbidden';
END IF;

-- The checks below cannot be performed on standby servers so
-- ensure the database is not in recovery
SELECT pg_is_in_recovery() INTO rec;
IF rec THEN
  RETURN;
END IF

-- 2. Assign some roles
IF hour BETWEEN 8 AND 20 THEN         -- at daytime grant the day_worker role
  EXECUTE 'REVOKE night_worker FROM ' || quote_ident(session_user);
  EXECUTE 'GRANT    day_worker TO '   || quote_ident(session_user);
ELSE                                  -- at other time grant the night_worker role
  EXECUTE 'REVOKE day_worker FROM ' || quote_ident(session_user);
  EXECUTE 'GRANT  night_worker TO ' || quote_ident(session_user);
END IF;

-- 3. Initialize some user session data
CREATE TEMP TABLE session_storage (x float, y integer);

-- 4. Log the connection time
INSERT INTO user_login_log VALUES (session_user, current_timestamp);

END;
$$;

-- trigger definition
CREATE EVENT TRIGGER init_session
  ON login
  EXECUTE FUNCTION init_session();