对 login 事件的事件触发器可用于记录用户登录，根据当前情况验证连接并分配角色，或者用于会话数据初始化。使用 login 事件的任一事件触发器在执行任何写入操作前检查数据库是否在恢复中非常重要。写入备用服务器将使其不可访问。

以下示例演示了这些选项。

-- 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 at time zone 'utc');
  rec boolean;
BEGIN
-- 1. Forbid logging in between 2AM and 4AM.
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 before we perform any
-- operations.
SELECT pg_is_in_recovery() INTO rec;
IF rec THEN
  RETURN;
END IF;

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

-- 3. Initialize user session data
CREATE TEMP TABLE session_storage (x float, y integer);
ALTER TABLE session_storage OWNER TO session_user;

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

END;
$$;

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