触发login事件的事件触发器对于记录用户登录、验证连接、 根据当前情况分配角色或初始化会话数据非常有用。非常重要的是，任何使用 login事件的事件触发器在执行写操作之前都必须检查数据库 是否处于恢复状态。写入备用服务器将导致其无法访问。

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;