postgresql创建的触发器没有被调用，求大神看看有什么问题

[一枝梅比花娇]

触发器：
create or REPLACE function trg_a_t() returns trigger as $body$
begin
if (tg_op='insert') then

update a_t t1
set t1.resume_id_uuid = uuid_generate_v4(),
t1.acc_id_uuid = uuid_generate_v4()
where t1.resume_id = NEW.resume_id;

end if;
return null;
end;
$body$
LANGUAGE 'plpgsql';

create trigger trg_a_t after insert on a_t for each row
EXECUTE PROCEDURE trg_a_t();

执行的insert语句
insert into a_t(resume_id,acc_id,data) select resume_id,acc_id,data from t;

说明：将t表中的resume_id、acc_id、data三个字段的全部数据导入到表a_t中，同时（a_t表中还有resume_id_uuid和acc_id_uuid字段）生成两个UUID分别放入resume_id_uuid和acc_id_uuid中。可是当我执行inert语句后查看表中信息，发现那两个字段全部为null。