在触发器里可以使用过渡表进行统计，示例如下：

create table test (id serial primary key, info varchar);

insert into test (info) values
    ('One'),
    ('Two'),
    ('Three'),
    ('Four'),
    ('Five');	

create function test_trigger() returns trigger 
as $$
declare
    rec record;
begin
    for rec in select * from v_old_table LOOP
        raise notice 'OLD: %', rec;
    end loop;
    for rec in select * from v_new_table LOOP
        raise notice 'NEW: %', rec;
    end loop;
    return NEW;
end;
$$ language plpgsql;

create trigger tg_test
    after update on test
    referencing new table as v_new_table
        old table as v_old_table
    for each statement
    execute procedure test_trigger();

执行update操作

postgres=# update test set info = 'test' where id between 2 and 4;
NOTICE:  OLD: (2,Two)
NOTICE:  OLD: (3,Three)
NOTICE:  OLD: (4,Four)
NOTICE:  NEW: (2,test)
NOTICE:  NEW: (3,test)
NOTICE:  NEW: (4,test)
UPDATE 3

对上面的v_old_table过度表进行处理即可。