postgresql/lightdb中WHERE CURRENT OF的使用

最近看PG源码，在语法分析中有看到CURRENT OF元素。在PG游标中，WHERE CURRENT OF可以用来使用游标更新或删除当前指向的行。

语法：​​UPDATE table SET ... WHERE CURRENT OF cursor;​​​ 语法：​​DELETE FROM table WHERE CURRENT OF cursor;​​

实例：

select * from tf1;
 c1 |  c2  |   c3   |    c4    |  c5   
----+------+--------+----------+-------
  1 | 1000 | China  | Dalian   | 23000
  2 | 4000 | Janpan | Tokio    | 45000
  3 | 1500 | China  | Xian     | 25000
  4 |  300 | China  | Changsha | 24000
  5 |  400 | USA    | New York | 35000
  6 | 5000 | USA    | Bostom   | 15000


CREATE  OR REPLACE FUNCTION tfun3() RETURNS int AS $$
DECLARE
    curs1 refcursor;
    y tf1%ROWTYPE;
BEGIN
    open curs1 FOR SELECT * FROM tf1;
    fetch last from curs1 into y; 
    RAISE NOTICE 'curs1 : %', y.c2;
    
    delete from tf1 WHERE CURRENT OF curs1;
    return 0;
END;
$$ LANGUAGE plpgsql;

select tfun3();
select * from tf1;
 c1 |  c2  |   c3   |    c4    |  c5   
----+------+--------+----------+-------
  1 | 1000 | China  | Dalian   | 23000
  2 | 4000 | Janpan | Tokio    | 45000
  3 | 1500 | China  | Xian     | 25000
  4 |  300 | China  | Changsha | 24000
  5 |  400 | USA    | New York | 35000

这样，最后一行就被删除。