Oracle9iPL/SQL编程的经验小结
正在看的ORACLE教程是:Oracle9iPL/SQL编程的经验小结。平时在PL/SQL中的编程中遇到一些问题,这里以问答的形式来进行把它们总结下来,以供大家分享。1、当需要向表中装载大量的数据流或者需要处理大量的数据流的时候,能否使用管道提高处理效率?
管道函数对于改善并行查询的性能非常方便,它加快往表中加载数据的速度。管道函数的使用总结如下两点:
每当在查询里使用PL/SQL函数时,该查询将被序列化,即一个处理器只能运行一个查询实例,那么在这种情况下不可能使用并行查询(比如在数据仓库中要经常使用这项技术)。因此,为了使用并行查询就必须使用管道函数,这样也就加快了执行的速度。
管道函数的输入参数必须是一个引用记录集类型(即ref cursor),而返回的是嵌套表类型(其表中每一行对应每一个引用记录)。在使用管道函数之前,必须先在程序头写上PARALLEL_ENABLE,这样才能在查询语句中使用管道函数来处理了。
2. 如何使PL/SQL程序等待一段时间执行?
方法就是使用DBMS_LOCK包的SLEEP函数,可以进行精确定时,其语法为:
DBMS_LOCK.SLEEP (seconds IN NUMBER);
3.需要在一张表插入一条记录之后等若干秒后再执行另外一个操作,如何在PL/SQL程序里进行定时操作?
一般的做法是用循环作延迟,利用 DBMS_UTILITY的gettime函数来检测当前的时间,程序代码如下:
DECLARE
v_delaytime CONSTANT INTEGER := 100;
v_starttime INTEGER ;
v_endtime INTEGER ;
BEGIN
V_starttime := DBMS_UTILITY.get_time;
V_endtime := DBMS_UTILITY.get_time;
While abs(V_endtime- V_starttime)/*空循环或者简单的耗时执行语句*/
End loop;
END;
/
另外如果是不同会话(session)之间的定时,就必须使用DBMS_PIPE包的函数来实现会话间的消息传递。
4.当PL/SQL返回一个数据集的时候,该使用集合还是游标?
一般情况下,有以下两点作为依据:
1) 如果PL/SQL程序返回多多行数据给另外一个PL/SQL程序的话,这里就建议使用集合,因为这样可以利用集合的批收集(bulk collection)来提高从数据库提取数据的速度。
2) 如果需要在PL/SQL程序的环境中把数据返回到宿主语言环境中(如Sql*plus,c,delphi等),这时应该使用游标变量来返回这些数据,因为几乎所有的宿主语言都支持游标变量,但不是所有的宿主语言都支持集合。这样可以增强程序的可移植性。
5.如何更有效的在PL/SQL中使用游标?
游标是PL/SQL中一个非常重要的概念,对数据库的检索主要依靠游标来操作。在PL/SQL中有两类游标,一类是隐式游标,如select clno into v_clno from table_detail.另外一类是显式游标,如cursor v_cur is select clno from table_detail。对于游标的使用这里给出以下几点建议:
1) 尽可能的使用bulk collection。它能够较大的提高运行性能,在Oracl9i的第二版,甚至可以使用bulk collection来直接将数据写入到记录表
2) 尽量使用显式游标来处理,因为相对于隐式游标来说,显式游标的速度更快一些。
3) 如果查询的表很小或者是静态的,可以把该表缓存到一个包级的集合里。这样,你的查询函数就直接从集合里(即进程全局区,PGA cache),而不是从系统全局区(SGA)来取数据,这样的处理速度会提升很多。

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