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<p><FONT class=normalfont><B><font color=blue>Oracle数据库系统使用经验六则</font></B></FONT><BR><FONT class=smallfont color=#ff9900>2004-04-23 15:18 pm</FONT><BR><FONT class=normalfont>作者：赵晋<br>来自：合肥市电信局计算中心<br>联系方式：无名<br><br>---- 笔者的工作与Oracle数据库"息息相关",在实践中学习和摸索了一些小经验,在此与大家共同探讨. <br>
<br>
---- 1.having 子句的用法 <br>
<br>
---- having 子句对 group by 子句所确定的行组进行控制,having 子句条件中只允许涉及常量,聚组函数或group by 子句中的列. <br>
<br>
---- 2.外部联接"+"的用法 <br>
<br>
---- 外部联接"+"按其在"="的左边或右边分左联接和右联接.若不带"+"运算符的表中的一个行不直接匹配于带"+"预算符的表中的任何行,则前者的行与后者中的一个空行相匹配并被返回.若二者均不带'+',则二者中无法匹配的均被返回.利用外部联接"+",可以替代效率十分低下的 not in 运算,大大提高运行速度.例如,下面这条命令执行起来很慢 <br>
<br>
select a.empno from emp a where a.empno not in<br>
(select empno from emp1 where job='SALE');<br>
<br>
---- 倘若利用外部联接,改写命令如下: <br>
<br>
select a.empno from emp a ,emp1 b <br>
where a.empno=b.empno(+) <br>
and b.empno is null <br>
and b.job='SALE';<br>
<br>
---- 可以发现,运行速度明显提高. <br>
<br>
---- 3.删除表内重复记录的方法 <br>
<br>
---- 可以利用这样的命令来删除表内重复记录: <br>
<br>
delete from table_name a <br>
where rowid< (select max(rowid) from table_name <br>
where column1=a.column1 and column2=a.column2 <br>
and colum3=a.colum3 and ...);<br>
<br>
---- 不过,当表比较大(例如50万条以上)时,这个方法的效率之差令人无法忍受,需要另想办法(可参看拙文《电信计费中长途重复话单的技术处理》,《计算机与通信》,1999-07). <br>
<br>
---- 4.set transaction 命令的用法 <br>
<br>
---- 在执行大事务时,有时oracle会报出如下的错误: <br>
<br>
ORA-01555:snapshot too old (rollback segment too small)<br>
<br>
---- 这说明oracle给此事务随机分配的回滚段太小了,这时可以为它指定一个足够大的回滚段,以确保这个事务的成功执行.例如 <br>
<br>
set transaction use rollback segment roll_abc;<br>
delete from table_name where ...<br>
commit;<br>
<br>
---- 回滚段roll_abc被指定给这个delete事务,commit命令则在事务结束之后取消了回滚段的指定. <br>
<br>
---- 5.使用索引的注意事项 <br>
<br>
---- select,update,delete 语句中的子查询应当有规律地查找少于20%的表行.如果一个语句查找的行数超过总行数的20%,它将不能通过使用索引获得性能上的提高. <br>
<br>
---- 索引可能产生碎片,因为记录从表中删除时,相应也从表的索引中删除.表释放的空间可以再用,而索引释放的空间却不能再用.频繁进行删除操作的被索引的表,应当阶段性地重建索引,以避免在索引中造成空间碎片,影响性能.在许可的条件下,也可以阶段性地truncate表,truncate命令删除表中所有记录,也删除索引碎片. <br>
<br>
---- 6.数据库重建应注意的问题 <br>
<br>
---- 在利用import进行数据库重建过程中,有些视图可能会带来问题,因为结构输入的顺序可能造成视图的输入先于它低层次表的输入,这样建立视图就会失败.要解决这一问题,可采取分两步走的方法:首先输入结构,然后输入数据.命令举例如下 (uesrname:jfcl,password:hfjf,host sting:ora1,数据文件:expdata.dmp): <br>
<br>
imp jfcl/hfjf@ora1 file=empdata.dmp rows=N<br>
imp jfcl/hfjf@ora1 file=empdata.dmp full=Y buffer=64000 <br>
commit=Y ignore=Y<br>
<br>
---- 第一条命令输入所有数据库结构,但无记录.第二次输入结构和数据,64000字节提交一次.ignore=Y选项保证第二次输入既使对象存在的情况下也能成功. <br>
<br>
结束<br>
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