oracle sql性能优化系列 (十一)--acnow_net.htm
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<H1>ORACLE SQL性能优化系列 (十一)</H1>
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<TD align=middle height=20>http://Tech.acnow.net 2005-4-29 <FONT color=#a20010>网络</FONT></TD>
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<TD class=l17><FONT class=f14 id=zoom><BR>36. 用UNION替换OR (适用于索引列)<BR><BR>通常情况下, 用UNION替换WHERE子句中的OR将会起到较好的效果. 对索引列使用OR将造成全表扫描. 注意, 以上规则只针对多个索引列有效. 如果有column没有被索引, 查询效率可能会因为你没有选择OR而降低. <BR><BR> 在下面的例子中, LOC_ID 和REGION上都建有索引.<BR><BR>高效:<BR><BR> SELECT LOC_ID , LOC_DESC , REGION<BR><BR> FROM LOCATION<BR><BR> WHERE LOC_ID = 10<BR><BR> UNION<BR><BR> SELECT LOC_ID , LOC_DESC , REGION<BR><BR> FROM LOCATION<BR><BR> WHERE REGION = “MELBOURNE”<BR><BR> <BR><BR>低效:<BR><BR> SELECT LOC_ID , LOC_DESC , REGION<BR><BR> FROM LOCATION<BR><BR> WHERE LOC_ID = 10 OR REGION = “MELBOURNE”<BR><BR> <BR><BR>如果你坚持要用OR, 那就需要返回记录最少的索引列写在最前面.<BR><BR> <BR><BR>注意:<BR><BR> <BR><BR>WHERE KEY1 = 10 (返回最少记录)<BR><BR>OR KEY2 = 20 (返回最多记录)<BR><BR> <BR><BR>ORACLE 内部将以上转换为<BR><BR>WHERE KEY1 = 10 AND<BR><BR>((NOT KEY1 = 10) AND KEY2 = 20) <BR><BR> <BR><BR>译者按: <BR><BR> <BR><BR>下面的测试数据仅供参考: (a = 1003 返回一条记录 , b = 1 返回1003条记录)<BR><BR>SQL> select * from unionvsor /*1st test*/<BR><BR> 2 where a = 1003 or b = 1;<BR><BR>1003 rows selected.<BR><BR>Execution Plan<BR><BR>----------------------------------------------------------<BR><BR> 0 SELECT STATEMENT Optimizer=CHOOSE<BR><BR> 1 0 CONCATENATION<BR><BR> 2 1 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR'<BR><BR> 3 2 INDEX (RANGE SCAN) OF 'UB' (NON-UNIQUE)<BR><BR> 4 1 TABLE ACCESS (BY INDEX ROWID) OF 'UNIONVSOR'<BR><BR> 5 4 INDEX (RANGE SCAN) OF 'UA' (NON-UNIQUE)<BR><BR>Statistics<BR><BR>----------------------------------------------------------<BR><BR> 0 recursive calls<BR><BR> 0 db block gets<BR><BR> 144 consistent gets<BR><BR> 0 physical reads<BR><BR> 0 redo size<BR><BR> 63749 bytes sent via SQL*Net to client<BR><BR> 7751 bytes received via SQL*Net from client<BR><BR> 68 SQL*Net roundtrips to/from client<BR><BR> 0 sorts (memory)<BR><BR> 0 sorts (disk)<BR><BR> 1003 rows processed<BR><BR>SQL> select * from unionvsor /*2nd test*/<BR><BR> 2 where b = 1 or a = 1003 ; <BR><BR>1003 rows selected.<BR><BR>Execution Plan<BR><BR>----------------------------------------------------------<BR><BR> 0 SELECT STATEMENT Optimizer=CHOOSE<BR><BR> 1 0 CONCATENATION<p><div align="right">本新闻共<font color=red>3</font>页,当前在第<font color=red>1</font>页 <font color="red">1</font> <a href="164559059816455975203_2.shtml">2</a> <a href="164559059816455975203_3.shtml">3</a> </div></p>
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上一篇：<a href='../../2005-4/29/164559059816455954036.shtml' title ='ORACLE常用傻瓜問題1000問(之五)'>ORACLE常用傻瓜問題1000問(之五)</a> 下一篇：<a href='../../2005-4/29/164559059816455980543.shtml' title ='ORACLE SQL性能优化系列 (十二)'>ORACLE SQL性能优化系列 (十二)</a>
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