📄 oracle sql性能优化系列 (九)-1-acnow_net.htm
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<H1>ORACLE SQL性能优化系列 (九)</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><BR> <BR><BR> 这里, ORACLE只用到了DEPT_NO索引. 执行路径如下:<BR><BR> <BR><BR> TABLE ACCESS BY ROWID ON EMP<BR><BR> INDEX RANGE SCAN ON DEPT_IDX<BR><BR> <BR><BR>译者按:<BR><BR>我们来试一下以下这种情况:<BR><BR>SQL> select index_name, uniqueness from user_indexes where table_name = 'EMP';<BR><BR> <BR><BR>INDEX_NAME UNIQUENES<BR><BR>------------------------------ ---------<BR><BR>EMPNO UNIQUE<BR><BR>EMPTYPE NONUNIQUE<BR><BR> <BR><BR>SQL> select * from emp where empno >= 2 and emp_type = 'A' ;<BR><BR> <BR><BR>no rows selected<BR><BR> <BR><BR> <BR><BR>Execution Plan<BR><BR>----------------------------------------------------------<BR><BR> 0 SELECT STATEMENT Optimizer=CHOOSE<BR><BR> 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'EMP'<BR><BR> 2 1 INDEX (RANGE SCAN) OF 'EMPTYPE' (NON-UNIQUE)<BR><BR> <BR><BR>虽然EMPNO是唯一性索引,但是由于它所做的是范围比较, 等级要比非唯一性索引的等式比较低!<BR><BR> <BR><BR> <BR><p><div align="right">本新闻共<font color=red>2</font>页,当前在第<font color=red>2</font>页 <a href="164558059816455832430.shtml">1</a> <font color="red">2</font> </div></p>
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上一篇：<a href='../../2005-4/29/164558059816455829048.shtml' title ='说一说Oracle的优化器(Optimizer)'>说一说Oracle的优化器(Optimizer)</a> 下一篇：<a href='../../2005-4/29/16455905981645599656.shtml' title ='ORACLE常用傻瓜問題1000問(之四)'>ORACLE常用傻瓜問題1000問(之四)</a>
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<td><img src="http://tech.acnow.net/Files/Img/list_nav_spacer.gif"><a Class="a2" href= http://tech.acnow.net/Html/DataBase/Oracle/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></td></tr>
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<td><img src="http://tech.acnow.net/Files/Img/list_nav_spacer.gif"><a Class="a2" href= http://tech.acnow.net/Html/DataBase/Oracle/2005-4/29/164558059816455832996.shtml title="ORACLE SQL性能优化系列 (四)">ORACLE SQL性能优化系列 (四)</a></td></tr>
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