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<H1>ORACLE SQL性能優(yōu)化系列 (十)</H1>
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<TD align=middle height=20>http://Tech.acnow.net 2005-4-29 <FONT color=#a20010>網(wǎng)絡(luò)</FONT></TD>
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<TD class=l17><FONT class=f14 id=zoom><BR><BR> 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'EMP'<BR><BR> 2 1 INDEX (RANGE SCAN) OF 'EMPNO' (UNIQUE) <BR><BR> <BR><BR>SQL> select * from emp where empno <= 1;<BR><BR>no rows selected<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 'EMPNO' (UNIQUE)<BR><BR> <BR><BR> 兩者的效率完全一樣，也許這符合作者關(guān)于” 在某些時候, ORACLE優(yōu)化器會自動將NOT轉(zhuǎn)化成相對應(yīng)的關(guān)系操作符” 的觀點．<BR><BR> <BR><BR>35. 用>=替代><BR><BR> <BR><BR>如果DEPTNO上有一個索引, <BR><BR> <BR><BR>高效:<BR><BR> <BR><BR> SELECT *<BR><BR> FROM EMP<BR><BR> WHERE DEPTNO >=4<BR><BR> <BR><BR> 低效:<BR><BR> <BR><BR> SELECT *<BR><BR> FROM EMP<BR><BR> WHERE DEPTNO >3<BR><BR> <BR><BR> 兩者的區(qū)別在于, 前者DBMS將直接跳到第一個DEPT等于4的記錄而后者將首先定位到DEPTNO=3的記錄并且向前掃描到第一個DEPT大于3的記錄.<BR><p><div align="right">本新聞共<font color=red>2</font>頁,當(dāng)前在第<font color=red>2</font>頁 <a href="164559059816455944859.shtml">1</a> <font color="red">2</font> </div></p>
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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/2005-4/29/164558059816455810544.shtml title="ORACLE SQL性能優(yōu)化系列 (一)">ORACLE SQL性能優(yōu)化系列 (一)</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/2005-4/29/164559059816455980543.shtml title="ORACLE SQL性能優(yōu)化系列 (十二)">ORACLE SQL性能優(yōu)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/2005-4/29/16455905981645592205.shtml title="ORACLE SQL性能優(yōu)化系列 (十三)">ORACLE SQL性能優(yōu)化系列 (十三)</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/2005-4/29/164558059816455862139.shtml title="ORACLE SQL性能優(yōu)化系列 (二)">ORACLE SQL性能優(yōu)化系列 (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/SoftWare/SoftwareEducate/OracleAuthentication/2006-5/29/150826355.shtml title="ORACLE SQL性能優(yōu)化系列 (一)">ORACLE SQL性能優(yōu)化系列 (一)</a></td></tr>
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