📄 oracle sql性能优化系列 (四)--acnow_net.htm
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<H1>ORACLE SQL性能优化系列 (四)</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>13. 计算记录条数<BR><BR> 和一般的观点相反, count(*) 比count(1)稍快 , 当然如果可以通过索引检索,对索引列的计数仍旧是最快的. 例如 COUNT(EMPNO)<BR><BR> <BR><BR>(译者按: 在CSDN论坛中,曾经对此有过相当热烈的讨论, 作者的观点并不十分准确,通过实际的测试,上述三种方法并没有显著的性能差别)<BR><BR> <BR><BR>14. 用Where子句替换HAVING子句<BR><BR> <BR><BR> 避免使用HAVING子句, HAVING 只会在检索出所有记录之后才对结果集进行过滤. 这个处理需要排序,总计等操作. 如果能通过WHERE子句限制记录的数目,那就能减少这方面的开销.<BR><BR> <BR><BR>例如:<BR><BR> <BR><BR> 低效:<BR><BR> SELECT REGION，AVG(LOG_SIZE)<BR><BR> FROM LOCATION<BR><BR> GROUP BY REGION<BR><BR> HAVING REGION REGION != ‘SYDNEY’<BR><BR> AND REGION != ‘PERTH’<BR><BR> <BR><BR> 高效<BR><BR> SELECT REGION，AVG(LOG_SIZE)<BR><BR> FROM LOCATION<BR><BR> WHERE REGION REGION != ‘SYDNEY’<BR><BR> AND REGION != ‘PERTH’<BR><BR> GROUP BY REGION<BR><BR>(译者按: HAVING 中的条件一般用于对一些集合函数的比较,如COUNT() 等等. 除此而外,一般的条件应该写在WHERE子句中)<BR><BR> <BR><BR>15. 减少对表的查询<BR><BR>在含有子查询的SQL语句中,要特别注意减少对表的查询.<BR><BR> <BR><BR>例如: <BR><BR> 低效<BR><BR> SELECT TAB_NAME<BR><BR> FROM TABLES<BR><BR> WHERE TAB_NAME = ( SELECT TAB_NAME <BR><BR> FROM TAB_COLUMNS<BR><BR> WHERE VERSION = 604)<BR><BR> AND DB_VER= ( SELECT DB_VER <BR><BR> FROM TAB_COLUMNS<BR><BR> WHERE VERSION = 604)<BR><BR> <BR><BR> 高效<BR><BR> SELECT TAB_NAME<BR><BR> FROM TABLES<BR><BR> WHERE (TAB_NAME,DB_VER)<BR><BR> = ( SELECT TAB_NAME,DB_VER) <BR><BR> FROM TAB_COLUMNS<BR><BR> WHERE VERSION = 604)<p><div align="right">本新闻共<font color=red>2</font>页,当前在第<font color=red>1</font>页 <font color="red">1</font> <a href="164558059816455832996_2.shtml">2</a> </div></p>
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