📄 oracle sql性能优化系列 (十二)--acnow_net.htm
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<H1>ORACLE SQL性能优化系列 (十二)</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>39. 总是使用索引的第一个列<BR><BR>如果索引是建立在多个列上, 只有在它的第一个列(leading column)被where子句引用时,优化器才会选择使用该索引. <BR><BR> <BR><BR>译者按:<BR><BR>这也是一条简单而重要的规则. 见以下实例.<BR><BR> <BR><BR>SQL> create table multiindexusage ( inda number , indb number , descr varchar2(10));<BR><BR>Table created.<BR><BR>SQL> create index multindex on multiindexusage(inda,indb);<BR><BR>Index created.<BR><BR>SQL> set autotrace traceonly<BR><BR> <BR><BR>SQL> select * from multiindexusage where inda = 1;<BR><BR>Execution Plan<BR><BR>----------------------------------------------------------<BR><BR> 0 SELECT STATEMENT Optimizer=CHOOSE<BR><BR> 1 0 TABLE ACCESS (BY INDEX ROWID) OF 'MULTIINDEXUSAGE'<BR><BR> 2 1 INDEX (RANGE SCAN) OF 'MULTINDEX' (NON-UNIQUE)<BR><BR> <BR><BR>SQL> select * from multiindexusage where indb = 1;<BR><BR>Execution Plan<BR><BR>----------------------------------------------------------<BR><BR> 0 SELECT STATEMENT Optimizer=CHOOSE<BR><BR> 1 0 TABLE ACCESS (FULL) OF 'MULTIINDEXUSAGE'<BR><BR> <BR><BR>很明显, 当仅引用索引的第二个列时,优化器使用了全表扫描而忽略了索引<BR><BR> <BR><BR> <BR><BR>40. ORACLE内部操作<BR><BR>当执行查询时,ORACLE采用了内部的操作. 下表显示了几种重要的内部操作.<BR><BR>ORACLE Clause<BR><BR>内部操作<BR><BR>ORDER BY<BR><BR>SORT ORDER BY<BR><BR>UNION<BR><BR>UNION-ALL<BR><BR>MINUS<BR><BR>MINUS<BR><BR>INTERSECT<BR><BR>INTERSECT<BR><BR>DISTINCT,MINUS,INTERSECT,UNION<BR><BR>SORT UNIQUE<BR><BR>MIN,MAX,COUNT<BR><BR>SORT AGGREGATE<BR><BR>GROUP BY<BR><BR>SORT GROUP BY<BR><BR>ROWNUM<BR><BR>COUNT or COUNT STOPKEY<BR><BR>Queries involving Joins<BR><BR>SORT JOIN,MERGE JOIN,NESTED LOOPS<BR><BR>CONNECT BY<BR><BR>CONNECT BY<BR><BR> <BR><BR> <BR><BR>41. 用UNION-ALL 替换UNION ( 如果有可能的话)<BR><BR> <BR><BR>当SQL语句需要UNION两个查询结果集合时,这两个结果集合会以UNION-ALL的方式被合并, 然后在输出最终结果前进行排序.<BR><BR>如果用UNION ALL替代UNION, 这样排序就不是必要了. 效率就会因此得到提高.<BR><BR> <BR><BR>举例:<BR><BR> 低效：<BR><BR> SELECT ACCT_NUM, BALANCE_AMT<BR><BR> FROM DEBIT_TRANSACTIONS<BR><BR> WHERE TRAN_DATE = ’31-DEC-95’<BR><BR> UNION<BR><BR> SELECT ACCT_NUM, BALANCE_AMT<BR><BR> FROM DEBIT_TRANSACTIONS<BR><BR> WHERE TRAN_DATE = ’31-DEC-95’<BR><BR>高效:<BR><BR> SELECT ACCT_NUM, BALANCE_AMT<BR><BR> FROM DEBIT_TRANSACTIONS<BR><BR> WHERE TRAN_DATE = ’31-DEC-95’<BR><BR> UNION ALL<BR><BR> SELECT ACCT_NUM, BALANCE_AMT<BR><BR> FROM DEBIT_TRANSACTIONS<BR><BR> WHERE TRAN_DATE = ’31-DEC-95’<BR><BR> <BR><BR>译者按:<BR><BR>需要注意的是，UNION ALL 将重复输出两个结果集合中相同记录. 因此各位还是<BR><BR>要从业务需求分析使用UNION ALL的可行性.<BR><BR>UNION 将对结果集合排序,这个操作会使用到SORT_AREA_SIZE这块内存. 对于这<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="164559059816455980543_2.shtml">2</a> </div></p>
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