转自：http://www.cnblogs.com/yuanjw/archive/2010/03/04/1678093.html
先看下面的一小段代码
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-->ORACLE采用自下而上的顺序解析WHERE子句,根据这个原理,表之间的连接必须写在其他WHERE条件之前,
那些可以过滤掉最大数量记录的条件必须写在WHERE子句的末尾.
例如:
(低效)
SELECT …
FROM EMP E
WHERE
SAL > 50000
AND JOB = ‘MANAGER’
AND 25 < (SELECT COUNT(*) FROM EMP WHERE MGR=E.EMPNO);
(高效)
SELECT …
FROM EMP E
WHERE
25 < (SELECT COUNT(*) FROM EMP WHERE MGR=E.EMPNO)
AND SAL > 50000
AND JOB = ‘MANAGER’;
举例说明:
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-->UPDATE XCSV_TEXT A SET ID05 = (
SELECT (COUNT(DISTINCT ID05) + 1)
FROM XCSV_TEXT B
WHERE
B.ID04 IN ('MD','RT')
AND TO_NUMBER(B.ID05) < TO_NUMBER(A.ID05)
)
WHERE A.ID04 IN ('MD','RT');
如果用这句代码来操作上面表中的数据,那么将会报错.
根据Oracle中Where条件的执行顺序,将会先执行TO_NUMBER(B.ID05) < TO_NUMBER(A.ID05)这一段, “Ver000017b65b65242dde1a9a014ac0e525ffb60”是不能To_Number的.
而改成
<!--
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-->UPDATE XCSV_TEXT A SET ID05 = (
SELECT (COUNT(DISTINCT ID05) + 1)
FROM XCSV_TEXT B
WHERE
TO_NUMBER(B.ID05) < TO_NUMBER(A.ID05)
AND B.ID04 IN ('MD','RT')
)
WHERE A.ID04 IN ('MD','RT');
就正常了,而且提高了执行效率(至于为什么提高了执行效率,不做讨论.).
结论:
ORACLE采用自下而上的顺序解析WHERE子句,编写Where条件需要注意顺序,为了避免不必要的错误,也为了提高执行效率!
来自 “ ITPUB博客 ” ，链接：http://blog.itpub.net/22980662/viewspace-708871/，如需转载，请注明出处，否则将追究法律责任。
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