The
problem is when you try to evaluate an expression by using: SELECT expression
FROM dual
If you
do this sometimes in a query that utilizes PQO on a big table then this
expression could be evaluated for every row retrieved!
Sometimes I still have to do it this way because of other performance
reasons that need a week to explain but instead I use the hint PUSH_SUBQ to
evaluate it first and once.
Regards,
Waleed
-----Original Message-----
From: Thomas Jeff [mailto:[EMAIL PROTECTED]]
Sent: Thursday, December 13, 2001 9:00 AM
To: Multiple recipients of list ORACLE-L
Subject: SELECT x FROM DUAL is bad for system performance?On page 92 of Oracle Performance Tuning 101, there's a recommendation
to avoid the use of SELECT X FROM DUAL, claiming that it can '..use up system
performance in a hurry', but does not go on to elaborate as to exactly *why*.Could someone shed a bit more light on this? Thanks.
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Jeffery D Thomas
DBA
Thomson Information Services
Thomson multimedia Inc.Email: [EMAIL PROTECTED]
DBA Quickplace: http://gkmqp.tce.com/tis_dba
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