RE: SELECT x FROM DUAL is bad for system performance?

From: Khedr, Waleed <Waleed.Khedr_at_FMR.COM>
Date: Thu, 13 Dec 2001 10:05:13 -0800
Message-ID: <F001.003DC115.20011213095106@fatcity.com>

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!
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size=2>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.
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size=2>Regards,
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size=2>Waleed

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  size=2>-----Original Message-----From: Thomas Jeff   [mailto:ThomasJe_at_tce.com]Sent: Thursday, December 13, 2001 9:00   AMTo: Multiple recipients of list ORACLE-LSubject:   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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  face="Courier New" size=2>Jeffery D Thomas <FONT face="Courier New"   size=2>DBA Thomson Information
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  face="Courier New" size=2>  Received on Thu Dec 13 2001 - 12:05:13 CST