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RE: Oracle position on hints

From: Jesse, Rich <Rich.Jesse_at_qtiworld.com>
Date: Mon, 10 Mar 2003 13:59:49 -0800
Message-ID: <F001.0056559A.20030310135949@fatcity.com>


Ewww! Could be dangerous on low cardinality b-tree columns (that's a whole other thread!) if an ad-hoc mistakenly filters on the "wrong" (majority) value.

Thanks for the heads-up! Time to RTFM (or wait until I get sent to 9i class. :)

Rich

Rich Jesse                        System/Database Administrator
rich.jesse_at_qtiworld.com           Quad/Tech International, Sussex, WI USA

-----Original Message-----
Sent: Monday, March 10, 2003 3:54 PM
To: oracle-l_at_fatcity.com
Cc: Jesse, Rich
Importance: High

>From the performance tuning guide:

The CBO peeks at the values of user-defined bind variables on the first invocation of a cursor. This feature lets the optimizer determine the selectivity of any WHERE clause condition, as well as if literals have been used instead of bind
variables. On subsequent invocations of the cursor, no peeking takes place, and the cursor is shared, based on the standard cursor-sharing criteria, even if subsequent invocations use different bind values.

CBO only peeks the bind variable once, so that wouldn't help much for star transforms and histograms.

Jared

"Jesse, Rich" <Rich.Jesse_at_qtiworld.com>
 03/10/2003 01:39 PM  

        To:     "'ORACLE-L_at_fatcity.com'" <ORACLE-L_at_fatcity.com>
        cc:     "'Jared.Still_at_radisys.com'" <Jared.Still_at_radisys.com>
        Subject:        RE: Oracle position on hints


I thought that went out the window with 9i because it can snoop at the bind
variables??

Rich

Rich Jesse                        System/Database Administrator
rich.jesse_at_qtiworld.com           Quad/Tech International, Sussex, WI USA


-----Original Message-----
Sent: Monday, March 10, 2003 12:09 PM
To: Multiple recipients of list ORACLE-L

Well, I wouldn't say you should *always* use bind variables.

Many situations in a database warehouse preclude that.

Potentially long running queries may need to have literals to help the CBO make the right choice.

Star transforms don't work with bind variables, and histograms can't be used with bind variables.

Jared

-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
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Author: Jesse, Rich
  INET: Rich.Jesse_at_qtiworld.com

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Received on Mon Mar 10 2003 - 15:59:49 CST

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