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RE: long runtimes cured by cutting hash_area_size by 90% - ???

From: C. Barr <cbarr_at_hotpop.com>
Date: Tue, 15 Apr 2003 06:14:14 -0800
Message-ID: <F001.0058151C.20030415061414@fatcity.com>


I'm very curious about 9i & CBO & hash-join-related params.

At 9i, 4 or so of these parameters are dynamically set (i.e. when unset or zero):

At 8i, only hash_multiblock_io_count should be "unset or zero". Changing this - alone - to "unset or zero" stopped a problem of filling up TEMP tablespace. But we had to lower hash_area_size to stop this 94M-row hash-join.

But the question remains, will CBO at 9i do a better job of fending off crazy cartesian-product joins. If params are set dynamically, it may use hash-join to get giant result sets which are skipped when, for example, a low hash_area_size is set.

> You specifically mentioned Oracle 8 (inclusive of 8i I assume?). Does your
> mention of 8 imply that 9i does a better job of exercising "damage
> limitation"?
>
> Regards,
>
> Larry G. Elkins
> elkinsl_at_flash.net
> 214.954.1781
>
> > -----Original Message-----
> > From: root_at_fatcity.com [mailto:root_at_fatcity.com]On Behalf Of Jonathan
> > Lewis
> > Sent: Monday, April 14, 2003 9:49 AM
> > To: Multiple recipients of list ORACLE-L
> > Subject: Re: long runtimes cured by cutting hash_area_size by 90% - ???
> >
> >
> >
> > The general principle is that multiple
> > hash joins can be running concurrently
> > and Oracle 8 is not very sensible about
> > choosing how to exercise damage limitation
> > when this occurs. (As it often does in
> > warehouse systems).
> >
> > The side-effects, which are very much
> > query and data depended, are extreme
> > uses of memory, or extreme thrashing
> > of temporary space.
> >
> >
> > Regards
> >
> > Jonathan Lewis
> > http://www.jlcomp.demon.co.uk
>
> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.net
> --
> Author: Larry Elkins
> INET: elkinsl_at_flash.net
>
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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: C. Barr
  INET: cbarr_at_hotpop.com

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Received on Tue Apr 15 2003 - 09:14:14 CDT

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