Re: Re: performance issue on select count(*)

"Exactly" my point! ;-)

You cannot use it for anymore than that, and neither should you ignore it completely.

• Original Message ----- To: "Multiple recipients of list ORACLE-L" <ORACLE-L_at_fatcity.com> Sent: Wednesday, October 29, 2003 6:39 AM

> yes bchr is only useful at extremes, but its based on interpretation. if
you have a very high BCHR, you probably have alot of very bad sql.
>
> if you have a very low one AND are in a type of application where you
should(namely OLTP) you may want to consider increasing your buffer cache.
>
> mladen is right. there is no 'exact' very high and very low. you have to
interpret it.
>
> that is about it. Anyone who uses it for anymore than that is wrong.
> >
> > From: Mladen Gogala <mladen_at_wangtrading.com>
> > Date: 2003/10/28 Tue PM 12:09:34 EST
> > To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
> > Subject: Re: performance issue on select count(*)
> >
> > So, what exactly is indicated by a high or low hit rate? What, exactly,
is "high"
> > and what do you consider "low"?
> > What "HR" are you talking about?
> > This would be the infamous BCHR:
> >
> > select 'bc_hit_ratio' ratio,( sum(decode(name, 'consistent
gets',value,0))
> > + sum(decode(name,'db block gets', value,0))
> > - sum(decode(name,'physical reads', value,0)))
> > / ( sum(decode(name, 'consistent gets',value,0))
> > + sum(decode(name,'db block gets', value,0)) ) * 100
> > from v$sysstat
> >
> > What exactly should the number returned by this query tell me?
> >
> >
> > On 10/28/2003 10:59:25 AM, Binley Lim wrote:
> > >
> > > The symptom suggests caching is a big factor here - most likely
> > > block-buffers.
> > >
> > > Contrary to ?current? popular beliefs, BCHR is still a very relevant
> > > performance indicator - either being very high, or being too low -
both of
> > > which gives a good indication of something that needs to be looked at.

> > >
> > >
> > > > I would be interested to know if there is a way to speed up the
initial
> > > > execution or how to diagnose what the delay was. It does not seems
right
> > > > that there is such a big difference in elapsed time between the
initial
> > > and
> > > > subsequent execution.
> > >
> > >
> > > --
> > > Please see the official ORACLE-L FAQ: http://www.orafaq.net
> > > --
> > > Author: Binley Lim
> > > INET: Binley.Lim_at_xtra.co.nz
> > >
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> > >
> >
> > Mladen Gogala
> > Oracle DBA
> >
> >
> >
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> > --
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> > --
> > Author: Mladen Gogala
> > INET: mladen_at_wangtrading.com
> >
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> Author: <ryan_oracle_at_cox.net
> INET: ryan_oracle_at_cox.net
>
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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.net
-- 
Author: Binley Lim
  INET: Binley.Lim_at_xtra.co.nz

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