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Home -> Community -> Mailing Lists -> Oracle-L -> RE: Currval and buffer gets
location type point (? means dubious value)..........
-------------------- -------- --------------------
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ksedmp()+168 CALL ksedst()+0 2D8 ? 1 ? EFFFBD98 ? EFFFB83C ? EFFFB820 ? 0 ?
-----Original Message-----
[mailto:Bruce.Reardon_at_comalco.riotinto.com.au]
Sent: Tuesday, April 23, 2002 8:39 PM
To: ORACLE-L_at_fatcity.com
Cc: Waleed.Khedr_at_FMR.COM
Waleed,
I am using 81714 under NT4.
I can NOT duplicate the problem you're seeing (but I ran it logged in as sys
and used sys.x$dual).
ie - I can run your script from 2 simultaneous windows & it works ok.
What platform & patch are you on?
Timing wise - the version using sys.x$dual took 500 or 600cs whilst the one using plain dual took 900 or 1000 cs.
I then created a view (x_$dual) on sys.x$dual and granted select to another
user.
Running from that other (DBA) user I again could not reproduce the problem
you see.
The performance in this case (using a view) was not noticeably different to being logged in as sys.
What error does your second session get?
Regards
Bruce Reardon
-----Original Message-----
Sent: Wednesday, 24 April 2002 10:43
No, I couldn't duplicate this effect (8.1.7.0.0/Linux and 9.0.1.1.1/Win2k) - 1) first test 2 simultaneous sessions and then 2) second test 3 simultaneous sessions running.
Regards
Lyubomir Petrov
-----Original Message-----
Sent: Wednesday, 24 April 2002 9:43
Never mess up with Oracle's memory:
It seems that x$dual is not designed for concurrent access. When I try to run the code below in two simultaneous windows the first one comes back and the second crash (8173):
declare
nn number;
ss1 date;
ss2 date;
begin
---
ss1 := sysdate;
for i in 1..100000 loop
select 2 into nn from sys.x_$dual;
end loop;
ss2 := sysdate;
dbms_output.put_line('run time using table dual in centiseconds='||(ss2 -
ss1 ) * 24 * 60 * 60 * 100);
end;
If the x$dual gets replaced by dual it works fine.
Can any one duplicate this?
Thanks
Waleed
-----Original Message-----
Sent: Tuesday, April 23, 2002 6:49 PM
I am writing a paper on application efficiency and I have a number of simple
tests in that paper that illustrates that in a very efficient application
the
switch to x$dual can make around a 70-80 percent improvent. But beaware: the
test suite only accesses x$dual and not other tables so the impact is large
and the application is written in the most efficient way. Running the same
test but in a worse efficiency case. shows only a 2-3 percent gain ......
Anjo.
"Khedr, Waleed" wrote:
> Can you guys tell me about all the overhead to resolve the view definition
> to reach the magical X$dual table?
>
> Waleed
>
> -----Original Message-----
> Sent: Tuesday, April 23, 2002 12:58 PM
> To: Multiple recipients of list ORACLE-L
>
> Redefine the view to work on dual and pay the price !!!
> Checked it on oracle 8.1.6 and 9.0.1.
> I will take the risk that maybe in some future date this will not work
> if it can save a lot of resources now.
>
> Yechiel Adar
> Mehish
>
> ----- Original Message -----
> To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
> Sent: Tuesday, April 23, 2002 3:13 PM
>
> > what would you do if Oracle removed this x$dual or made it multiple row
> > table in future upgrades?
> >
> > -----Original Message-----
> > To: Multiple recipients of list ORACLE-L
> > Sent: 4/23/02 4:08 AM
> >
> > Hello Gaja
> >
> > Thanks for the detailed info.
> > I created a view and did grant to public.
> > I did 10 selects and they did only 2 buffers get.
> > BTW - describe on x$dual does not work but select * works.
> >
> > Yechiel Adar
> > Mehish
> >
> > ----- Original Message -----
> > To: Multiple recipients of list ORACLE-L <ORACLE-L_at_fatcity.com>
> > Sent: Monday, April 22, 2002 8:15 PM
> >
> >
> > > Hello Yechiel,
> > >
> > > X$DUAL is an Oracle-internal table "in the SGA" and
> > > will not be shown in an ALL_OBJECTS listing.
> > > Obviously, you need to be SYS to see this. You can do
> > > a describe as SYS and you will see it. Which is the
> > > reason why I recommended creating a view and a public
> > > synonym on the view, so that the application may
> > > reference it without any issues.
> > >
> > > Cheers,
> > >
> > > Gaja
> > >
> > > --- Yechiel Adar <adaryechiel_at_hotmail.com> wrote:
> > > > Hello Gaja
> > > >
> > > > I could not find x$dual. Did select on all_objects
> > > > got zip.
> > > > Oracle 8.1.6.3.4 on NT.
> > > >
> > > > Yechiel Adar
> > > > Mehish
> > > >
> > > > ----- Original Message -----
> > > > To: Multiple recipients of list ORACLE-L
> > > > <ORACLE-L_at_fatcity.com>
> > > > Sent: Sunday, April 21, 2002 8:28 PM
> > > >
> > > >
> > > > > Hi Yechiel,
> > > > >
> > > > > Any full-table-scan in Oracle 8i (or below)
> > > > consumes 4
> > > > > LIOs to the segment header. This number has
> > > > reduced to
> > > > > 2 in 9i. Given that the 1 row that you are going
> > > > after
> > > > > is in 1 data block, there is 1 LIO for the data
> > > > block
> > > > > itself, given you a total of 5 LIOs. You can
> > > > verify
> > > > > this by setting 10046 for the session and looking
> > > > at
> > > > > the trace output.
> > > > >
> > > > > The workaround is to reference x$dual in your
> > > > > application. Alternatively, you can create a view
> > > > on
> > > > > x$dual, create a synonym for it and then go from
> > > > > there. You will incur some I/O for the first
> > > > access of
> > > > > the query (with the synonym), but subsequent
> > > > accesses
> > > > > will incur 0 LIOs against x$dual.
> > > > >
> > > > > Cheers,
> > > > >
> > > > > Gaja
> > > > > --- Yechiel Adar <adaryechiel_at_hotmail.com> wrote:
> > > > > > I did two statspack snapshots, one hour and
> > > > forty
> > > > > > minutes apart.
> > > > > > Then I generated a report and loaded it into
> > > > > > oraperf.com.
> > > > > > In the report I saw that the two SQL statements
> > > > that
> > > > > > where executed the most
> > > > > > times where:
> > > > > >
> > > > > > Select xxxx.currval from dual;
> > > > > >
> > > > > > Select xxxx.nextval from dual;.
> > > > > >
> > > > > > Each one was executed about 90,000 times with 5
> > > > > > buffer gets per execution.
> > > > > > The net result was about 950,000 buffer get for
> > > > > > nextval and currval.
> > > > > >
> > > > > > My question is:
> > > > > > Why should there be about 5 buffer gets per
> > > > > > execution?
> > > > > >
> > > > > > Yechiel Adar
> > > > > > Mehish
--
Please see the official ORACLE-L FAQ: http://www.orafaq.com
--
Author: Khedr, Waleed
INET: Waleed.Khedr_at_FMR.COM
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Received on Tue Apr 23 2002 - 20:48:28 CDT
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