RE: Currval and buffer gets

Thanks for the test.

I ran this test to compare the performance difference dealing with dual and x$dual:

declare
nn number;
ss1 date;
ss2 date;
begin
ss1 := sysdate;
for i in 1..100000 loop
select 2 into nn from sys.x_$dual;
end loop;
ss2 := sysdate;
dbms_output.put_line('run time using view x_$dual in centiseconds='||(ss2 - ss1 ) * 24 * 60 * 60 * 100);

---
ss1 := sysdate;
for i in 1..100000 loop
select 2 into nn from dual;
end loop;
ss2 := sysdate;
dbms_output.put_line('run time using table dual in centiseconds='||(ss2 -
ss1 ) * 24 * 60 * 60 * 100);
end;

 
Here is the results:

run time using view x_$dual in
centiseconds=1100.000000000000000000000000000000000002

run time using table dual in
centiseconds=1799.999999999999999999999999999999999997



For 100,000 executions it took 11 sec using x$dual and took 18 sec using
dual.

 
Regards,
 
Waleed



-----Original Message-----
mailto:lpetrov_at_yahoo.com]
Sent: Tuesday, April 23, 2002 8:43 PM
To: Multiple recipients of list ORACLE-L


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

--- "Khedr, Waleed" <Waleed.Khedr_at_FMR.COM> wrote:


> 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

> To: Multiple recipients of list ORACLE-L

>

>

> 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

>

=== message truncated ===


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Please see the official ORACLE-L FAQ:  <http://www.orafaq.com>
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Author: Lyubomir Petrov
  INET: lpetrov_at_yahoo.com

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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.com
-- 
Author: Khedr, Waleed
  INET: Waleed.Khedr_at_FMR.COM

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