Re: Currval and buffer gets

Hello Gaja,

Here is the line to talk about:

FETCH #1:c=0,e=0,p=0,cr=0,cu=0,mis=0,r=1,dep=0,og=4,tim=5656332

As we see, cu+cr=0 and returned raw is r=1. Normally, it's not possible to return a raw without touching a block. But there are some cases such as X$ tables that these statistics are zero.

I had tested V$SESSION, V$PROCESS and I had seen logical IO is 0 since they are based on X$ tables.

I did not document it, but I think, Oracle doesn't account logical IO for X$ tables. Also, How can Oracle account it in blocks ? X$ tables are not organized in DB_BLOCK_SIZE.

I may be wrong, I've not done detailed tests on this issue. I'm looking forward to hearing a confirmation on that.

regards...

Gaja Krishna Vaidyanatha wrote:

> Hi Dan and list,
>
> Here is the output from a test after setting 10046 at
> level 12. Please tell me whether this is really 0 or
> not accounted for. If it is not accounted for, then I
> stand corrected.
>
> Thanks,
>
> Gaja
>
> ----Trace file output starts here----
> *** 2002-04-22 16:04:14.090
> *** SESSION ID:(11.752) 2002-04-22 16:04:14.060
> =====================
> PARSING IN CURSOR #1 len=70 dep=0 uid=0 oct=42 lid=0
> tim=5655178 hv=347037164 ad='51d70a0'
> alter session set events '10046 trace name context
> forever, level 12'
> END OF STMT
> EXEC
> #1:c=2,e=4,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=4,tim=5655179
> WAIT #1: nam='SQL*Net message to client' ela= 0
> p1=1111838976 p2=1 p3=0
> *** 2002-04-22 16:04:25.617
> WAIT #1: nam='SQL*Net message from client' ela= 1152
> p1=1111838976 p2=1 p3=0
> =====================
> PARSING IN CURSOR #1 len=21 dep=0 uid=0 oct=3 lid=0
> tim=5656331 hv=2119980703 ad='51d5564'
> select * from x$dual
> END OF STMT
> PARSE
> #1:c=0,e=0,p=0,cr=0,cu=0,mis=1,r=0,dep=0,og=4,tim=5656331
> BINDS #1:
> EXEC
> #1:c=0,e=0,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=4,tim=5656332
> WAIT #1: nam='SQL*Net message to client' ela= 0
> p1=1111838976 p2=1 p3=0
> FETCH
> #1:c=0,e=0,p=0,cr=0,cu=0,mis=0,r=1,dep=0,og=4,tim=5656332
> WAIT #1: nam='SQL*Net message from client' ela= 2
> p1=1111838976 p2=1 p3=0
> FETCH
> #1:c=0,e=0,p=0,cr=0,cu=0,mis=0,r=0,dep=0,og=4,tim=5656334
> WAIT #1: nam='SQL*Net message to client' ela= 0
> p1=1111838976 p2=1 p3=0
> WAIT #1: nam='SQL*Net message from client' ela= 815
> p1=1111838976 p2=1 p3=0
> STAT #1 id=1 cnt=1 pid=0 pos=0 obj=0 op='FIXED TABLE
> FULL X$DUAL '
> =====================
> ----Trace file output ends here----
>
> --- Danisment Gazi Unal <dunal_at_ubTools.com> wrote:
> > Hello Gaja,
> >
> > am I missing something ?
> >
> > is it 0 logical IO indeed ? or it's not accounted
> > for X$ tables ?
> >
> > regards...
> >
> >
> >
> > Gaja Krishna Vaidyanatha wrote:
> >
> > > 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: Yechiel Adar
> > > > INET: adaryechiel_at_hotmail.com
> > > >
> > > > Fat City Network Services -- (858) 538-5051
> > FAX:
> > > > (858) 538-5051
> > > > San Diego, California -- Public Internet
> > > > access / Mailing Lists
> > > >
> > >
> >
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> > > =====
> > > Gaja Krishna Vaidyanatha
> > > Director, Storage Management Products,
> > > Quest Software, Inc.
> > > Co-author - Oracle Performance Tuning 101
> > >
> >
> http://www.osborne.com/database_erp/0072131454/0072131454.shtml
> > >
> > > __________________________________________________
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> > --
> > Danisment Gazi Unal
> > http://www.ubTools.com
> >
> >
> > --
> > Please see the official ORACLE-L FAQ:
> > http://www.orafaq.com
> > --
> > Author: Danisment Gazi Unal
> > INET: dunal_at_ubTools.com
> >
> > Fat City Network Services -- (858) 538-5051 FAX:
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> =====
> Gaja Krishna Vaidyanatha
> Director, Storage Management Products,
> Quest Software, Inc.
> Co-author - Oracle Performance Tuning 101
> http://www.osborne.com/database_erp/0072131454/0072131454.shtml
>
> __________________________________________________
> Do You Yahoo!?
> Yahoo! Games - play chess, backgammon, pool and more
> http://games.yahoo.com/
> --
> Please see the official ORACLE-L FAQ: http://www.orafaq.com
> --
> Author: Gaja Krishna Vaidyanatha
> INET: oraperfman_at_yahoo.com
>
> Fat City Network Services -- (858) 538-5051 FAX: (858) 538-5051
> San Diego, California -- Public Internet access / Mailing Lists
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--
Danisment Gazi Unal
http://www.ubTools.com



-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.com
-- 
Author: Danisment Gazi Unal
  INET: dunal_at_ubTools.com

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