Re: Currval and buffer gets

From: Gaja Krishna Vaidyanatha <oraperfman_at_yahoo.com>
Date: Sun, 21 Apr 2002 10:28:18 -0800
Message-ID: <F001.0044A76E.20020421102818@fatcity.com>

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:
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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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-- 
Please see the official ORACLE-L FAQ: http://www.orafaq.com
-- 
Author: Gaja Krishna Vaidyanatha
  INET: oraperfman_at_yahoo.com

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Received on Sun Apr 21 2002 - 13:28:18 CDT