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Re[2]: Re[2]: Dirty reads

From: <dgoulet_at_vicr.com>
Date: Wed, 4 Oct 2000 12:08:40 -0400
Message-Id: <10639.118482@fatcity.com>


Jim,

    I can agree & disagree at the same time. Consider the following:

    A batch job is only interested in historical data in an OLTP table. = This
job will run for several hours due to volume of data and other considerat= ions. =

Now the job starts at SCN 1000, but at the same time one or more jobs/processes/forms/panels (I think you get the idea) insert data into t= he
table. Now according to Oracle's default workings rollback is retained s= tarting
at SCN 1000 for this job to provide a "read consistent view" of the table= =2E =

Nice, except that this particular job will never look at those new rows o= f data,
so why should it get a "ORA-1555 Snapshot too old" error after 2 hours of=

running?? Having the ability to "dirty read" would be a benefit in this = case
(one of about 12 I have).

Dick Goulet

____________________Reply Separator____________________
Subject: Re: Re[2]: Dirty reads
Author: "Jim Conboy" <Jim.Conboy_at_trw.com> Date: 10/4/00 8:15 AM

Am I the only one with no imagination? How is uncommitted data of any pr= actical
value? Of course I can see the desire to read data as up-to-date as poss= ible,
but if the possibility exists that the data read is 'wrong' I can't see a= ny
advantage to this, and loads of potential harm.

Jim

>>> dgoulet_at_vicr.com 10/04/00 11:06AM >>>
The subject of "dirty reads" has been around for as long as I've worked w= ith
Oracle, which started back in 1985. It's basically a forbidden subject. =  I know
DB2 allows this and there are others who would like it since rollback seg= ments
would become unnecessary from their point of view (it would also make som= e batch
processing a little more error free). But in looking at the ANSI SQL sta= ndard
book that I've got here, dirty reads are NOT standard behavior, actually = it's
prohibited!!

Dick Goulet.

____________________Reply Separator____________________
Author: "Ron Rogers" <RROGERS_at_galottery.org>
Date:       10/3/00 1:11 PM

I don't think that Oracle would allow it because until the commit is issu= ed the
other users see the "old" data and the user making the changes sees the n= ew
data. If the user wants to see both copies of the data have them login tw= ice.
One to make the changes and the other to view the original data. To circu= mvent
Oracle's method of data integrity would destroy the foundation building b= locks
of the database
ROR =AA=BF=AA
>>> turner_at_tellme.com 10/03/00 03:51PM >>>
Is there any way to say I want a select that won't look at the rollback segment? The user doesn't care if it's a dirty read.

Dave Turner

Please see the official ORACLE-L FAQ: http://www.orafaq.com =

Author: David Turner
  INET: turner_at_tellme.com =

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Author: Jim Conboy
  INET: Jim.Conboy_at_trw.com

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Received on Wed Oct 04 2000 - 11:08:40 CDT

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