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RE: Antw2: RE: Re: Informix isolation levels

Posted By: Dan Michaelis
Date: Wednesday, 17 September 2003, at 2:04 p.m.

In Response To: RE: Antw2: RE: Re: Informix isolation levels (NormaJean.Sebastian@tellabs.com)

Well, it's a little inconceivable that I'm defending Oracle, but the
snapshot-too-old error is in a strange way equivalent to the long
transaction error that you can get with Informix. An Oracle
"snapshot-too-old" happens when a read process requires rows from the
rollback segments that it can no longer retrieve, since the rollback
segments have been over-written by subsequent transactions. This happens
because while write operations in Oracle will cause the creation of
additional segments within the rollback segment, read operations do not.
Therefore, it's possible to need a row for a read that: 1. Has been
changed since the beginning of the read operation and 2. is no longer
accurately represented in the rollback segment because that segment has been
re-used.

In general, I think that the snapshot-too-old errors are a little less
"harmful" than long transactions (only the offending read session is
affected; the rest of the sessions roll merrily along... Not always the case
with Informix -- hitting the exclusive high water mark causes all update
activity to halt while the offender is rolled back). I'll admit that the
dynamic logfile allocation in 9.2 and above does a fair bit to resolve long
transactions, but it's still possible to get them, and still possible to do
yourself a fair amount of damage with them. Oracle's "snapshot-too-old"
error is generally more frequent ('specially if you have really BAD select
statements), but isn't as damaging.

Regarding the transaction management, I suppose I'm greedy... I'd like to
see an isolation level in Informix that would allow me to read the
pre-altered images from the logical logs in the same way that Oracle reads
from the rollback segments, but I still want the ability to do dirty reads,
and use the other isolation levels that Informix has (which are absent in
Oracle).

Just my two cents.



Dan Michaelis
407-758-3395





>From: "NormaJean.S...." <NormaJean.Sebastian@tellabs.com>
>To: ids@iiug.org
>Subject: RE: Antw2: RE: Re: Informix isolation levels [1867] Date: Sat,
>13 Sep 2003 19:54:45 -0400 (EDT)
>
>If oracle is so perfect (pardon me, I have only DBA'd on it for less
>than 2 years), why does it error with a "timestamp too old" message?
>
>If I am an end user, I want my data... I don't want my transaction to
>abend because the database is confused...
>
>Informix gives me dirty read.. I get my data as of timestamp X... it's
>all consistent as of timestamp X. If someone changed the data at
>timestamp X+5 (while my rpt is still running), who cares... .. my report
>wanted timestamp X, and I got it. Oracle goes poopy on timestamp too
>old?
>
>Pardon my ignorance, I could be totally wrong, but I have seen the
>'timestamp too old'. I don't really understand it... and I think any
>good database should not have that problem.
>
>Norma Jean
>
>
>
>-----Original Message-----
>From: andreas.kutsche@spar.at [mailto:andreas.kutsche@spar.at]
>Sent: Saturday, September 13, 2003 7:57 AM
>To: ids@iiug.org; forum.subscriber@iiug.org
>Subject: Antw2: RE: Re: Informix isolation levels [1866]
>
>
>----LNX_Sat_Sep_13_2003_14:54:13_V3.33--
>Content-Transfer-Encoding: quoted-printable
>
> >Datum: 2003.09.13 01:45:25
> >Sender: Obnoxio The.... <obnoxio@hotmail.com>
> >Betreff: RE: Re: Informix isolation levels [1863]
> >
> >Really. Oracle are wrong. Trust me on this.
> >
> >--
> >Bye now,
> >Obnoxio
> =
>
> >>>
> >>>The developer is wrong. The way Informix handles transactions is
>right=
>,
> >>>the way Oracle handles them is wrong.
> >>>
> =
>
> >>Andreas Kutsche:
> >>
> >>I think that's not correct. It's a different approach.
> >>
> =
>
>I think I should add that my opionion that Oracle is right (too), only
>is=
> valid
>for COMMITTED READ (the most used isolation level in applications I
>suppo=
>se).
>When it comes to SERIALIZABLE than Oracle is really wrong (or not really
>serializable under all circumstances) - see
>http://www.cs.umb.edu/~isotest/snaptest/snaptest.pdf
>or
>http://www-dbs.cs.uni-sb.de/papers/tdd99.pdf
>for examples (keyword: snapshot isolation ).
> =
>
>Regards,
>Andreas Kutsche
>
>------------------------------------------------
>SPAR Oesterreichische Warenhandels-AG
>Hauptzentrale
>Europastra=DFe 3
>A-5015 Salzburg
> =
>
>Telefon : +43 662 4470 24423
>E-Mail : Andreas.KUTSCHE@spar.at
>Internet: http://www.spar.at
>------------------------------------------------
>
> =
>
>
>----LNX_Sat_Sep_13_2003_14:54:13_V3.33----
>
>
>
>--openmail-part-41e19bc9-00000002
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>

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