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Re: oracle - mysql comparison

From: VC <>
Date: Fri, 16 Jul 2004 17:54:51 GMT
Message-ID: <LDUJc.105848$Oq2.2189@attbi_s52>

"Alex Filonov" <> wrote in message
> "VC" <> wrote in message
> > "Alex Filonov" <> wrote in message
> >
> >
> > >As for correct concurrency
> > > model,
> >
> > A correct concurrency control/model ensures that all the permitted
> > transactions are serializable. In this sense, databases like DB2 or
> > implement a correct concurrency model albeit at the expense of lower
> > concurrency in some circumstances. Funnily enough, none of the Oracle
> > isolation levels can make the same promise, i.e. ensure serializable
> > transaction histories, in any of its isolation levels. Usually, it's
> > easy to obtain correct results by augmenting an isolation level with
> > something like 'select for update', though..
> >
> Hm, what about SERIALIZABLE isolation level in Oracle? I don't see that
> it's any different from similar isolation level in MSSQL.

Oracle does not have SERIALIZABLE despite their assurances to the contrary. It's trivial to imagine a transaction history where Oracle's SERIALIZABLE won't produce correct results. For starters, try 'insert into t1 select max(ID)+1 from t1'. The issue with Oracle's serializable being not quite what it's labeled was discussed extensively in this newsgroup a couple of years ago. Unfortunately, I do not remeber the subject name -- something to do with airline reservations.

> > 2. On the other hand, if the implementation was a mixed OLTP/DSS
> > then judging by your story, the database architect was not qualified to
> > design a system like that. His/her being surprised that a locking
> > scheduler's reads block writes, and vice versa, is akin to a swimmer's
> > amazed that water is wet. There are several well-known approaches to the
> > reporting problem such as replicating the main database to a reporting
> > database, transaction log shipping, scheduling reports off-hours, etc.
> >
> Yeah, right. Now, tell me where to find managers who understand what
> kind of DB architects they really need. And who prefer architects
> proposing more expensive, although correct, solutions.

Well, one can hardly blame the hammer if one wants to use it as a screwdriver can one ? Oracle has quite a few pecularities of its own that can jeopardize a project if those responsible for design/implementation have no clue as to what they are doing.

>And, of course,
> offline reporting (and all solutions you propose are actually variants of
> it) is not always what customer wants.

What's wrong with those solutions for long running reports, quick queries not being a problem ? Besides, the situation is not very much different from running a long report under Oracle where the results won't be actual due to the very nature of the beast -- 'read consistency'.

There is no argument that a multiversioning scheduler provides higher concurrency in many cases, after all that's its raison d'etre, but one should not forget about trade-offs/pitfalls and think that alternative, locking, approaches won't work.

> > VC
Received on Fri Jul 16 2004 - 12:54:51 CDT

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