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RE: AW: Re: [SQL] possible row locking bug in 7.0.3 & 7.1

From: "Mikheev, Vadim" <vmikheev(at)SECTORBASE(dot)COM>
To: "'Tom Lane'" <tgl(at)sss(dot)pgh(dot)pa(dot)us>, Zeugswetter Andreas SB <ZeugswetterA(at)Wien(dot)Spardat(dot)at>
Cc: "'Hiroshi Inoue'" <Inoue(at)tpf(dot)co(dot)jp>, Philip Warner <pjw(at)rhyme(dot)com(dot)au>, pgsql-hackers(at)postgresql(dot)org
Subject: RE: AW: Re: [SQL] possible row locking bug in 7.0.3 & 7.1
Date: 2001-03-30 20:02:34
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
> > It is intuitive. The bug was iirc, that you saw 2 versions 
> > of the same row in the second select statement (= 2 rows
> > returned by second select).
> I think we should be extremely wary of assuming that we have a clear
> characterization of "what the bug is", let alone "how to fix it".
> The real issue here is that SELECT has different MVCC visibility rules
> from UPDATE and SELECT FOR UPDATE.  I suspect that that *must* be so

This is not correct - SELECT has same rules. Are you able to reproduce
this bad behaviour without running queries in functions? I assume
the answer is NO. I just overlooked function case two years ago.
But SELECT/UPDATE visibility rules are same!
rollback in the event of concurrent modification? Because of concurrent
modifications make visibility of SELECT and UPDATE different and this
means *unconsistent* view of database for applications.
In READ COMMITTED mode a query must see changes made by previous
queries - the only one rule we have to follow to provide consistent
result for applications.

> in any mode that allows more concurrency than full serializable mode.
> Thus, the question we are really facing is how we might alter the
> visibility rules in a way that will make the results more intuitive
> and/or useful while still allowing concurrency.
> This will take thought, research and discussion.  A quick fix is the
> last thing that should be on our minds.

I agreed to leave it as Known Bug for 7.1.

> A first question: where did the MVCC rules come from 
> originally, anyway?

From the fact that I've used Oracle before Postgres'95,
liked it and had time to read its documentation -:)

> Is there any academic research to look at?

There is academic Theorem of Serializability but it's
different from SERIALIZABLE mode definitions in standard.
Probably, this difference was caused by lobbying from



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Subject: RE: AW: Re: [SQL] possible row locking bug in 7.0.3 & 7 .1
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Subject: 7.1 pg_dump fails for user-defined types (release stopper?)

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