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Re: incoherent view of serializable transactions

From: "Kevin Grittner" <Kevin(dot)Grittner(at)wicourts(dot)gov>
To: "Robert Haas" <robertmhaas(at)gmail(dot)com>
Cc: "Gregory Stark" <stark(at)enterprisedb(dot)com>,"Emmanuel Cecchet" <manu(at)frogthinker(dot)org>,<pgsql-hackers(at)postgresql(dot)org>,"Tom Lane" <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Subject: Re: incoherent view of serializable transactions
Date: 2008-12-23 21:06:01
Message-ID: 4950FE59.EE98.0025.0@wicourts.gov (view raw or flat)
Thread:
Lists: pgsql-hackers
>>> "Robert Haas" <robertmhaas(at)gmail(dot)com> wrote: 
 
> For most users, the artifacts that have been
> introduced by these fine-grained locks are outweighed by the
> performance benefits of greater concurrency - but, as you point out,
> not necessarily always.
 
That's what I don't understand.  I never did point that out.  I never
suggested it.  I never requested a change to the software.  I just
suggested that we document the artifacts.
 
Ah, well; thanks for the feedback.
 
> With respect to predicate locking, what you're describing is NOT
> predicate locking.
 
I never would have considered it to be such except for some people
insisting that a true serializable implementation is impossible
without predicate locking.  Either that assertion is false or this is
a form of predicate locking, crude as it might be.
 
> Maybe our
> documentation could say something along the lines of "PostgreSQL's
> MVCC framework and row-level locking permit a greater degree of
> concurrency than some other databases.  Even when the transaction
> isolation level is set to serializable, serialization anomalies can
> occur in the following situations.  When it is important to prevent
> these anomalies, explicit row-level or table-level locking can be
used
> at the expense of reduced concurrency."
 
That's responsive to my concern and nicely worded.  Thanks.
 
-Kevin

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