From: | "Michael Paesold" <mpaesold(at)gmx(dot)at> |
---|---|
To: | "Tsirkin Evgeny" <tsurkin(at)mail(dot)jct(dot)ac(dot)il> |
Cc: | <pgsql-admin(at)postgresql(dot)org> |
Subject: | Re: duplicates |
Date: | 2004-09-06 10:51:45 |
Message-ID: | 00be01c493ff$82814530$ad01a8c0@zaphod |
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Lists: | pgsql-admin |
I wrote:
> BEGIN;
> SET TRANSACTION ISOLATION LEVEL SERIALIZABLE;
>
> DELETE FROM schedule WHERE studentid = ... ;
> INSERT INTO schedule (studentid, ...) VALUES (... );
> INSERT INTO schedule (studentid, ...) VALUES (... );
>
> COMMIT;
>
> If you do it like in the above sql code, there is still a problem. The
> serializable checking here only works, if DELETE FROM schedule... really
> finds at least one row. If it does not, it will not recognize the
> serialization problem. So it's still possible that your programm creates
> duplicates.
>
> I have tested this here. I don't really know if this is just the case with
> PostgreSQL serializable transactions (MVCC limitation) or a general
problem.
There is a detailed description about this problem in Section 12.2.2.1. of
the PostgreSQL 8 docs here:
http://developer.postgresql.org/docs/postgres/transaction-iso.html
(Serializable Isolation versus True Serializability)
Best Regards,
Michael Paesold
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