Re: duplicates

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