Re: CLUSTER and MVCC

Tom Lane wrote:
> Heikki Linnakangas <heikki(at)enterprisedb(dot)com> writes:
> > We wouldn't clean up tuples that are visible to a transaction, but if
> > you have one long-running transaction like pg_dump in a database with
> > otherwise short transaction, you'll have a lot of tuples that are not
> > vacuumable because of the long-running process, but are not in fact
> > visible to any transaction.
>
> It sounds to me like you are proposing to remove the middles of update
> chains, which would break READ-COMMITTED updates initiated by the older
> transactions. Now admittedly pg_dump isn't going to issue any such
> updates, but VACUUM doesn't know that.

Since a multi-statement transaction can't change its transaction
isolation level after its first statement, would adding a boolean to
PGPROC help VACUUM be more aggressive about removing rows? I am
thinking something like PGPROC.cannot_be_serializable.

--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://www.enterprisedb.com

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