Re: CLUSTER and MVCC

Tom Lane wrote:
> Heikki Linnakangas <heikki@enterprisedb.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.

You could restrict this to serializable transactions, or even to
read-only transactions. Or maybe the tuple could be reduced to
just it's header - doesn't HOT do something similar?

greetings, Florian Pflug