Tom Lane wrote:
Heikki Linnakangas <[EMAIL PROTECTED]> 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.

I was thinking of inserts+deletes. Updates are harder, you'd need to change the ctid of the old version to skip the middle part of the chain, atomically, but I suppose they could be handled as well.

Isolation level doesn't really matter. We just need a global view of in-use *snapshots* in the system, serializable or not. Not that that's an easy thing to do...

--
  Heikki Linnakangas
  EnterpriseDB   http://www.enterprisedb.com

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