Isolated transactions?

Hi,

I have a number of large tables in a schema all of which are related by
foreign keys.

Now, what I would like to be able to do is run some long-running queries
(mostly selects, but also some updates and inserts into the tables), to
generate some reports which will eventually be rolled-back.

Now, while this works just fine, I would like to be able to avoid
holding other transactions back while this is happening.

Is there a way to tell postgresql that a transaction is eventually going
to be rolled back, so that changes there do not delay other transactions?

Basically, the changes made in the transaction used for generating the
reports will only ever be visible in that transaction, which will get
rolled back.

Sorry for the weird description, I hope you understand what I mean.

Thanks,
LL