On Mon, 2012-12-10 at 08:16 -0500, Stephen Frost wrote: > I'm trying to figure out why there are all the constraints around this > command to begin with. If we're going to support this, why do we > require the user to create or truncate the table in the same > transaction? Clearly that's going to reduce the usefulness of this > feature and it's not clear to me why that constraint is needed, > code-wise.
There is a very specific reason: If you insert frozen tuples into a table that already has tuples in it, then you no longer have just an isolation issue, you have an *atomicity* issue (and probably a number of other serious issues) because you can't roll back. Doing it in the same transaction as the table was created leaves you with a way to roll back: just delete the table (which will happen implicitly because the creation is part of the same transaction anyway). Perhaps we can take some partial steps toward MVCC-safe access? For instance, how about trying to recheck the xmin of a pg_class tuple when starting a scan? Regards, Jeff Davis -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers