On Thu, Aug 26, 2021 at 12:51 PM Bruce Momjian <bruce@momjian.us> wrote:
> I just don't want to add requirements/complexity to pg_upgrade without
> clearly stated reasons because future database changes will need to
> honor this new preservation behavior.
Well, I agree that it's good to have reasons clearly stated and I hope
that at this point you agree that they have been. Whether you agree
with them is another question, but I hope you at least agree that they
have been stated.
As far as the other part of your concern, what I think makes this
change pretty safe is that we are preserving more things rather than
fewer. I can imagine some server behavior depending on something being
the same between the old and the new clusters, but it is harder to
imagine a dependency on something not being preserved. For example, we
know that the OIDs of pg_type rows have to be the same in the old and
new cluster because arrays are stored on disk with the type OIDs
included. Therefore those need to be preserved. If in the future we
changed things so that arrays - and other container types - did not
include the type OIDs in the on-disk representation, then perhaps it
would no longer be necessary to preserve the OIDs of pg_type rows
across a pg_upgrade. However, it would not be harmful to do so. It
just might not be required.
So I think this proposed change is in the safe direction. If
relfilenodes were currently preserved and we wanted to make them not
be preserved, then I think you would be quite right to say "whoa,
whoa, that could be a problem." Indeed it could. If anyone then in the
future wanted to introduce a dependency on them staying the same, they
would have a problem. However, nothing in the server itself can care
about relfilenodes - or anything else - being *different* across a
pg_upgrade. The whole point of pg_upgrade is to make it feel like you
have the same database after you run it as you did before you ran it,
even though under the hood a lot of surgery has been done. Barring
bugs, you can never be sad about there being too LITTLE difference
between the post-upgrade database and the pre-upgrade database.
--
Robert Haas
EDB: http://www.enterprisedb.com