Re: [HACKERS] Clarification in pg10'spgupgrade.html step 10 (upgrading standby servers)

Andreas,

* Andreas Joseph Krogh (andreas@visena.com) wrote:
> I have to ask; Why not run pg_upgrade on standby, after verifying that it's in
> sync with primary and promoting it to primary if necessary and then making it
> standby again after pg_upgrade is finished?

I don't think that we could be guaranteed that the catalog tables would
be the same on the replica as on the primary if they were actually
created by pg_upgrade.

The catalog tables *must* be identical between the primary and the
replica because they are updated subsequently through WAL replay, not
through SQL commands (which is how pg_upgrade creates them in the first
place).

Perhaps we could have some mode for pg_upgrade where it handles the
update to replicas (with the additional checks that I outlined and using
the methodology discussed for rsync --hard-links), but that would still
require solving the communicate-over-the-network problem between the
primary and the replicas, which is the hard part.  Whether it's an
independent utility or something built into pg_upgrade isn't really that
big of a distinction, though it doesn't seem to me like there'd be much
code reuse there.

Thanks!

Stephen