Re: increasing the default WAL segment size

On Thu, Jan 5, 2017 at 8:39 PM, Beena Emerson <memissemerson(at)gmail(dot)com> wrote:
> On Tue, Jan 3, 2017 at 5:46 PM, Michael Paquier <michael(dot)paquier(at)gmail(dot)com>
> wrote:
>> Actually, why not just having an equivalent of the SQL
>> command and be able to query parameter values?
>
> This patch only needed the wal_segment_size and hence I made this specific
> command.
> How often and why would we need other parameter values in the replication
> connection?
> Making it a more general command to fetch any parameter can be a separate
> topic. If it gets consensus, maybe it could be done and used here.

I concur that for this patch it may not be necessary. But let's not
narrow us in a corner when designing things. Being able to query the
value of parameters is something that I think is actually useful for
cases where custom GUCs are loaded on the server's
shared_preload_libraries to do validation checks (one case is a
logical decoder on backend, with streaming receiver as client
expecting the logical decoder to do a minimum). This can allow a
client to do checks only using a replication stream. Another case that
I have in mind is for utilities like pg_rewind, we have been
discussing about being able to not need a superuser when querying the
target server. Having such a command would allow for example pg_rewind
to do a 'SHOW full_page_writes' without the need of an extra
connection.
--
Michael