Re: [PATCH v1] Add and report the new "in_hot_standby" GUC pseudo-variable.

On Wed, Apr 5, 2017 at 6:22 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Thu, Mar 23, 2017 at 4:50 AM, Magnus Hagander <magnus(at)hagander(dot)net>
> wrote:
> > One thing we might want to consider around this -- in 10 we have
> > target_session_attrs=read-write (since
> > 721f7bd3cbccaf8c07cad2707826b83f84694832), which will issue a SHOW
> > transaction_read_only on the connection.
> >
> > We should probably consider if there is some way we can implement these
> two
> > things the same way. If we're inventing a new variable that gets pushed
> on
> > each connection, perhaps we can use that one and avoid the SHOW command?
>
> I think that would be a good idea. It was, in fact, proposed to do
> exactly that as part of the patch that added
> target_session_attrs=read-write, but we ended up not doing anything
> about it because the SHOW mechanism would still be needed when
> connecting to pre-10 versions of PostgreSQL. Therefore, it seemed
> like a separate improvement. But if we're adding a GUC_REPORT value
> that could be used for the same or a similar purpose, I think it would
> make sense to consider revising that mechanism to leverage it as well,
> obviously only on releases that have the GUC.
>
>
Based on that we seem to agree here, should we add this as an open item?
Clearly if we want to change this, we should do so before 10.

I also came up with another case where the current one won't work but it
could be really useful -- if we make a replication connection (with say
pg_receivewal) it would be good to be able to say "i want the master" (or
"i want a standby") the same way. And that will fail today if it needs SHOW
to work, right?

So having it send that information across in the startup package when
talking to a 10 server, but falling back to using SHOW if talking to an
earlier server, would make a lot of sense I think.

--
Magnus Hagander
Me: https://www.hagander.net/ <http://www.hagander.net/>
Work: https://www.redpill-linpro.com/ <http://www.redpill-linpro.com/>