Re: PoC/WIP: Extended statistics on expressions (\d in old client)

On Fri, Jan 22, 2021 at 02:09:04PM +0100, Tomas Vondra wrote:
> On 1/22/21 5:01 AM, Justin Pryzby wrote:
> > On Fri, Jan 22, 2021 at 04:49:51AM +0100, Tomas Vondra wrote:
> > > > > | Statistics objects:
> > > > > | "public"."s2" (ndistinct, dependencies, mcv) ON FROM t
> > >
> > > Umm, for me that prints:
> >
> > > "public"."s2" ON ((i + 1)), (((i + 1) + 0)) FROM t
> > >
> > > which I think is OK. But maybe there's something else to trigger the
> > > problem?
> >
> > Oh. It's because I was using /usr/bin/psql and not ./src/bin/psql.
> > I think it's considered ok if old client's \d commands don't work on new
> > server, but it's not clear to me if it's ok if they misbehave. It's almost
> > better it made an ERROR.
> >
>
> Well, how would the server know to throw an error? We can't quite patch the
> old psql (if we could, we could just tweak the query).

To refresh: stats objects on a v14 server which include expressions are shown
by pre-v14 psql client with the expressions elided (because the attnums don't
correspond to anything in pg_attribute).

I'm mentioning it again since, even though I knew about this earlier in the
year, it caused some confusion for me again just now while testing our
application. I had the v14 server installed but the psql symlink still pointed
to the v13 client.

There may not be anything we can do about it.
And it may not be a significant issue outside the beta period: more typically,
the client version would match the server version.

--
Justin