Re: postgres_fdw: using TABLESAMPLE to collect remote sample

On Fri, Feb 11, 2022 at 12:39 PM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> writes:
> > So here we go. The patch does a very simple thing - it uses TABLESAMPLE
> > to collect/transfer just a small sample from the remote node, saving
> > both CPU and network.
>
> This is great if the remote end has TABLESAMPLE, but pre-9.5 servers
> don't, and postgres_fdw is supposed to still work with old servers.
> So you need some conditionality for that.

I think it's going to be necessary to compromise on that at some
point. I don't, for example, think it would be reasonable for
postgres_fdw to have detailed knowledge of which operators can be
pushed down as a function of the remote PostgreSQL version. Nor do I
think that we care about whether this works at all against, say,
PostgreSQL 8.0. I'm not sure where it's reasonable to draw a line and
say we're not going to expend any more effort, and maybe 15 with 9.5
is a small enough gap that we still care at least somewhat about
compatibility. But even that is not 100% obvious to me.

--
Robert Haas
EDB: http://www.enterprisedb.com