Re: postgres_fdw: using TABLESAMPLE to collect remote sample

Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> writes:
> The one difference is that I realized the relkind check does not
> actually say we can't do sampling - it just means we can't use
> TABLESAMPLE to do it. We could still use "random()" ...

> Furthermore, I don't think we should silently disable sampling when the
> user explicitly requests TABLESAMPLE by specifying bernoulli/system for
> the table - IMHO it's less surprising to just fail in that case.

Agreed on both points. This patch looks good to me.

> Of course, all relkinds that don't support TABLESAMPLE currently have
> reltuples value that will disable sampling anyway (e.g. views have -1).
> So we won't actually fallback to random() anyway, because we can't
> calculate the sample fraction.
> That's a bit annoying for foreign tables pointing at a view, which is a
> more likely use case than table pointing at a sequence.

Right, that's a case worth being concerned about.

> But I realized we could actually still do "random()" sampling:
> SELECT * FROM t ORDER BY random() LIMIT $X;

Hmm, interesting idea, but it would totally bollix our correlation
estimates. Not sure that those are worth anything for remote views,
but still...

regards, tom lane