From: Tom Lane <tgl@sss.pgh.pa.us>
To: Tomas Vondra <tomas.vondra@enterprisedb.com>
cc: James Finnerty <jfinnert@amazon.com>, pgsql-hackers@lists.postgresql.org
Subject: Re: postgres_fdw: using TABLESAMPLE to collect remote sample
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Comments: In-reply-to Tomas Vondra <tomas.vondra@enterprisedb.com>
	message dated "Fri, 06 Jan 2023 15:40:36 +0100"
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Date: Fri, 06 Jan 2023 11:58:52 -0500
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Tomas Vondra <tomas.vondra@enterprisedb.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