Re: postgres_fdw: using TABLESAMPLE to collect remote sample

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com>
Cc: James Finnerty <jfinnert(at)amazon(dot)com>, pgsql-hackers(at)lists(dot)postgresql(dot)org
Subject: Re: postgres_fdw: using TABLESAMPLE to collect remote sample
Date: 2022-12-31 04:42:24
Message-ID: 951485.1672461744@sss.pgh.pa.us
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Tomas Vondra <tomas(dot)vondra(at)enterprisedb(dot)com> writes:
> After thinking about it a bit more I decided to rip out the 10% sampling
> rate inflation.

+1. I'm not sure if there's anything more we need to do there, but
that didn't seem like that was it.

I notice that the committed patch still has a reference to that hack
though:

+ * Ensure the sampling rate is between 0.0 and 1.0, even after the
+ * 10% adjustment above. (Clamping to 0.0 is just paranoia.)

Clamping still seems like a wise idea, but the comment is just
confusing now.

Also, I wonder if there is any possibility of ANALYZE failing
with

ERROR: TABLESAMPLE clause can only be applied to tables and materialized views

I think the patch avoids that, but only accidentally, because
reltuples will be 0 or -1 for a view. Maybe it'd be a good
idea to pull back relkind along with reltuples, and check
that too?

regards, tom lane

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