Re: Why not represent "never vacuumed" accurately wrt pg_class.relpages?

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2018-12-11 09:47:38 -0500, Tom Lane wrote:
>> And why do you blame it on this representation? We don't believe that
>> relpages is the actual size of the table.

> No, but we assume that there's 10 pages. Even if both relpages and the
> actual relation stats say there's not. And we assume there's as many
> tuples on the page as can fit on it, using get_rel_data_width(). So if
> you have a small table with a handful of entries at most, you suddenly
> get estimates of a few hundred to ~a thousand rows.

That's intentional, and not particularly constrained by the representation
used in pg_class. The downsides of incorrectly assuming a table is tiny
are a lot worse than those of assuming the opposite.

> I guess you could argue that the relation would potentially not be be
> empty anymore by the time the plan is executed, but if that were part of
> the logic it a) wouldn't just be relevant if relpages = 0, and b) should
> be documented.

Yeah, that's the case where you can get hurt the worst. As for "not
documented", the first para in the comment is trying to say that.

regards, tom lane