Re: RT3.4 query needed a lot more tuning with 9.2 than it did with 8.1

On Mon, May 13, 2013 at 4:33 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Mon, May 13, 2013 at 4:14 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> You know, of course, that the join size estimate isn't arrived at that
>>> way.  Still, this point does make it seem more like a planner bug and
>>> less like bad input stats.  It would be nice to see a self-contained
>>> example ...
>
>> Yeah, I remember there have been examples like this that have come up
>> before.  Unfortunately, I haven't fully grokked what's actually going
>> on here that allows this kind of thing to happen.  Refresh my memory
>> on where the relevant code is?
>
> The point is that we estimate the size of a joinrel independently of
> any particular input paths for it, and indeed before we've built any
> such paths.  So this seems like a bug somewhere in selectivity
> estimation, but I'm not prepared to speculate as to just where.

Hmm.  I went looking for the relevant code and found
calc_joinrel_size_estimate().  If that's actually the right place to
be looking, it's hard to escape the conclusion that pselec > 1.

--
Robert Haas
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