Re: Cost estimates for parameterized paths

On Fri, Sep 3, 2010 at 5:53 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> On Fri, Sep 3, 2010 at 2:04 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>>> On reflection I think that for parameterized paths the problem won't be
>>> too bad, because (a) we'll ignore parameterized paths except when
>>> considering a join to the right outer rel, so their presence in the
>>> rel's pathlist won't cost much otherwise,
>
>> Hmm.  Maybe they should go into a separate path list, and perhaps we
>> could do the min/max calculations only with that pathlist (at least
>> for now), thus avoiding taking a generalized penalty to handle this
>> specific case.  IIUC, a parameterized path should never cause an
>> unparamaterized path to be thrown out,
>
> Yeah, but the converse isn't true.  I had considered the idea of keeping
> parameterized paths in a separate list, but you'd still have to examine
> the main list to look for unparameterized paths that might dominate them.

Definitely true, but if it avoids slowing down add_path() in the
common case, it's worth it.

--
Robert Haas
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