Re: postgres_fdw: another oddity in costing aggregate pushdown paths

(2019/02/25 19:59), Etsuro Fujita wrote:
> (2019/02/22 23:10), Antonin Houska wrote:
>> Etsuro Fujita<fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>>> As mentioned in the near thread, I think there is another oversight in
>>> the cost estimation for aggregate pushdown paths in postgres_fdw, IIUC.
>>> When costing an aggregate pushdown path using local statistics, we
>>> re-use the estimated costs of implementing the underlying scan/join
>>> relation, cached in the relation's PgFdwRelationInfo (ie,
>>> rel_startup_cost and rel_total_cost). Since these costs wouldn't yet
>>> contain the costs of evaluating the final scan/join target, as tlist
>>> replacement by apply_scanjoin_target_to_paths() is performed afterwards.
>>> So I think we need to adjust these costs so that the tlist eval costs
>>> are included, but ISTM that estimate_path_cost_size() forgot to do so.
>>> Attached is a patch for fixing this issue.
>>
>> I think the following comment in apply_scanjoin_target_to_paths() should
>> mention that FDWs rely on the new value of reltarget.
>>
>> /*
>> * Update the reltarget. This may not be strictly necessary in all cases,
>> * but it is at least necessary when create_append_path() gets called
>> * below directly or indirectly, since that function uses the reltarget as
>> * the pathtarget for the resulting path. It seems like a good idea to do
>> * it unconditionally.
>> */
>> rel->reltarget = llast_node(PathTarget, scanjoin_targets);
>
> Agreed. How about mentioning that like the attached? In addition, I
> added another assertion to estimate_path_cost_size() in that patch.

This doesn't get applied cleanly after commit 1d33858406. Here is a
rebased version of the patch. I also modified the comments a little
bit. If there are no objections from Antonin or anyone else, I'll
commit the patch.

Best regards,
Etsuro Fujita