Re: Problems with plan estimates in postgres_fdw - Mailing list pgsql-hackers

(2019/03/01 20:00), Antonin Houska wrote:
> Etsuro Fujita<fujita.etsuro@lab.ntt.co.jp>  wrote:
>> (2019/02/22 22:54), Antonin Houska wrote:
>>> Etsuro Fujita<fujita.etsuro@lab.ntt.co.jp>   wrote:
>>>> So, the two changes are handling different
>>>> cases, hence both changes would be required.

>> +       /*
>> +        * If this is an UPPERREL_ORDERED step performed on the final
>> +        * scan/join relation, the costs obtained from the cache wouldn't yet
>> +        * contain the eval costs for the final scan/join target, which would
>> +        * have been updated by apply_scanjoin_target_to_paths(); add the eval
>> +        * costs now.
>> +        */
>> +       if (fpextra&&  !IS_UPPER_REL(foreignrel))
>> +       {
>> +           /* The costs should have been obtained from the cache. */
>> +           Assert(fpinfo->rel_startup_cost>= 0&&
>> +                  fpinfo->rel_total_cost>= 0);
>> +
>> +           startup_cost += foreignrel->reltarget->cost.startup;
>> +           run_cost += foreignrel->reltarget->cost.per_tuple * rows;
>> +       }
>
>> Yeah, but I think the code bit cited above is needed.  Let me explain using
>> yet another example with grouping and ordering:
>>
>>      SELECT a+b, random() FROM foreign_table GROUP BY a+b ORDER BY a+b;
>>
>> For this query, the sort_input_target would be {a+b}, (to which the
>> foreignrel->reltarget would have been set when called from
>> estimate_path_cost_size() called from postgresGetForeignUpperPaths() with the
>> UPPERREL_ORDERED stage), but the final target would be {a+b, random()}, so the
>> sort_input_target isn't the same as the final target in that case; the costs
>> needs to be adjusted so that the costs include the ones of generating the
>> final target.  That's the reason why I added the code bit you cited.
>
> I used gdb to help me understand, however the condition
>
>     if (fpextra&&  !IS_UPPER_REL(foreignrel))
>
> never evaluated to true with the query above.

Sorry, my explanation was not enough again, but I showed that query 
("SELECT a+b, random() FROM foreign_table GROUP BY a+b ORDER BY a+b;") 
to explain why the following code bit is needed:

+       /*
+        * If this includes an UPPERREL_ORDERED step, the given target, 
which
+        * would be the final target to be applied to the resulting 
path, might
+        * have different expressions from the underlying relation's 
reltarget
+        * (see make_sort_input_target()); adjust tlist eval costs.
+        */
+       if (fpextra&&  fpextra->target != foreignrel->reltarget)
+       {
+               QualCost        oldcost = foreignrel->reltarget->cost;
+               QualCost        newcost = fpextra->target->cost;
+
+               startup_cost += newcost.startup - oldcost.startup;
+               total_cost += newcost.startup - oldcost.startup;
+               total_cost += (newcost.per_tuple - oldcost.per_tuple) * 
rows;
+       }

I think that the condition

     if (fpextra && fpextra->target != foreignrel->reltarget)

would be evaluated to true for that query.

> It's still unclear to me why add_foreign_ordered_paths() passes the input
> relation (input_rel) to estimate_path_cost_size(). If it passed the output rel
> (i.e. ordered_rel in this case) like add_foreign_grouping_paths() does, then
> foreignrel->reltarget should contain the random() function. (What I see now is
> that create_ordered_paths() leaves ordered_rel->reltarget empty, but that's
> another problem to be fixed.)
>
> Do you still see a reason to call estimate_path_cost_size() this way?

Yeah, the reason for that is because we can estimate the costs of 
sorting for the final scan/join relation using the existing code as-is 
that estimates the costs of sorting for base or join relations, except 
for tlist eval cost adjustment.  As you mentioned, we could pass 
ordered_rel to estimate_path_cost_size(), but if so, I think we would 
need to get input_rel (ie, the final scan/join relation) from 
ordered_rel within estimate_path_cost_size(), to use that code, which 
would not be great.

Best regards,
Etsuro Fujita