Re: postgres_fdw join pushdown (was Re: Custom/Foreign-Join-APIs)

>
> If so, my concern is, the helper function probably wouldn't
>> extend to the parameterized-foreign-join-path cases, though that
>> would work well for the unparameterized-foreign-join-path cases. We
>> don't support parameterized-foreign-join paths for 9.6?
>>
>
> If we do not find a local path with given parameterization, it means
>> there are other local parameterized paths which are superior to it. This
>> possibly indicates that there will be foreign join parameterised paths
>> which are superior to this parameterized path, so we basically do not
>> create foreign join path with that parameterization.
>>
>
> The latest version of the postgres_fdw join pushdown patch will support
> only the unparameterized-path case, so we don't have to consider this, but
> why do you think the superiority of parameterizations is preserved between
> remote joining and local joining?
>

AFAIU, parameterization for local paths bubbles up from base relations. For
foreign relations, we calculate the cost of parameterization when
use_remote_estimate is ON, which means it's accurate. So, except that we
will get clause selectivity wrong (if foreign tables were analyzed
regularly even that won't be the case, I guess) resulting in some small
sway in the costs as compared to those of parameterized foreign join paths.
So, I am guessing that the local estimates for parameterized join paths
would be closer to parameterized foreign paths (if we were to produce
those). Hence my statement. There is always a possibility that those two
costs are way too different, hence I have used phrase "possibly" there. I
could be wrong.

--
Best Wishes,
Ashutosh Bapat
EnterpriseDB Corporation
The Postgres Database Company