Re: [HACKERS] postgres_fdw bug in 9.6

On 2017/01/13 0:43, Robert Haas wrote:
> On Wed, Jan 11, 2017 at 2:45 AM, Etsuro Fujita
> <fujita.etsuro@lab.ntt.co.jp> wrote:
>> As I said before, that might be fine for 9.6, but I don't think it's a good
>> idea to search the pathlist because once we support parameterized foreign
>> join paths, which is on my TODOs, we would have to traverse through the
>> possibly-lengthy pathlist to find a local-join path, as mentioned in [3].

> I'm not sure that's really going to be a problem.  The number of
> possible parameterizations that need to be considered isn't usually
> very big.  I bet the path list will have ten or a few tens of entries
> in it, not a hundred or a thousand.  Traversing it isn't that
> expensive.
>
> That having been said, I haven't read the patches, so I'm not really
> up to speed on the bigger issues here.  But surely it's more important
> to get the overall design right than to worry about the cost of
> walking the pathlist or worrying about the cost of an extra function
> call (?!).

My biggest concern about GetExistingLocalJoinPath is that might not be 
extendable to the case of foreign-join paths with parameterization; in 
which case, fdw_outerpath for a given foreign-join path would need to 
have the same parameterization as the foreign-join path, but there might 
not be any existing paths with the same parameterization in the path 
list.  You might think we could get the fdw_outerpath by getting an 
existing path with no parameterization as in GetExistingLocalJoinPath 
and then modifying the path's param_info to match the parameterization 
of the foreign-join path.  I don't know that really works, but that 
might be inefficient.

What I have in mind to support foreign-join paths with parameterization 
for postgres_fdw like this: (1) generate parameterized paths from any 
joinable combination of the outer/inner cheapest-parameterized paths 
that have pushed down the outer/inner relation to the remote server, in 
a similar way as postgresGetForeignJoinPaths creates unparameterized 
paths, and (2) create fdw_outerpath for each parameterized path from the 
outer/inner paths used to generate the parameterized path, by 
create_nestloop_path (or, create_hashjoin_path or create_mergejoin_path 
if full join), so that the resulting fdw_outerpath has the same 
parameterization as the paramterized path.  This would probably work and 
might be more efficient.  And the patch I proposed would be easily 
extended to this, by replacing the outer/inner cheapest-total paths with 
the outer/inner cheapest-parameterized paths.  Attached is the latest 
version of the patch.

Best regards,
Etsuro Fujita

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