Re: postgres_fdw bug in 9.6

From: Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>
To: Robert Haas <robertmhaas(at)gmail(dot)com>
Cc: Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org>
Subject: Re: postgres_fdw bug in 9.6
Date: 2017-01-13 11:22:45
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On 2017/01/13 0:43, Robert Haas wrote:
> On Wed, Jan 11, 2017 at 2:45 AM, Etsuro Fujita
> <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)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

Attachment Content-Type Size
epqpath-for-foreignjoin-3.patch text/x-patch 42.6 KB

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