Re: [idea] more aggressive join pushdown on postgres_fdw

On Sat, May 30, 2015 at 9:03 PM, Kouhei Kaigai <kaigai@ak.jp.nec.com> wrote:
> Yesterday, JPUG held an unconference event at Tokyo, and
> Hanada-san had a talk about join-pushdown feature of
> postgres_fdw.
> At this talk, someone proposed an interesting idea to
> make join pushdown more aggressive/effective.
> Let me share it with pgsql-hackers.
>
> He said, we may have a workload to join a large foreign-
> scan and a small local-scan regardless of the plan type.
>
> For example:
>   joinrel (expected nrows = 5)
>     + outerrel ForeignScan (expected nrows = 1000000)
>     + innerrel LocalScan (expected nrows = 5)
>
> In this case, we may be able to run the entire joinrel
> on the remote side then fetch just 5 rows, if fdw-driver
> construct VALUES() clause according to the contents of
> LocalScan then makes an entire join query with another
> one kept in ForeignScan.
>
> If above ForeignScan have the following remote query,
>   SELECT a, b, c FROM t0 WHERE d < 1000000
> we may be able to construct the query below to run remote
> join with local (small) relation.
>
>   SELECT a, b, c, x, y FROM
>     (SELECT a, b, c FROM t0 WHERE d < 1000000) AS ft
>     JOIN
>     (VALUES (1,'aaa'), (2,'bbb'), (3,'ccc'),
>             (4,'ddd'), (5,'eee')) AS lt (x, y)
>     ON ft.a = lt.x
>
> The VALUES clauses can be mechanically constructed according
> to the result set of LocalScan, and it is not difficult to
> make such a remote query on top of the existing ForeignScan.
> In the result, it will reduce amount of network traffic and
> CPU cycles to form/deform tuples dramatically.
>
> I don't intend to implement this idea urgently (of course,
> join pushdown for both ForeignScan case has higher priority),
> however, it makes sense to keep the future direction in mind.
>
> Also, as an aside, even though Hanada-san mentioned ForeignScan
> does not need an infrastructure to initialize child path nodes,
> this idea may require ForeignScan to have local child path.

Neat idea.  This ties into something I've thought about and mentioned
before: what if the innerrel is local, but there's a replicated copy
on the remote server?  Perhaps both cases are worth thinking about at
some point.

-- 
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company