Re: pgsql: postgres_fdw: Add ORDER BY to some remote SQL queries.

On Tue, Dec 1, 2015 at 11:30 AM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> On Tue, Nov 3, 2015 at 06:05:11PM +0000, Robert Haas wrote:
>> postgres_fdw: Add ORDER BY to some remote SQL queries.
>>
>> If the join problem's entire ORDER BY clause can be pushed to the
>> remote server, consider a path that adds this ORDER BY clause. If
>> use_remote_estimate is on, we cost this path using an additional
>> remote EXPLAIN. If not, we just estimate that the path costs 20%
>> more, which is intended to be large enough that we won't request a
>> remote sort when it's not helpful, but small enough that we'll have
>> the remote side do the sort when in doubt. In some cases, the remote
>> sort might actually be free, because the remote query plan might
>> happen to produce output that is ordered the way we need, but without
>> remote estimates we have no way of knowing that.
>>
>> It might also be useful to request sorted output from the remote side
>> if it enables an efficient merge join, but this patch doesn't attempt
>> to handle that case.
>
> Does this complete FDW join pushdown?

It's the most important part, but we may want to do other things. See

http://www.postgresql.org/message-id/CAFjFpRfeKHiCmwJ72p4=ZvuZRQsaU9tbfyW7vwr-5PPvrCbcQQ@mail.gmail.com

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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