Subject: Re: BUG #15724: Can't create foreign table as partition
References: <20190329141050.GA22336@alvherre.pgsql>
 <5CA1A320.4050707@lab.ntt.co.jp>
From: Amit Langote <Langote_Amit_f8@lab.ntt.co.jp>
Message-ID: <038a0fec-6684-c654-bade-bcf80cb8daad@lab.ntt.co.jp>
Date: Mon, 1 Apr 2019 15:23:15 +0900
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To: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>,
        Alvaro Herrera <alvherre@2ndquadrant.com>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, stepya@ukr.net,
        pgsql-bugs@lists.postgresql.org
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On 2019/04/01 14:35, Etsuro Fujita wrote:
> (2019/03/29 23:10), Alvaro Herrera wrote:
>> On 2019-Mar-29, Tom Lane wrote:
>>
>>> Alvaro Herrera<alvherre@2ndquadrant.com>  writes:
>>>> On 2019-Mar-29, PG Bug reporting form wrote:
>>>>> SQL Error [42809]: ERROR: cannot create index on foreign table
>>>>> "msg_json"
> 
>>> Really, what *ought* to happen in such a case is that the FDW gets
>>> told about the command and has the opportunity to try to do something
>>> corresponding to making an index.
> 
> Interesting!

+1

Maybe, CHECK constraints on foreign tables are similar -- it merely
asserts the condition that all of the rows in the remote table *must*
satisfy so that the local server can do something useful with it, such as
trying to exclude the table with constraint exclusion.

I guess the intended benefit with allowing indexes would be similar -- a
locally-defined index on a foreign table will help an FDW estimate the
cost of a query accessing the table more efficiently than, for example,
having to rely on some other relatively expensive alternative, such as
use_remote_estimate in postgres_fdw's case.

Thanks,
Amit