Re: Problems with plan estimates in postgres_fdw

From: Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Andrew Gierth <andrew(at)tao11(dot)riddles(dot)org(dot)uk>, pgsql-hackers(at)postgresql(dot)org
Subject: Re: Problems with plan estimates in postgres_fdw
Date: 2018-10-09 05:48:48
Message-ID: 5BBC4140.50403@lab.ntt.co.jp
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(2018/10/05 19:15), Etsuro Fujita wrote:
> (2018/08/02 23:41), Tom Lane wrote:
>> Andrew Gierth<andrew(at)tao11(dot)riddles(dot)org(dot)uk> writes:
>>> [ postgres_fdw is not smart about exploiting fast-start plans ]
>>
>> Yeah, that's basically not accounted for at all in the current design.
>>
>>> One possibility: would it be worth adding an option to EXPLAIN that
>>> makes it assume cursor_tuple_fraction?
>>
>> [ handwaving ahead ]
>>
>> I wonder whether it could be done without destroying postgres_fdw's
>> support for old servers, by instead including a LIMIT in the query sent
>> for explaining. The trick would be to know what value to put as the
>> limit, though. It'd be easy to do if we were willing to explain the query
>> twice (the second time with a limit chosen as a fraction of the rowcount
>> seen the first time), but man that's an expensive solution.
>>
>> Another component of any real fix here would be to issue "SET
>> cursor_tuple_fraction" before opening the execution cursor, so as to
>> ensure that we actually get an appropriate plan on the remote side.
>>
>> If we could tell whether there's going to be any use in fast-start plans,
>> it might make sense to build two scan paths for a foreign table, one
>> based
>> on a full-table scan and one based on EXPLAIN ... LIMIT 1. This still
>> means two explain requests, which is why I'm not thrilled about doing it
>> unless there's a high probability of the extra explain being useful.
>
> Agreed, but ISTM that to address the original issue, it would be enough
> to jsut add LIMIT (or ORDER BY LIMIT) pushdown to postgres_fdw based on
> the upper-planner-pathification work.

Will work on it unless somebody else wants to.

Best regards,
Etsuro Fujita

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