From: Etsuro Fujita <fujita.etsuro@lab.ntt.co.jp>
To: Pg Hackers <pgsql-hackers@postgresql.org>
Subject: Re: postgres_fdw: estimate_path_cost_size fails to re-use cached costs
Date: Tue, 29 Jan 2019 12:33:17 +0900
Message-ID: <5C4FC97D.2080305@lab.ntt.co.jp> (raw)
In-Reply-To: <5C4EDB7C.2020406@lab.ntt.co.jp>
References: <5C4AF3F3.4060409@lab.ntt.co.jp>
<5C4EDB7C.2020406@lab.ntt.co.jp>
(2019/01/28 19:37), Etsuro Fujita wrote:
> (2019/01/25 20:33), Etsuro Fujita wrote:
>> I noticed yet another thing while updating the patch for pushing down
>> ORDER BY LIMIT. Let me explain. When costing foreign paths on the
>> basis of local statistics, we calculate/cache the costs of an unsorted
>> foreign path, and re-use them to estimate the costs of presorted foreign
>> paths, as shown below. BUT: we fail to re-use them for some typical
>> queries, such as "select * from ft1 order by a", due to
>> fpinfo->rel_startup_cost=0, leading to doing the same cost calculation
>> repeatedly.
>>
>> /*
>> * We will come here again and again with different set of pathkeys
>> * that caller wants to cost. We don't need to calculate the cost of
>> * bare scan each time. Instead, use the costs if we have cached
>> them
>> * already.
>> */
>> if (fpinfo->rel_startup_cost> 0&& fpinfo->rel_total_cost> 0)
>> {
>> startup_cost = fpinfo->rel_startup_cost;
>> run_cost = fpinfo->rel_total_cost - fpinfo->rel_startup_cost;
>> }
>>
>> I think we should use "fpinfo->rel_startup_cost>= 0" here, not
>> "fpinfo->rel_startup_cost> 0". Also, it would be possible that the
>> total cost calculated is zero in corner cases (eg, seq_page_cost=0 and
>> cpu_tuple_cost=0 for the example), so I think we should change the total
>> cost part as well. Attached is a patch for that.
>
> I added the commit message. Updated patch attached. If no objections,
> I'll apply that to HEAD only as there are no reports of actual trouble
> from this, as far as I know.
Pushed.
Best regards,
Etsuro Fujita
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To: pgsql-hackers@postgresql.org
Cc: fujita.etsuro@lab.ntt.co.jp
Subject: Re: postgres_fdw: estimate_path_cost_size fails to re-use cached costs
In-Reply-To: <5C4FC97D.2080305@lab.ntt.co.jp>
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