From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
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To: | Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> |
Cc: | Ashutosh Bapat <ashutosh(dot)bapat(at)enterprisedb(dot)com>, Jeff Janes <jeff(dot)janes(at)gmail(dot)com>, Robert Haas <robertmhaas(at)gmail(dot)com>, pgsql-hackers <pgsql-hackers(at)postgresql(dot)org> |
Subject: | Re: postgres_fdw bug in 9.6 |
Date: | 2016-12-19 15:37:32 |
Message-ID: | 10840.1482161852@sss.pgh.pa.us |
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Etsuro Fujita <fujita(dot)etsuro(at)lab(dot)ntt(dot)co(dot)jp> writes:
> On 2016/12/17 1:13, Tom Lane wrote:
>> So I think the rule could be
>> "When first asked to produce a path for a given foreign joinrel, collect
>> the cheapest paths for its left and right inputs, and make a nestloop path
>> (or hashjoin path, if full join) from those, using the join quals needed
>> for the current input relation pair.
> Seems reasonable.
>> Use this as the fdw_outerpath for
>> all foreign paths made for the joinrel."
> I'm not sure that would work well for foreign joins with sort orders.
> Consider a merge join, whose left input is a 2-way foreign join with a
> sort order that implements a full join and whose right input is a sorted
> local table scan. If the EPQ subplan for the foreign join wouldn't
> produce the right sort order, the merge join might break during EPQ
> rechecks (note that in this case the EPQ subplan for the foreign join
> might produce more than a single row during an EPQ recheck).
How so? We only recheck one row at a time, therefore it can be claimed to
have any sort order you care about.
regards, tom lane
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