On 2016/12/20 0:37, Tom Lane wrote:
> Etsuro Fujita <fujita.etsuro@lab.ntt.co.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.
I'll have second thoughts about that. I agree with you except for that,
so I've created a patch; I removed GetExistingLocalJoinPath and added a
helper function, CreateLocalJoinPath, that generates a local join path
for a given foreign join, as described above. Please find attached a patch.
Best regards,
Etsuro Fujita
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