Etsuro Fujita <> 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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