On 2016/12/21 21:44, Etsuro Fujita wrote:
On 2016/12/20 0:37, Tom Lane wrote:
Etsuro Fujita <fujita.ets...@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,
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
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 noticed I was wrong and you are right.  Sorry for the noise.

Best regards,
Etsuro Fujita

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