On 2016/12/19 13:59, Ashutosh Bapat wrote:
On Fri, Dec 16, 2016 at 9:43 PM, Tom Lane <t...@sss.pgh.pa.us> wrote:
Etsuro Fujita <fujita.ets...@lab.ntt.co.jp> writes:
On 2016/12/16 11:25, Etsuro Fujita wrote:
As I said upthread, an alternative I am thinking is (1) to create an
equivalent nestloop join path using inner/outer paths of a foreign join
path, except when that join path implements a full join, in which case a
merge/hash join path is used, (2) store it in fdw_outerpath as-is, and
(3) during an EPQ recheck, apply postgresRecheckForeignScan to the outer
subplan created from the fdw_outerpath as-is.  What do you think about

Let me explain about #1 and #2 a bit more.  What I have in mind is:

* modify postgresGetForeignPaths so that a simple foreign table scan
path is stored into the base relation's PgFdwRelationInfo.
* modify postgresGetForeignJoinPaths so that
     (a) a local join path for a 2-way foreign join is created using
simple foreign table scan paths stored in the base relations'
PgFdwRelationInfos, and stored into the join relation's PgFdwRelationInfo.
     (b) a local join path for a 3-way foreign join, whose left input is
a 2-way foreign join, is created using a local join path stored in the
left input join relation's PgFdwRelationInfo and a simple foreign table
scan path stored into the right input base relation's PgFdwRelationInfo.
     (c) Likewise for higher level foreign joins.
     (d) local join paths created are passed to create_foreignscan_path
and stored into the fdw_outerpaths of the resulting ForeignPahts.

Hm, isn't this overcomplicated?  As you said earlier, we don't really
care all that much whether the fdw_outerpath is free of lower foreign
joins, because EPQ setup will select those lower join's substitute EPQ
plans anyway.  All that we need is that the EPQ tree be a legal
implementation of the join order with join quals applied at the right
places.  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.  Use this as the fdw_outerpath for
all foreign paths made for the joinrel."

We could use fdw_outerpath of the left and right inputs instead of
looking for the cheapest paths. For base relations as left or right
relations, use the unparameterized cheapest foreign path. This will
ensure that all joins in fdw_outerpath are local joins, making EPQ
checks slightly efficient by avoiding redirection at every foreign

That seems close to what I had in mind described above. One additional work required for that would to store the fdw_outerpath into the RelOptInfo's fdw_private such as PgFdwRelationInfo before add_path for the foreign-join path containing the fdw_outerpath, because add_path might reject the foreign-join path. I think the additional work would make that rather complicated.

Best regards,
Etsuro Fujita

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