pgsql: Always build a custom plan node's targetlist from the path's pat

Always build a custom plan node's targetlist from the path's pathtarget.

We were applying the use_physical_tlist optimization to all relation
scan plans, even those implemented by custom scan providers. However,
that's a bad idea for a couple of reasons. The custom provider might
be unable to provide columns that it hadn't expected to be asked for
(for example, the custom scan might depend on an index-only scan).
Even more to the point, there's no good reason to suppose that this
"optimization" is a win for a custom scan; whatever the custom provider
is doing is likely not based on simply returning physical heap tuples.
(As a counterexample, if the custom scan is an interface to a column store,
demanding all columns would be a huge loss.) If it is a win, the custom
provider could make that decision for itself and insert a suitable
pathtarget into the path, anyway.

Per discussion with Dmitry Ivanov. Back-patch to 9.5 where custom scan
support was introduced. The argument that the custom provider can adjust
the behavior by changing the pathtarget only applies to 9.6+, but on
balance it seems more likely that use_physical_tlist will hurt custom
scans than help them.

Discussion: https://postgr.es/m/e29ddd30-8ef9-4da5-a50b-2bb7b8c7198d@postgrespro.ru

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/6f0f98bb0bced134c09b7acf8fe35ff3a6f1bbd2

Modified Files
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src/backend/optimizer/plan/createplan.c | 14 +++++++++++---
1 file changed, 11 insertions(+), 3 deletions(-)