pgsql: Improve planner's handling of SetOp plans.

Improve planner's handling of SetOp plans.

Remove the code for inserting flag columns in the inputs of a SetOp.
That was the only reason why there would be resjunk columns in a
set-operations plan tree, so we can get rid of some code that
supported that, too.

Get rid of choose_hashed_setop() in favor of building Paths for
the hashed and sorted alternatives, and letting them fight it out
within add_path().

Remove set_operation_ordered_results_useful(), which was giving wrong
answers due to examining the wrong ancestor node: we need to examine
the immediate SetOperationStmt parent not the topmost node. Instead
make each caller of recurse_set_operations() pass down the relevant
parent node. (This thinko seems to have led only to wasted planning
cycles and possibly-inferior plans, not wrong query answers. Perhaps
we should back-patch it, but I'm not doing so right now.)

Teach generate_nonunion_paths() to consider pre-sorted inputs for
sorted SetOps, rather than always generating a Sort node.

Patch by me; thanks to Richard Guo and David Rowley for review.

Discussion: https://postgr.es/m/1850138.1731549611@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8d96f57d5cc79c0c51050bb707c19bf07d2895eb

Modified Files
--------------
src/backend/optimizer/plan/planner.c | 7 +-
src/backend/optimizer/prep/prepunion.c | 478 ++++++++++++--------------------
src/backend/optimizer/util/pathnode.c | 69 ++++-
src/include/optimizer/prep.h | 1 -
src/test/regress/expected/subselect.out | 48 +++-
src/test/regress/expected/union.out | 53 ++--
src/test/regress/sql/subselect.sql | 32 ++-
src/test/regress/sql/union.sql | 9 +
8 files changed, 365 insertions(+), 332 deletions(-)