pgsql: Add more tests for hashing and hash-based plans

Add more tests for hashing and hash-based plans

- Test hashing of an array of a non-hashable element type.

- Test UNION [DISTINCT] with hash- and sort-based plans. (Previously,
only INTERSECT and EXCEPT where tested there.)

- Test UNION [DISTINCT] with a non-hashable column type. This
currently reverts to a sort-based plan even if enable_hashagg is on.

- Test UNION/INTERSECT/EXCEPT hash- and sort-based plans with arrays
as column types. Also test an array with a non-hashable element
type.

- Test UNION/INTERSECT/EXCEPT similarly with row types as column
types. Currently, this uses only sort-based plans because there is
no hashing support for row types.

- Add a test case that shows that recursive queries using UNION
[DISTINCT] require hashable column types.

- Add a currently failing test that uses UNION DISTINCT in a
cycle-detection use case using row types as column types.

Discussion: /messages/by-id/38eccd35-4e2d-6767-1b3c-dada1eac3124@2ndquadrant.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/6dd8b00807871bfd0c3ce8e18fd1f6ba1a71fbf9

Modified Files
--------------
src/test/regress/expected/hash_func.out | 7 +
src/test/regress/expected/union.out | 357 +++++++++++++++++++++++++++++++-
src/test/regress/expected/with.out | 20 ++
src/test/regress/sql/hash_func.sql | 6 +
src/test/regress/sql/union.sql | 92 +++++++-
src/test/regress/sql/with.sql | 18 ++
6 files changed, 498 insertions(+), 2 deletions(-)