pgsql: Fix EquivalenceClass processing for nested append relations.

Fix EquivalenceClass processing for nested append relations.

The original coding of EquivalenceClasses didn't foresee that appendrel
child relations might themselves be appendrels; but this is possible for
example when a UNION ALL subquery scans a table with inheritance children.
The oversight led to failure to optimize ordering-related issues very well
for the grandchild tables.  After some false starts involving explicitly
flattening the appendrel representation, we found that this could be fixed
easily by removing a few implicit assumptions about appendrel parent rels
not being children themselves.

Kyotaro Horiguchi and Tom Lane, reviewed by Noah Misch

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/a87c729153e372f3731689a7be007bc2b53f1410

Modified Files
--------------
src/backend/optimizer/path/allpaths.c   |   20 ++++++++++---
src/backend/optimizer/path/equivclass.c |   12 +++++---
src/backend/optimizer/plan/createplan.c |    2 +-
src/test/regress/expected/union.out     |   47 +++++++++++++++++++++++++++++++
src/test/regress/sql/union.sql          |   29 +++++++++++++++++++
5 files changed, 101 insertions(+), 9 deletions(-)