commit two of two. this reorganizes ColumnCollection
to build a new index up front that's used to optimize
the corresponding_column() method.
Additional performance enhancements within ORM-enabled SQL statements,
specifically targeting callcounts within the construction of ORM
statements, using combinations of :func:`_orm.aliased` with
:func:`_sql.union` and similar "compound" constructs, in addition to direct
performance improvements to the ``corresponding_column()`` internal method
that is used heavily by the ORM by constructs like :func:`_orm.aliased` and
similar.
Fixes: #8796
Change-Id: I4a76788007d5a802b9a4081e6a0f6e4b52497b50
Mike Bayer
d5be2cc139