simple but unusual system allows for a dramatic savings in Python
overhead for the construction and processing of orm :class:`.Query`
objects, from query construction up through rendering of a string
SQL statement.
fixes #3054

:attr:`.Query.column_descriptions`.  This refers to the primary ORM
mapped class or aliased class that is referred to by the expression.
Compared to the existing entry for ``"type"``, it will always be
a mapped entity, even if extracted from a column expression, or
None if the given expression is a pure core expression.
references #3320

DROP TYPE instruction when a plain ``table.drop()`` is called,
assuming the object is not associated directly with a
:class:`.MetaData` object.   In order to accomodate the use case of
an enumerated type shared between multiple tables, the type should
be associated directly with the :class:`.MetaData` object; in this
case the type will only be created at the metadata level, or if
created directly.  The rules for create/drop of
Postgresql enumerated types have been highly reworked in general.
fixes #3319

:meth:`.QueryEvents.before_compile` event allows the creation
of functions which may place additional modifications to
:class:`.Query` objects before the construction of the SELECT
statement.   It is hoped that this event be made much more
useful via the advent of a new inspection system that will
allow for detailed modifications to be made against
:class:`.Query` objects in an automated fashion.
fixes #3317

is used with a one-to-many query that also features LIMIT,
OFFSET, or DISTINCT has been disabled in the case of a one-to-one
relationship, that is a one-to-many with
:paramref:`.relationship.uselist` set to False.  This will produce
more efficient queries in these cases.
fixes #3249

attached to session X" would fail to prevent the object from
also being attached to the new session, in the case that execution
continued after the error raise occurred.
fixes #3301

of a declarative inheritance hierarchy would prevent attributes
and configuration being correctly propagated from the base class
to the inheriting class.
fixes #3219 fixes #3240