same target more than once; it implicitly dedupes only in the case of
a relationship join, and due to :ticket:`3233`, in 1.0 a join
to the same table twice behaves differently than 0.9 in that it no
longer erroneously aliases.   To help document this change,
the verbiage regarding :ticket:`3233` in the migration notes has
been generalized, and a warning has been added when :meth:`.Query.join`
is called against the same target relationship more than once.
fixes #3367

FROM clauses when using the :meth:`.Query.update` or
:meth:`.Query.delete` methods; instead of silently ignoring these
fields if methods like :meth:`.Query.join` or
:meth:`.Query.select_from` has been called, an error is raised.
In 0.9.10 this only emits a warning.
fixes #3349
- don't needlessly call _compile_context() and build up a
whole statement that we never need.  Construct QueryContext
as it's part of the event contract, but don't actually call upon
mapper attributes; use more direct systems of determining the
update or delete table.
- don't realy need _no_select_modifiers anymore

This is useful to be able to pass in mysql_limit=1 from using the
ORM.

to be using getattr

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

: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