- The change in :ticket:`2721`, which is that the ``deferrable`` keyword
of :class:`.ForeignKeyConstraint` is silently ignored on the MySQL
backend, will be reverted as of 0.9; this keyword will now render again, raising
errors on MySQL as it is not understood - the same behavior will also
apply to the ``initially`` keyword.  In 0.8, the keywords will remain
ignored but a warning is emitted.   Additionally, the ``match`` keyword
now raises a :class:`.CompileError` on 0.9 and emits a warning on 0.8;
this keyword is not only silently ignored by MySQL but also breaks
the ON UPDATE/ON DELETE options.

To use a :class:`.ForeignKeyConstraint`
that does not render or renders differently on MySQL, use a custom
compilation option.  An example of this usage has been added to the
documentation, see :ref:`mysql_foreign_keys`.
[ticket:2721] [ticket:2839]