return type is not strictly assumed to be boolean; it now
returns a :class:`.Boolean` subclass called :class:`.MatchType`.
The type will still produce boolean behavior when used in Python
expressions, however the dialect can override its behavior at
result time.  In the case of MySQL, while the MATCH operator
is typically used in a boolean context within an expression,
if one actually queries for the value of a match expression, a
floating point value is returned; this value is not compatible
with SQLAlchemy's C-based boolean processor, so MySQL's result-set
behavior now follows that of the :class:`.Float` type.
A new operator object ``notmatch_op`` is also added to better allow
dialects to define the negation of a match operation.
fixes #3263

- add a test for PG reflection of unique index without any unique
constraint
- for PG, don't include 'duplicates_constraint' in the entry
if the index does not actually mirror a constraint
- use a distinct method for unique constraint reflection within table
- catch unique constraint not implemented condition; this may
be within some dialects and also is expected to be supported by
Alembic tests
- migration + changelogs for #3184
- add individual doc notes as well to MySQL, Postgreql
fixes #3184

Calls to reflect a table did not create any UniqueConstraint objects.
The reflection core made no calls to get_unique_constraints and as
a result, the sqlite dialect would never reflect any unique constraints.

MySQL transparently converts unique constraints into unique indexes, but
SQLAlchemy would reflect those as an Index object and as a
UniqueConstraint. The reflection core will now deduplicate the unique
constraints.

PostgreSQL would reflect unique constraints as an Index object and as
a UniqueConstraint object. The reflection core will now deduplicate
the unique indexes.

in :ticket:`2682` disallowed the MySQL dialect from making use of the
"true" and "false" symbols in the context of "IS" / "IS NOT", but
MySQL supports this syntax even though it has no boolean type.
MySQL remains "non native boolean", but the :func:`.true`
and :func:`.false` symbols again produce the
keywords "true" and "false", so that an expression like
``column.is_(true())`` again works on MySQL.
fixes #3186