Fixes: #6217
Change-Id: Icd8275c49f30ea77d00f76209fb71fb2b8fa9528

Added accessors ``.sqlstate`` and synonym ``.pgcode`` to the ``.orig``
attribute of the SQLAlchemy exception class raised by the asyncpg DBAPI
adapter, that is, the intermediary exception object that wraps on top of
that raised by the asyncpg library itself, but below the level of the
SQLAlchemy dialect.

Fixes: #6199
Change-Id: Ie0f1ffaaff47c7a50dd1fbccdbe588cdc5322b70

Added a new flag to the :class:`_engine.Dialect` class called
:attr:`_engine.Dialect.supports_statement_cache`. This flag now needs to be present
directly on a dialect class in order for SQLAlchemy's
:ref:`query cache <sql_caching>` to take effect for that dialect. The
rationale is based on discovered issues such as :ticket:`6173` revealing
that dialects which hardcode literal values from the compiled statement,
often the numerical parameters used for LIMIT / OFFSET, will not be
compatible with caching until these dialects are revised to use the
parameters present in the statement only. For third party dialects where
this flag is not applied, the SQL logging will show the message "dialect
does not support caching", indicating the dialect should seek to apply this
flag once they have verified that no per-statement literal values are being
rendered within the compilation phase.

Fixes: #6184
Change-Id: I6fd5b5d94200458d4cb0e14f2f556dbc25e27e22

Introduce a new parameter :paramref:`_types.Enum.omit_aliases` in
:class:`_types.Enum` type allow filtering aliases when using a pep435 Enum.
Previous versions of SQLAlchemy kept aliases in all cases, creating
database enum type with additional states, meaning that they were treated
as different values in the db. For backward compatibility this flag
defaults to ``False`` in the 1.4 series, but will be switched to ``True``
in a future version. A deprecation warning is raise if this flag is not
specified and the passed enum contains aliases.

Fixes: #6146
Change-Id: I547322ffa90d0273d91bb3bf8bfea6ec934d48b9

Fixed regression in the MySQL dialect where the reflection query used to
detect if a table exists would fail on very old MySQL 5.0 and 5.1 versions.

Fixes: #6151
Change-Id: I48e98542c3a1a49519a5d113e43ba6f917e8580e

Fixed regression caused by :ticket:`6023` where the PostgreSQL cast
operator applied to elements within an :class:`_types.ARRAY` when using
psycopg2 would fail to use the correct type in the case that the datatype
were also embedded within an instance of the :class:`_types.Variant`
adapter.

Additionally, repairs support for the correct CREATE TYPE to be emitted
when using a ``Variant(ARRAY(some_schema_type))``.

Fixes: #6182
Change-Id: I1b9ba7c876980d4650715a0b0801b46bdc72860d

Fixed a regression in MSSQL 2012+ that prevented the order clause
to be rendered when ``offset=0`` is used in a subquery.

Fixed critical regression where the Oracle compiler would not maintain the
correct parameter values in the LIMIT/OFFSET for a select due to a caching
issue.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #6163
Fixes: #6173
Change-Id: Ieb12354271d09ad935d684ee0db4fa0128837215