Fixed #17266 -- Skipped the "SET TIME ZONE" query for PostgreSQL when it isn't necessary.

                conn_params['port'] = settings_dict['PORT']

            self.connection = Database.connect(**conn_params)

            self.connection.set_client_encoding('UTF8')

            # Set the time zone in autocommit mode (see #17062)

            tz = 'UTC' if settings.USE_TZ else settings_dict.get('TIME_ZONE')

            if tz:

                try:

                    get_parameter_status = self.connection.get_parameter_status

                except AttributeError:

                    # psycopg2 < 2.0.12 doesn't have get_parameter_status

                    conn_tz = None

                else:

                    conn_tz = get_parameter_status('TimeZone')

                if conn_tz != tz:

                    # Set the time zone in autocommit mode (see #17062)

                    self.connection.set_isolation_level(

                            psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT)

                    self.connection.cursor().execute("SET TIME ZONE %s", [tz])

.. _known to be faulty: http://archives.postgresql.org/pgsql-bugs/2007-07/msg00046.php

.. _Release 8.2.5: http://developer.postgresql.org/pgdocs/postgres/release-8-2-5.html

Optimizing PostgreSQL's configuration

-------------------------------------

Django needs the following parameters for its database connections:

- ``client_encoding``: ``'UTF8'``,

- ``default_transaction_isolation``: ``'read committed'``,

- ``timezone``: ``'UTC'`` when :setting:`USE_TZ` is ``True``, value of

  :setting:`TIME_ZONE` otherwise.

If these parameters already have the correct values, Django won't set them for

every new connection, which improves performance slightly. You can configure

them directly in :file:`postgresql.conf` or more conveniently per database

user with `ALTER ROLE`_.

Django will work just fine without this optimization, but each new connection

will do some additional queries to set these parameters.

.. _ALTER ROLE: http://www.postgresql.org/docs/current/interactive/sql-alterrole.html

Transaction handling

---------------------

If your application is particularly read-heavy and doesn't make many

database writes, the overhead of a constantly open transaction can

sometimes be noticeable. For those situations, if you're using the

``postgresql_psycopg2`` backend, you can configure Django to use

*"autocommit"* behavior for the connection, meaning that each database

sometimes be noticeable. For those situations, you can configure Django

to use *"autocommit"* behavior for the connection, meaning that each database

operation will normally be in its own transaction, rather than having

the transaction extend over multiple operations. In this case, you can

still manually start a transaction if you're doing something that