fix up the isolation level docs which were a mess

lib/sqlalchemy/dialects/postgresql/base.py

@@ -41,24 +41,40 @@ case.
 To force the usage of RETURNING by default off, specify the flag
 ``implicit_returning=False`` to :func:`.create_engine`.
 
+.. _postgresql_isolation_level:
+
 Transaction Isolation Level
 ---------------------------
 
-:func:`.create_engine` accepts an ``isolation_level`` parameter which results
-in the command ``SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL
-<level>`` being invoked for every new connection. Valid values for this
-parameter include ``READ COMMITTED``, ``READ UNCOMMITTED``, ``REPEATABLE READ``,
-and ``SERIALIZABLE``::
+All Postgresql dialects support setting of transaction isolation level
+both via a dialect-specific parameter ``isolation_level``
+accepted by :func:`.create_engine`,
+as well as the ``isolation_level`` argument as passed to :meth:`.Connection.execution_options`.
+When using a non-psycopg2 dialect, this feature works by issuing the
+command ``SET SESSION CHARACTERISTICS AS TRANSACTION ISOLATION LEVEL
+<level>`` for each new connection.
+
+To set isolation level using :func:`.create_engine`::
 
     engine = create_engine(
                     "postgresql+pg8000://scott:tiger@localhost/test",
                     isolation_level="READ UNCOMMITTED"
                 )
 
-When using the psycopg2 dialect, a psycopg2-specific method of setting
-transaction isolation level is used, but the API of ``isolation_level``
-remains the same.  The psycopg2 dialect also includes support
-for ``AUTOCOMMIT`` isolation - see :ref:`psycopg2_isolation`.
+To set using per-connection execution options::
+
+    connection = engine.connect()
+    connection = connection.execution_options(isolation_level="READ COMMITTED")
+
+Valid values for ``isolation_level`` include:
+
+* ``READ COMMITTED``
+* ``READ UNCOMMITTED``
+* ``REPEATABLE READ``
+* ``SERIALIZABLE``
+
+The :mod:`~sqlalchemy.dialects.postgresql.psycopg2` dialect also offers the special level ``AUTOCOMMIT``.  See
+:ref:`psycopg2_isolation_level` for details.
 
 
 Remote / Cross-Schema Table Introspection

lib/sqlalchemy/dialects/postgresql/psycopg2.py

@@ -17,7 +17,7 @@ psycopg2 Connect Arguments
 psycopg2-specific keyword arguments which are accepted by
 :func:`.create_engine()` are:
 
-* *server_side_cursors* - Enable the usage of "server side cursors" for SQL
+* ``server_side_cursors``: Enable the usage of "server side cursors" for SQL
   statements which support this feature. What this essentially means from a
   psycopg2 point of view is that the cursor is created using a name, e.g.
   ``connection.cursor('some name')``, which has the effect that result rows are
@@ -28,8 +28,12 @@ psycopg2-specific keyword arguments which are accepted by
   time are fetched over the wire to reduce conversational overhead.
   Note that the ``stream_results=True`` execution option is a more targeted
   way of enabling this mode on a per-execution basis.
-* *use_native_unicode* - Enable the usage of Psycopg2 "native unicode" mode
-  per connection. True by default.
+* ``use_native_unicode``: Enable the usage of Psycopg2 "native unicode" mode
+  per connection.  True by default.
+* ``isolation_level``: This option, available for all Posgtresql dialects,
+  includes the ``AUTOCOMMIT`` isolation level when using the psycopg2
+  dialect.  See :ref:`psycopg2_isolation_level`.
+
 
 Unix Domain Connections
 ------------------------
@@ -60,13 +64,11 @@ The following DBAPI-specific options are respected when used with
 
 * isolation_level - Set the transaction isolation level for the lifespan of a
   :class:`.Connection` (can only be set on a connection, not a statement
-  or query). This includes the options ``SERIALIZABLE``, ``READ COMMITTED``,
-  ``READ UNCOMMITTED``, ``REPEATABLE READ``, and ``AUTOCOMMIT``.
+  or query).   See :ref:`psycopg2_isolation_level`.
 
-  .. versionadded:: 0.8.2 support for AUTOCOMMIT isolation level when using
-     psycopg2.
-
-* stream_results - Enable or disable usage of server side cursors.
+* stream_results - Enable or disable usage of psycopg2 server side cursors -
+  this feature makes use of "named" cursors in combination with special
+  result handling methods so that result rows are not fully buffered.
   If ``None`` or not set, the ``server_side_cursors`` option of the
   :class:`.Engine` is used.
 
@@ -117,16 +119,31 @@ Transactions
 
 The psycopg2 dialect fully supports SAVEPOINT and two-phase commit operations.
 
-.. _psycopg2_isolation:
+.. _psycopg2_isolation_level:
 
-Transaction Isolation Level
----------------------------
+Psycopg2 Transaction Isolation Level
+-------------------------------------
 
-The ``isolation_level`` parameter of :func:`.create_engine` here makes use
+As discussed in :ref:`postgresql_isolation_level`,
+all Postgresql dialects support setting of transaction isolation level
+both via the ``isolation_level`` parameter passed to :func:`.create_engine`,
+as well as the ``isolation_level`` argument used by :meth:`.Connection.execution_options`.
+When using the psycopg2 dialect, these options make use of
 psycopg2's ``set_isolation_level()`` connection method, rather than
-issuing a ``SET SESSION CHARACTERISTICS`` command.   This because psycopg2
-resets the isolation level on each new transaction, and needs to know
-at the API level what level should be used.
+emitting a Postgresql directive; this is because psycopg2's API-level
+setting is always emitted at the start of each transaction in any case.
+
+The psycopg2 dialect supports these constants for isolation level:
+
+* ``READ COMMITTED``
+* ``READ UNCOMMITTED``
+* ``REPEATABLE READ``
+* ``SERIALIZABLE``
+* ``AUTOCOMMIT``
+
+.. versionadded:: 0.8.2 support for AUTOCOMMIT isolation level when using
+   psycopg2.
+
 
 NOTICE logging
 ---------------