query for  "show standard_conforming_strings"; as this variable was
added as of PG 8.2, we skip the query for PG versions older than
that as well as for backends like Redshift. #2946

reflection query were updated to take into account primary key constraints
that were renamed; the newer query fails on very old versions of
Postgresql such as version 7, so the old query is restored in those cases
when server_version_info < (8, 0) is detected. #2291

to support dialect-level reflection options for all :class:`.Table`
objects reflected.

- Added a new dialect-level argument ``postgresql_ignore_search_path``;
this argument is accepted by both the :class:`.Table` constructor
as well as by the :meth:`.MetaData.reflect` method.  When in use
against Postgresql, a foreign-key referenced table which specifies
a remote schema name will retain that schema name even if the name
is present in the ``search_path``; the default behavior since 0.7.3
has been that schemas present in ``search_path`` would not be copied
to reflected :class:`.ForeignKey` objects.  The documentation has been
updated to describe in detail the behavior of the ``pg_get_constraintdef()``
function and how the ``postgresql_ignore_search_path`` feature essentially
determines if we will honor the schema qualification reported by
this function or not. [ticket:2922]

arguments; [ticket:2866]
- add dialect specific kwarg functionality to ForeignKeyConstraint, ForeignKey

the indexing.  this is for more natural operation.
- also add cast() to the JSON expression to complement astext. This integrates
the CAST call which will be needed frequently.  Part of [ticket:2687].
- it's a little unclear how more advanced unicode attribute-access is going to go,
some quick attempts at testing yielded strange error messages from psycopg2.
- do other cross linking as mentioned in [ticket:2687].

compatible with psycopg2's per-connection/cursor approach.  add round trip tests for
both native and non-native.

- add support for "standalone" JSON objects; this involves getting CAST
to upgrade the given type of a bound parameter.  should add a core-only test
for this.
- add tests for "standalone" json round trips both with and without unicode
- add mechanism by which we remove psycopg2's "json" handler in order to get
the effect of using our non-native result handlers

doing the isinstance() check - currently used only by psycopg2 + native enum + py2k.
- didn't realize psycopg2 had UNICODEARRAY extension all this time; replace _PGArray
with just using UNICODEARRAY instead.
- replace unnecessary/inconsistent __import__ in _isolation_lookup.