Encoding session data was definitely the wrong solution. Session data is binary text (esp. when using other session.serialize_handler) that must stay as-is and thus must also be safed in a binary column. Base64 encoding session data just decreses performance and increases storage costs and is semantically wrong because it does not have a character encoding.
That saving null bytes in Posgres won't work on a character column is also documented

First, binary strings specifically allow storing octets of value zero and other "non-printable" octets (usually, octets outside the range 32 to 126). Character strings disallow zero octets, and also disallow any other octet values and sequences of octet values that are invalid according to the database's selected character set encoding.
http://www.postgresql.org/docs/9.1/static/datatype-binary.html#DATATYPE-BINARY-TABLE

Implement lazy connections that are only opened when session is used by either passing a dsn string explicitly or falling back to session.save_path ini setting.

add a create table method that creates the correct table depending on database vendor. This makes the class self-documenting and standalone useable.

add lifetime column to session table which allows to have different lifetimes for each session

added upgrade and changelog notes