Database for storing heterogeneous "message" objects

[Jason Stokes]

I want to use a database to store sets of heterogeneous "message" objects,
which currently cover email and news messages, but will be extended more
generally to include messages in web-accessible archives and other
"email-like" things.  Queries can be made on a message database based on
their header properties.

The Python standard relational database API doesn't quite fit this domain
because the structure of such a database isn't quite tabular.  Although the
messages will have certain mandatory headers like from, subject etc. in
common, individual messages will also have optional headers that aren't
defined for all messages.  I want to allow queries on optional headers -- if
a header is defined for a message, any selection rule based on that message
will be applied to it, otherwise the test automatically fails.