Transaction context switching?

Hi,

we plan to use PostgresSQL it as a database back-end for multi-threaded
CORBA server(s) written in C++. From the DB point of view, that's just a
multithreaded C++ client. This multithreaded client can now use several
different transactions "simultaneously" (meaning each thread may have it's
'own' active transaction). It seems to me the only way of telling PostgreSQL
that a command should be executed in the context of a certain transaction is
by assigning one distinct DB connection to each thread.

Is this correct? If so, this could become a problem, since a DB is a rather
'expensive' resource, as far as I understand these things.
Or is there another possibility to explicitly refer to a specific
transaction when issuing commands to the DB via the C++ API?

TIA and regards
-Philipp Leibfried
Speech Design GmbH