A 2-3 line description:
The VisualWorks EXDI is a core API of the VisualWorks Smalltalk
development environment. It provides a standard set of interfaces to
SQL-compliant databases. The PosgreSQL Smalltalk driver + EXDI interface
libarries provide an EXDI implementation for VisualWorks programmers
wishing to access PostgreSQL databases.
Is this the kind of thing you had in mind?