Hello Ryszard,
As you are probably aware, the SQL standard is quite limited, and all database systems I've ever used go beyond it and provide extra functionality. They tend to all use different syntax for these extensions, and provide slightly different ones, so I didn't want to bother with making my library support these different syntaxes or be some kind of lowest common denominator -- partially because I am lazy, and partially because I think PostgreSQL is a very solid system which is pretty easy to set up -- there is rarely a very strong need to use anything else.
I am afraid that I couldn't quite give a precise list of Postgres-specific features that Postmodern uses, but just writing a cl-postgres replacement for SQLite will definitely not be enough -- that only concerns the client/server protocol, and doesn't solve the problem of SQL syntax and differences in supported features.
I have no idea how well the SQLite interface is documented, and whether you'd want to implement it in CL or use FFI. Postgres has good documentation about its protocol, so after studying that, writing the client was not that hard (apart from having to implement some stuff like binary floating point decoding).
Cheers, Marijn