Postgres can't be embedded or serverless. Firebird has the embedded feature.
Most of the databases have this capability (hsqldb, derby,oracle,mysql,
firebird, and db2). Derby and hsqldb are the only free embedded databases
for commercial use.
I recently ported a schema from postgres to firebird and found name size
limitations. Firebird has a limitation on the size of it's column names,
table names, constraint names and index names. I think the size limitation
on firebird is 31 characters. Postgres doesn't have this limitation.
Steve