How is FireBird rated when compared with PostgreSQL?
I don't know how it is rated, but honestly I looked at Firebird and it is way too "quirky" and I could never get it to compile embedded. The documentation is very very poor for that project when it comes to the embedded model, if you ask me. Also, Firebird has odd license which I think may limit your ability to commercially embed it without paying licensing -- not sure about that -- it certainly isn't as simple as a BSD license.
For embedded solutions I ended up looking at SQLite, which seems to work well enough.
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Chad
http://www.postgresqlforums.com/
