ParAccel, like a large percentage of parallel analytical databases, are forked off the excellent PostgreSQL code base because those internals were designed to be easy to extend and modify. Netezza, Vertica, EMC/Greenplum, Teradata/Aster, et al are all PostgreSQL derivatives as well with varying degrees of divergence. I've designed and built custom parallel derivatives of PostgreSQL for companies too, it is surprisingly straightforward.

There are only a handful of original, high-quality database kernels out there because it is enormously difficult to design one from scratch. Most good databases copy an existing design, or even more conveniently, fork the mature, easily modifiable, BSD-licensed, Stonebraker-designed PostgreSQL kernel. Every basic kernel design has distinctive characteristics that tend to stick with everything derived from them, which leaves an identifiable "fingerprint" on a new database if you know what to look for. You inherit both the strengths and weaknesses of the underlying kernel design.

(Source: I've designed analytical databases engines for a long time.)