You don't need a low-level language like C for the query planner and SQL parser.

Linspire is doing OS development in Haskell http://lambda-the-ultimate.org/node/1506 and SQLite, with it's 45MLoC of test code for its 67 KLoC of source code, tests a lot of memory allocation bugs. http://www.sqlite.org/testing.html

The bigger question is, even though these guys are selling a software package that's probably never going open-source, how long until we use compiled high-level-languages instead of C, just like C replaced assembler?