My CTO wrote custom PostgreSQL functions to generate all the JSON that our API returns. First time I saw it, I said "Holy crap, that query is two pages long. You could just tell Rails to serialize the object in 3 lines of code." He replied "I know. That's how we did it at first. But generating the JSON directly in postgres is about 700x faster."

Eventually we even moved a very complex scoring algorithm from an overnight Hadoop Java process, into Postgres as a compiled extension in C. We used to spend all night calculating those scoring results and caching them in an enormous table. But with the Postgres+C solution, we could calculate and store it in realtime by having Postgres do the hard part.