Clustrix wrote a compiler. It was able to call out to C for string intrinsics and communication and btree operations, so it primarily handled extracting data from serialized formats, simple arithmetic, constructing serializations from downstream.

this was to replace a little byte code VM, and as far I recall took a very good programmer about 2 weeks for the basic version.

I dont see any reason to even try to bring something like a general purpose JIT. there's a huge benefit from just doing the basics, and you can move the line between precompiled functions and dynamically compiled ones on an ongoing basis. its also cheap enough that I wouldn't get hung up on needing to amortize that cost with prepared statements.