You don't need to recompile it actually! It's enough to compile your extension and Postgres can then dynamically link to it, which makes for a perfectly good feedback loop. pgfdb is built with pgrx: https://github.com/pgcentralfoundation/pgrx, which is a framework for building Postgres extensions in Rust that handles all the compiling and linking for you. Highly recommend that if you want to try writing extensions and are also a fan of Rust!

> Also, how do the deployment semantics of FDB affect this. If I remember correctly, you typically run one FDB process per CPU on a machine. How do PG processes map to those

You would run the two separately, so PG processes would run and scale separately from FDB processes. Postgres spawns a new process for each connection so you would not want to map Postgres itself 1:1 with CPUs!