Executable code is literally just data that you mark as executable. It did the JIT code, and the idea that it can't then share it between processes is incomprehensible.

I was actually confused by this submission as it puts so much of an emphasis on initial compilation time, when every DB (apparently except for pgsql) caches that result and shares it/reuses it until invalidation. Invalidation can occur for a wide variety of reasons (data composition changing, age, etc), but still the idea of redoing it on every query, where most DBs see the same queries endlessly, is insane.

The emphasis on compilation time there is because the JIT provider that comes with Postgres (LLVM-based) is broken in that particular area. But you're right, JITed code can be cached, if some conditions are met (it's position independent, for one). Not all JIT providers do that, but many do. Caching is on the table, but if your JIT-compilation takes microseconds, caching could be rather a burden in many cases. Still for some cases useful.

No a lot of jitted code has pointers to addresses specific to that process which makes no sense in another process.

To make code shareable between processes takes effort and will have tradeoff in performance since it is not specialized to the process.

If the query plan where at least serializable which is more like a AST then at least that part could be reused and then maybe have jitted code in each processes cached in memory that the plan can reference by some key.

DB's like MSSQL avoid the problem because they run a single OS process with multiple threads instead. This is also why it can handle more connections easily since each connection is not a whole process.