The "byte-code" coming from the query planner typically only has a handful of steps in a linear sequence. Joins, filters, and such. But the individual steps can be very costly.

So there is not much to gain from JITing the query plan execution only.

JITing begins to make more sense, when the individual query plan steps (join, filter, ...) themselves be specialized/recompiled/improved/merged by knowing the context of the query plan.