So far I've avoided any sort of "apply this python function" operation.  This will be powerful at some point though.  Right now we have the following potential approaches for the following backends:

Python - works naturally
PySpark - also works pretty naturally (in the mapping/filtering case)
Pandas - Should be ok, won't be vectorized (use Numba?)
DyND - (use Numba?)
SQL - Postgres apparently has an interface for this for C++ functions,  again, can we use Numba?