A pachadi is not a single dish and describes different types of side dish accompaniments, depending where you are on the subcontinent.
In Kerala and Tamil Nadu, a pachadi is like a raita (raitha) and is mainly made from yogurt.
In Andhra Pradesh, a pachadi is like a chutney (or pickle).
And in the rest of south India, a pachadi is a vegetable side dish.
The Curry Focus staff cannot remember having seen pachadi on a menu in one of the many curry houses that we enjoy.
So if somebody offers you a pachadi, be sure and ask exactly what it is.