Why json

Once there's a json lookup operator (say implementing the [] operator for json to get the value of a key, or a path lookup function kind of like xpath provides for xml) then a functional index might be reasonably practical, at least if the result is a scalar.

Right now the lack of the concept of a "scalar json value" (due to an IMO over-eager interpretation of the JSON "spec") is frustrating all sorts of things, including proper json constructor functions.

More general indexing might be done through GiST, but I imagine it'd be a pretty complicated job involving extracting paths-to-values.