Sometimes it matters whether serialization to a string is a deterministic function. If the JSON is the same, you want the output to be the same. A JSON parser won't care, but it's useful to do a string comparison without parsing (for example when diffing output).
But that's not really a correct thing to expect is it? If an object is an unordered set of name/value pairs then multiple serialized versions of the object may indeed be the same object data.
AFAIK the default serializer does not guarantee the order to be stable, but I haven't participated in the development of this feature that closely. So maybe I'm wrong.
Anyway, it should be possible to write a simple serializer on your own - a trivial PL/V8 function should suffice, I guess. And then you can define a CAST using that function (but generally adding casts is a bit dangerous, as it may have unexpected consequences).
