ERROR: PL/pgSQL functions cannot return type myrange
Am I misunderstanding the docs, or are they out-of-date, or is something else going on? How can I create a canonicalization function for my range type?
In short, I'm pretty sure shell types can only be referenced in C language functions. The allowed types listing for pl/pgsql doesn't mention shell types and this error reinforces the fact that the omission is indeed a reflection of the fact that shell types are not allowed.
Thus one needs to write a C language function if one wishes to implement a custom range.
