This is unlikely to be implemented anytime soon.
Because of the way that RETURNING is implemented, metadata about the
returned columns is not easily available when the bytecode for the
RETURNING clause is generated. A major refactoring will be required to
make that information available. Furthermore, the metadata is of
limited utility, and it uses CPU cycles to compute and heap space
to store, even in the common case where it is never accessed.
So the down-side of adding metadata to RETURNING columns includes:
* A lot of work for the developers.
* Probably a whole bunch of new bugs.
* Additional CPU cycles required to prepare statements with RETURNING
clauses, even if the metadata is never accessed.
* Additional heap memory used by prepared statements with RETURNING
clauses, even if the metadata is never accessed.
And there seems to be very little offsetting up-side. In all of the
programs I've written in 20 years of using SQLite, I don't recall ever
having used the metadata information (except for programs whose purpose
is to test the metadata APIs).