http://www.postgresql.org/message-id/E1WRpmB-0002et-MT@gemul... links to the git commit at http://git.postgresql.org/gitweb/?p=postgresql.git;a=commitd.... The obviously interesting file is src/include/utils/jsonb.h (a new header file named jsonb).

It has a reasonably decent (highly decent for a header file) description.

One thing I had to take a guess at is that, in JEntry, the header is a combination of a bit mask and an offset. I also would guess that offsets for identical keys (can happen in nested json), and maybe offsets for other identical data will be identical. I didn't check either.