What version are you using? What is your usage pattern for
insert/update/deletes? If sometimes the JSON data gets too big and the data
is moved from in-line storage to TOASTED, then that would be opening up
gaps. Or if you are doing deletes. Perhaps adjusting your fillfactor
and/or TOAST_TUPLE_TARGET would influence that behavior in the direction
you want to go. As best I understand though if you are doing deletes, you
won't be able to prevent those gaps from showing up and being re-used. I
don't believe there is any way to influence Postgres to append-only the
tuples and pages.