Rebase after pg_indent. Besides, off the list there was a suggestion that this
could be useful to accept more than one data type as a key for subscripting.
E.g. for jsonb it probably makes sense to understand both a simple key name and
jsonpath:
jsonb['a'] and jsonb['$.a']
While to implement it can be technically relatively straightforward I guess, I
wonder if there is any opinion about how valuable it could be and what it
should looks like from the syntax point of view (since I believe a user needs
to specify which type needs to be used).
