Re: Casts from jsonb to other types should cope with json null

I wrote:
> OK. Nobody has spoken against the 0001 patch (replace errors with
> return-a-null), so I think I'll go ahead and commit that one.
> Then I'll return to this thread with a fleshed-out patch for 0002.

0001 is pushed, and as promised, here's a version of 0002 extended
to cover all the string-category types.

However ... the more I play with 0002, the less enchanted I get.
The problem is that there are two different use-cases to serve:

1. General conversion of any jsonb value to string form.
2. Conversion of a jsonb scalar string to a SQL string.

For use-case #1, it makes sense that we do

regression=# select '"hello"'::jsonb::text;
text
---------
"hello"
(1 row)

but for use-case #2, you'd probably prefer that the quotes
weren't included. (We can't do that in use-case #1 because
the string contents might look too much like some other
sort of JSON value.) So it seems like two separate conversion
functions are needed to serve these two use-cases, and for
better or worse we've already decided that casting jsonb to text
is meant for use-case #1. (It was sort of a decision by default,
I suspect, but not deciding is still a decision.)

What I am realizing is that "JSON null becomes SQL NULL" is a rule
that is adapted to use-case #2 but not so much to use-case #1.
For example, the existing behavior

regression=# select null::jsonb::text;
text
------

(1 row)

regression=# select 'null'::jsonb::text;
text
------
null
(1 row)

actually makes plenty of sense if you hope to be able to round-trip
the result. It's only after rejecting non-scalar JSON values that
it makes sense to special-case a JSON null.

So here's the patch, just because I promised it, but I'm now
thinking about withdrawing it.

What would make more sense for use-case #2 is something that
produces NULL for JSON null, a de-quoted string for a JSON
string value, and an error otherwise. The ->> operator is
about halfway there (it won't throw an error for non-scalar
input), but of course that only works when the value you want
to extract is in a JSON object field. I guess what would
be wanted is a new function f(jsonb) returns text, but I'm
unsure about a good name.

regards, tom lane