Proposal for JSONB functions for internal representation casting insted text-casting

The usefulness of ->> operator is indisputable, but even with boolean or
numeric values, with good binary internal representation, it returns JSONB
value as text data type.

The simple *(myJSONB->>'myField')::expectedType* is not enough because:

1) there are no internal optimization, need two-step casting, first
bynary-to-text, them text-to-expectedType.

2) if expectedType is not the expected (in the associated jsonb_typeof),
generates an error... The ideal "return NULL" convention is not easy to
implement with usual casting.

More details and some examples at
http://stackoverflow.com/q/34579758/287948

- - - -
CONTEXT OF USEFULNESS

As section "8.14. JSON Types" in the pg9.4 guide,
"Representing data as JSON can be considerably more flexible (...) is quite
possible for both approaches to co-exist and complement each other (...)
However, even for applications where maximal flexibility is desired, it is
still recommended that JSON documents have a somewhat fixed structure".

The proposed casting functions of JSONB is a part of "predictable but fluid
structure" demands in JSON representation, and makes it easier to write
queries that mix usual data types with JSONB.

- - - -
Formal requeriment <https://en.wikipedia.org/wiki/Requirements_analysis> for
a C implementation below

CREATE FUNCTION jbval_to_numeric(JSONB, varchar) RETURNS numeric AS $f$
SELECT CASE
WHEN jsonb_typeof($1->$2)='number' THEN ($1->>$2)::numeric
ELSE NULL::numeric
END;$f$ LANGUAGE sql IMMUTABLE;
CREATE FUNCTION jbval_to_float(JSONB, varchar) RETURNS float AS $f$
SELECT CASE
WHEN jsonb_typeof($1->$2)='number' THEN ($1->>$2)::float
ELSE NULL::float
END;$f$ LANGUAGE sql IMMUTABLE;
CREATE FUNCTION jbval_to_int(JSONB, varchar, boolean DEFAULT true)
RETURNS int AS $f$
SELECT CASE
WHEN jsonb_typeof($1->$2)='number' THEN
CASE WHEN $3 THEN ($1->>$2)::int ELSE ($1->>$2)::float::int END
ELSE NULL::int
END;$f$ LANGUAGE sql IMMUTABLE;
CREATE FUNCTION jbval_to_boolean(JSONB, varchar) RETURNS boolean AS $f$
SELECT CASE
WHEN jsonb_typeof($1->$2)='boolean' THEN ($1->>$2)::boolean
ELSE NULL::boolean
END;$f$ LANGUAGE sql IMMUTABLE;