Extract numeric filed in JSONB more effectively

Hi:

Currently if we want to extract a numeric field in jsonb, we need to use
the following expression: cast (a->>'a' as numeric). It will turn a numeric
to text first and then turn the text to numeric again. See
jsonb_object_field_text and JsonbValueAsText. However the binary format
of numeric in JSONB is compatible with the numeric in SQL, so I think we
can have an operator to extract the numeric directly. If the value of a
given
field is not a numeric data type, an error will be raised, this can be
documented.

In this patch, I added a new operator for this purpose, here is the
performance gain because of this.

create table tb (a jsonb);
insert into tb select '{"a": 1}'::jsonb from generate_series(1, 100000)i;

current method:
select count(*) from tb where cast (a->>'a' as numeric) = 2;
167ms.

new method:
select count(*) from tb where a(at)->'a' = 2;
65ms.

Is this the right way to go? Testcase, document and catalog version are
updated.

--
Best Regards
Andy Fan