Обсуждение: BUG #15763: JSON nulls not handled properly

The following bug has been logged on the website:

Bug reference:      15763
Logged by:          Jacob Crell
Email address:      jacobcrell@gmail.com
PostgreSQL version: 9.6.8
Operating system:   AWS RDS
Description:

SELECT '{"test":null}'::json->>'test' will return a null
SELECT '{"test":null}'::json->'test' will return a string 'null'
The 2nd option seems like it should also return a null.

PG Bug reporting form <noreply@postgresql.org> writes:
> SELECT '{"test":null}'::json->>'test' will return a null
> SELECT '{"test":null}'::json->'test' will return a string 'null'
> The 2nd option seems like it should also return a null.

I don't claim to be a JSON expert, but the -> operator is specified
to give back a JSON value (not a text string).  So 'null'::json seems
like the right answer there.  Also, if we had it return a NULL, then
you couldn't distinguish the case where the field isn't present:

regression=# SELECT '{"test":null}'::json->'notthere';
 ?column? 
----------
 
(1 row)

regression=# SELECT '{"test":null}'::json->'notthere' is null;
 ?column? 
----------
 t
(1 row)

            regards, tom lane

The following bug has been logged on the website:

Bug reference:      15763
Logged by:          Jacob Crell
Email address:      jacobcrell@gmail.com
PostgreSQL version: 9.6.8
Operating system:   AWS RDS
Description:       

SELECT '{"test":null}'::json->>'test' will return a null
SELECT '{"test":null}'::json->'test' will return a string 'null'
The 2nd option seems like it should also return a null.

Jacob Crell <jacobcrell@gmail.com> writes:
> Thanks for the response. I may have been a bit off in my diagnosis of what
> was going wrong. My bug report stemmed from the fact that the below returns
> different results, the first throwing an error and the second returning no
> rows:

> SELECT json_array_elements('{"key":null}'::json->'key')
> SELECT json_array_elements(null::json)

> This seems unintuitive. Is it potentially a bug?

No, because null::json is not the same thing as 'null'::json.
They're related ideas, but not interchangeable.  In your
second example, json_array_elements() never gets called at all
because it's marked strict and strict functions are not invoked
on SQL nulls.  In the first example, it is called and it complains
because the JSON value it's called on isn't an array, but a
scalar null value.

You could make a case that the function should have been defined
to return zero rows for a JSON-null input ... but it'd be at best
a debatable point, so we're unlikely to change the function
definition now.

If you need that behavior, it is available in the jsonb world,
with something like

regression=# SELECT jsonb_array_elements(nullif('{"key":null}'::jsonb->'key',
                                                'null'::jsonb));
 jsonb_array_elements 
----------------------
(0 rows)

But nullif() doesn't work on plain json, for lack of an equality
operator :-(

            regards, tom lane