Re: to_json(NULL) should to return JSON null instead NULL

On 08/29/2015 04:27 PM, Tom Lane wrote:
> Jim Nasby <Jim.Nasby@BlueTreble.com> writes:
>> On 8/29/15 12:29 PM, Pavel Stehule wrote:
>>> what is correct from JSON perspective? All fields with NULL
>> ISTM that the whole purpose of to_json is to properly jsonify something,
>> and the proper json form for "undefined" is 'null', is it not?
> What's not entirely clear is what we should do with cases like
>
> regression=#  select array_to_json(null::int[]);
>   array_to_json
> ---------------
>   
> (1 row)
>
> regression=#  select row_to_json(null::record);
>   row_to_json
> -------------
>   
> (1 row)
>
> If we leave those alone (and in the latter case, in particular, there is
> not enough information available to do much else) then it's not so clear
> that changing to_json() is really improving consistency overall.
> For instance, do we really want row_to_json(null::record) and
> to_json(null::record) giving different results?  Or if we make them
> both return "null", that breaks the previous invariant that row_to_json
> always yields a JSON object.
>
> An advantage of leaving these things as strict is that the user can easily
> substitute whatever specific behavior she wants for NULLs via coalesce(),
> as was shown upthread.  If we put in a different behavior, then the
> only way to override it would be with a CASE, which is tedious and creates
> multiple-evaluation issues.
>
> I'm not necessarily against changing it --- but it doesn't seem entirely
> black-and-white to me, and we do now have a couple of versions worth
> of precedent we'd be breaking with.
>
> If we do vote to change it, I'd want to do so now (ie in 9.5) rather than
> create yet another year's worth of precedent.
>
>             

I agree with pretty much all of this. My fairly strong inclination is to 
leave it as it is and document the behaviour more clearly. Changing it 
seems likely to introduce a different inconsistency which is harder to 
understand.

cheers

andrew