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.
I agree so there is not clear solution - and both possible solution can have a real base. On second hand, the fix with COALESCE, NULLIF, .. is not hard and it is has not a performance impact - so better documentation can be good enough fix. The custom solution is ugly named but simple function to_json2 CREATE OR REPLACE FUNCTION to_json2(anyelement) RETURNS json AS $$ SELECT COALESCE(to_json($1), json 'null') $$ LANGUAGE sql;
