Re: json generation enhancements - Mailing list pgsql-hackers

On 02/24/2013 02:15 AM, Craig Ringer wrote:
> On 02/24/2013 02:09 PM, Steve Singer wrote:
>> Here is a review of this patch.,
>>
>> Overview
>> ---------------------
>> This patch adds a set of functions to convert types to json. Specifically
>>
>> * An aggregate that take the elements and builds up a json array.
>> * Conversions from an hstore to json
>>
>> For non-builtin types the text conversion is used unless a cast has
>> specifically been defined from the type to json.
>>
>> There was some discussion last year on this patch that raised three
>> issues
>>
>> a) Robert was concerned that if someone added a cast from a non-core
>> type like citext to json that it would change the behaviour of this
>> function. No one else offered an opinion on this at the time.  I don't
>> see this as a problem, if I add a cast between citext (or any other
>> non-core datatype) to json I would expect it to effect how that
>> datatype is generated as a json object in any functions that generate
>> json.   I don't see why this would be surprising behaviour.  If I add
>> a cast between my datatype and json to generate a json representation
>> that differs from the textout representation then I would expect that
>> representation to be used in the json_agg function as well.
> I'm not thrilled about that, because we're likely to want to add more
> JSON-specific casts to built-in or extension types in the future. If
> doing so changes behaviour, causing something that used to work to
> continue to work but produce a different result, that'll result in
> considerable arguments about backward compatibility.
>
> I'd be happier to require explicit casts to text or require the user to
> explicitly CREATE CAST where no JSON-aware cast is already defined.
>

Adding a cast to json for a builtin type will have no effect unless you 
also change this code. We can relax that but my view was that we should 
know how to generate JSON from builtin types and just do it. But if we 
wanted to allow someone to create a cast from, say, xml to json, and 
have it take effect in json_agg then it might make sense to honor casts 
for all types.

Your requirement of an explicit cast to text or json would result in a 
class of type that could not be represented as json at all. I'm very 
strongly opposed to that. If you proposed instead to prefer a cast to 
text and only fall back on the type's default output format if one 
doesn't exist I could live with that, although I strongly suspect it 
will be mostly pointless.

cheers

andrew