Re: json function question

Yes I meant equivalence in the roundtrip conversion sense.

And of course the "feature complete" solution which can handle deep
structures would be really nice to have.

Best Regards
Dan S

2016-02-23 21:11 GMT+01:00 David G. Johnston <david(dot)g(dot)johnston(at)gmail(dot)com>:

> On Tue, Feb 23, 2016 at 12:54 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>
>> Dan S <strd911(at)gmail(dot)com> writes:
>> > I have this table, data and query:
>>
>> > create table test
>> > (
>> > id int,
>> > txt text,
>> > txt_arr text[],
>> > f float
>> > );
>>
>> > insert into test
>> > values
>> >
>> (1,'jkl','{abc,def,fgh}',3.14159),(2,'hij','{abc,def,fgh}',3.14159),(2,null,null,null),(3,'def',null,0);
>>
>> > select j, json_populate_record(null::test, j)
>> > from
>> > (
>> > select to_json(t) as j from test t
>> > ) r;
>>
>> > ERROR: malformed array literal: "["abc","def","fgh"]"
>> > DETAIL: "[" must introduce explicitly-specified array dimensions.
>>
>> > Is it a bug or how am I supposed to use the populate function ?
>>
>> AFAICS, json_populate_record has no intelligence about nested container
>> situations. It'll basically just push the JSON text representation of any
>> field of the top-level object at the input converter for the corresponding
>> composite-type column. That doesn't work if you're trying to convert a
>> JSON array to a Postgres array, and it wouldn't work for sub-object to
>> composite column either, because of syntax discrepancies.
>>
>> Ideally this would work for arbitrarily-deeply-nested array+record
>> structures, but it looks like a less than trivial amount of work to make
>> that happen.
>>
>> > If I try an equivalent example with hstore it works well.
>>
>> hstore hasn't got any concept of substructure in its field values, so
>> it's hard to see how you'd create an "equivalent" situation.
>>
>
> ​Equivalent in the "ability to round-trip" sense. Since hstore doesn't
> have nested containers internal serialization of a record to hstore is
> forced to "stringify" the array which can then be fed back in as-is. But
> the [row_]to_json​
>
> ​logic converts the PostgreSQL arrays to JSON arrays and then we fail to
> handle them on the return portion of the trip.
>
> Arrays are likely to be a much for common scenario but I agree that
> dealing with arbitrary depths and objects would make the feature complete.
>
> And yes, back-patching should only occur (and ideally behavior changing)
> for situations that today raise errors - as the example does.
>
> ​David J.
> ​
> ​
>