Re: Emitting JSON to file using COPY TO

On 2023-12-01 Fr 14:28, Joe Conway wrote:
> On 11/29/23 10:32, Davin Shearer wrote:
>> Thanks for the responses everyone.
>>
>> I worked around the issue using the `psql -tc` method as Filip
>> described.
>>
>> I think it would be great to support writing JSON using COPY TO at
>> some point so I can emit JSON to files using a PostgreSQL function
>> directly.
>>
>> -Davin
>>
>> On Tue, Nov 28, 2023 at 2:36 AM Filip Sedlák <filip(at)sedlakovi(dot)org
>> <mailto:filip(at)sedlakovi(dot)org>> wrote:
>>
>>     This would be a very special case for COPY. It applies only to a
>> single
>>     column of JSON values. The original problem can be solved with psql
>>     --tuples-only as David wrote earlier.
>>
>>
>>     $ psql -tc 'select json_agg(row_to_json(t))
>>                    from (select * from public.tbl_json_test) t;'
>>
>>        [{"id":1,"t_test":"here's a \"string\""}]
>>
>>
>>     Special-casing any encoding/escaping scheme leads to bugs and harder
>>     parsing.
>
> (moved to hackers)
>
> I did a quick PoC patch (attached) -- if there interest and no hard
> objections I would like to get it up to speed for the January commitfest.
>
> Currently the patch lacks documentation and regression test support.
>
> Questions:
> ----------
> 1. Is supporting JSON array format sufficient, or does it need to
> support some other options? How flexible does the support scheme need
> to be?
>
> 2. This only supports COPY TO and we would undoubtedly want to support
> COPY FROM for JSON as well, but is that required from the start?
>
> Thanks for any feedback.

I  realize this is just a POC, but I'd prefer to see composite_to_json()
not exposed. You could use the already public datum_to_json() instead,
passing JSONTYPE_COMPOSITE and F_RECORD_OUT as the second and third
arguments.

I think JSON array format is sufficient.

I can see both sides of the COPY FROM argument, but I think insisting on
that makes this less doable for release 17. On balance I would stick to
COPY TO for now.

cheers

andrew

--
Andrew Dunstan
EDB: https://www.enterprisedb.com