Re: Emitting JSON to file using COPY TO

Please be sure to include single and double quotes in the test values since
that was the original problem (double quoting in COPY TO breaking the JSON
syntax).

On Sun, Dec 3, 2023, 10:11 Andrew Dunstan <andrew(at)dunslane(dot)net> wrote:

>
> 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
>
>