Re: Emitting JSON to file using COPY TO

On 12/1/23 18:09, Nathan Bossart wrote:
> On Fri, Dec 01, 2023 at 02:28:55PM -0500, Joe Conway wrote:
>> 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.
>
> Cool. I would expect there to be interest, given all the other JSON
> support that has been added thus far.

Thanks for the review

> I noticed that, with the PoC patch, "json" is the only format that must be
> quoted. Without quotes, I see a syntax error. I'm assuming there's a
> conflict with another json-related rule somewhere in gram.y, but I haven't
> tracked down exactly which one is causing it.

It seems to be because 'json' is also a type name ($$ =
SystemTypeName("json")).

What do you think about using 'json_array' instead? It is more specific
and accurate, and avoids the need to quote.

test=# copy foo to stdout (format json_array);
[
{"id":1,"f1":"line: 1","f2":"2023-12-01T12:58:16.776863-05:00"}
,{"id":2,"f1":"line: 2","f2":"2023-12-01T12:58:16.777084-05:00"}
,{"id":3,"f1":"line: 3","f2":"2023-12-01T12:58:16.777096-05:00"}
,{"id":4,"f1":"line: 4","f2":"2023-12-01T12:58:16.777103-05:00"}
]

>> 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?
>
> I don't presently have a strong opinion on this one. My instinct would be
> start with something simple, though. I don't think we offer any special
> options for log_destination...

WFM

>> 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?
>
> I would vote for including COPY FROM support from the start.

Check. My thought is to only accept the same format we emit -- i.e. only
take a json array.

>> ! if (!cstate->opts.json_mode)
>
> I think it's unfortunate that this further complicates the branching in
> CopyOneRowTo(), but after some quick glances at the code, I'm not sure it's
> worth refactoring a bunch of stuff to make this nicer.

Yeah that was my conclusion.

--
Joe Conway
PostgreSQL Contributors Team
RDS Open Source Databases
Amazon Web Services: https://aws.amazon.com