Re: Emitting JSON to file using COPY TO

On 12/6/23 14:47, Joe Conway wrote:
> On 12/6/23 13:59, Daniel Verite wrote:
>> Andrew Dunstan wrote:
>>
>>> IMNSHO, we should produce either a single JSON
>>> document (the ARRAY case) or a series of JSON documents, one per row
>>> (the LINES case).
>>
>> "COPY Operations" in the doc says:
>>
>> " The backend sends a CopyOutResponse message to the frontend, followed
>> by zero or more CopyData messages (always one per row), followed by
>> CopyDone".
>>
>> In the ARRAY case, the first messages with the copyjsontest
>> regression test look like this (tshark output):
>>
>> PostgreSQL
>> Type: CopyOut response
>> Length: 13
>> Format: Text (0)
>> Columns: 3
>> Format: Text (0)
>> PostgreSQL
>> Type: Copy data
>> Length: 6
>> Copy data: 5b0a
>> PostgreSQL
>> Type: Copy data
>> Length: 76
>> Copy data:
>> 207b226964223a312c226631223a226c696e652077697468205c2220696e2069743a2031…
>>
>> The first Copy data message with contents "5b0a" does not qualify
>> as a row of data with 3 columns as advertised in the CopyOut
>> message. Isn't that a problem?
>
>
> Is it a real problem, or just a bit of documentation change that I missed?
>
> Anything receiving this and looking for a json array should know how to
> assemble the data correctly despite the extra CopyData messages.

Hmm, maybe the real problem here is that Columns do not equal "3" for
the json mode case -- that should really say "1" I think, because the
row is not represented as 3 columns but rather 1 json object.

Does that sound correct?

Assuming yes, there is still maybe an issue that there are two more
"rows" that actual output rows (the "[" and the "]"), but maybe those
are less likely to cause some hazard?

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