Re: Emitting JSON to file using COPY TO

On 12/6/23 18:09, Joe Conway wrote:
> 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?

The attached should fix the CopyOut response to say one column. I.e. it
ought to look something like:

PostgreSQL
Type: CopyOut response
Length: 13
Format: Text (0)
Columns: 1
Format: Text (0)
PostgreSQL
Type: Copy data
Length: 6
Copy data: 5b0a
PostgreSQL
Type: Copy data
Length: 76
Copy data: [...]

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