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From: Joe Conway <[email protected]>
To: Daniel Verite <[email protected]>
To: Andrew Dunstan <[email protected]>
Cc: Davin Shearer <[email protected]>
Cc: PostgreSQL-development <[email protected]>
Subject: Re: Emitting JSON to file using COPY TO
Date: Wed, 6 Dec 2023 18:09:30 -0500
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
	<[email protected]>

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

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