Message-ID: <86650b9d-ff52-4ec8-964d-4b9352971ecd@joeconway.com>
Date: Tue, 5 Dec 2023 16:15:29 -0500
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Subject: Re: Emitting JSON to file using COPY TO
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To: Andrew Dunstan <andrew@dunslane.net>, Davin Shearer <davin@apache.org>,
 PostgreSQL-development <pgsql-hackers@postgresql.org>
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From: Joe Conway <mail@joeconway.com>
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Precedence: bulk

On 12/5/23 16:12, Andrew Dunstan wrote:
> 
> On 2023-12-05 Tu 16:02, Joe Conway wrote:
>> On 12/5/23 15:55, Andrew Dunstan wrote:
>>>
>>> On 2023-12-05 Tu 14:50, Davin Shearer wrote:
>>>> Hi Joe,
>>>>
>>>> In reviewing the 005 patch, I think that when used with FORCE ARRAY, 
>>>> we should also _imply_ FORCE ROW DELIMITER.  I can't envision a use 
>>>> case where someone would want to use FORCE ARRAY without also using 
>>>> FORCE ROW DELIMITER.  I can, however, envision a use case where 
>>>> someone would want FORCE ROW DELIMITER without FORCE ARRAY, like 
>>>> maybe including into a larger array.  I definitely appreciate these 
>>>> options and the flexibility that they afford from a user perspective.
>>>>
>>>> In the test output, will you also show the different variations with 
>>>> FORCE ARRAY and FORCE ROW DELIMITER => {(false, false), (true, 
>>>> false), (false, true), (true, true)}? Technically you've already 
>>>> shown me the (false, false) case as those are the defaults.
>>>>
>>>>
>>>
>>> I don't understand the point of FORCE_ROW_DELIMITER at all. There is
>>> only one legal delimiter of array items in JSON, and that's a comma.
>>> There's no alternative and it's not optional. So in the array case you
>>> MUST have commas and in any other case (e.g. LINES) I can't see why you
>>> would have them.
>>
>> The current patch already *does* imply row delimiters in the array 
>> case. It says so here:
>> 8<---------------------------
>> +    <varlistentry>
>> + <term><literal>FORCE_ARRAY</literal></term>
>> +     <listitem>
>> +      <para>
>> +       Force output of array decorations at the beginning and end of 
>> output.
>> +       This option implies the <literal>FORCE_ROW_DELIMITER</literal>
>> +       option. It is allowed only in <command>COPY TO</command>, and 
>> only
>> +       when using <literal>JSON</literal> format.
>> +       The default is <literal>false</literal>.
>> +      </para>
>> 8<---------------------------
>>
>> and it does so here:
>> 8<---------------------------
>> +         if (opts_out->force_array)
>> +             opts_out->force_row_delimiter = true;
>> 8<---------------------------
>>
>> and it shows that here:
>> 8<---------------------------
>> + copy copytest to stdout (format json, force_array);
>> + [
>> +  {"style":"DOS","test":"abc\r\ndef","filler":1}
>> + ,{"style":"Unix","test":"abc\ndef","filler":2}
>> + ,{"style":"Mac","test":"abc\rdef","filler":3}
>> + ,{"style":"esc\\ape","test":"a\\r\\\r\\\n\\nb","filler":4}
>> + ]
>> 8<---------------------------
>>
>> It also does not allow explicitly setting row delimiters false while 
>> force_array is true here:
>> 8<---------------------------
>>
>> +         if (opts_out->force_array &&
>> +             force_row_delimiter_specified &&
>> +             !opts_out->force_row_delimiter)
>> +             ereport(ERROR,
>> +                     (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
>> +                      errmsg("cannot specify FORCE_ROW_DELIMITER 
>> false with FORCE_ARRAY true")));
>> 8<---------------------------
>>
>> Am I understanding something incorrectly?
> 
> 
> But what's the point of having it if you're not using FORCE_ARRAY?


See the follow up email -- other databases support it so why not? It 
seems to be a thing...

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