Message-ID: <90984087-723c-48e6-88ca-661ed15b2dd6@joeconway.com>
Date: Thu, 19 Mar 2026 12:06:13 -0400
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User-Agent: Mozilla Thunderbird
Subject: Re: Emitting JSON to file using COPY TO
To: Andrew Dunstan <andrew@dunslane.net>,
 jian he <jian.universality@gmail.com>,
 Daniel Verite <daniel@manitou-mail.org>
Cc: Masahiko Sawada <sawada.mshk@gmail.com>, Junwang Zhao
 <zhjwpku@gmail.com>, Florents Tselai <florents.tselai@gmail.com>,
 "Andrey M. Borodin" <x4mmm@yandex-team.ru>,
 Dean Rasheed <dean.a.rasheed@gmail.com>, Davin Shearer <davin@apache.org>,
 PostgreSQL development <pgsql-hackers@postgresql.org>
References: <964a3377-d957-4a06-ad02-8228aefa65fc@dunslane.net>
 <fc61429f-76a1-4385-a2bd-999967da306f@manitou-mail.org>
 <CACJufxGAQk5cr3zcqX-PfN-9S+qnJgLL+OmNGhKeG_iG5mvddw@mail.gmail.com>
 <8428eb0f-f467-4cbd-a510-5be4f29b9ad7@dunslane.net>
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From: Joe Conway <mail@joeconway.com>
In-Reply-To: <8428eb0f-f467-4cbd-a510-5be4f29b9ad7@dunslane.net>
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On 3/19/26 11:02, Andrew Dunstan wrote:
> 
> On 2026-03-18 We 9:58 PM, jian he wrote:
>> On Wed, Mar 18, 2026 at 10:37 PM Daniel Verite <daniel@manitou-mail.org> wrote:
>>> Currently there's no difference in output between the null
>>> json value and the SQL null.
>>>
>>> postgres=# create table tbl  (j jsonb);
>>> postgres=# insert into tbl values('null');
>>> postgres=# insert into tbl values(null);
>>> postgres=# copy tbl to stdout with (format json);
>>> {"j":null}
>>> {"j":null}
>>>
>>> Does it have to be that way or are there valid distinct outputs
>>> that  we could use to avoid this ambiguity?
>>>
>> This is an existing (quite old) behavior of
>> composite_to_json->datum_to_json_internal, IMHO.
>>
>> ```
>>      if (is_null)
>>      {
>>          appendBinaryStringInfo(result, "null", strlen("null"));
>>          return;
>>      }
>> ```
>> produce the same results as
>> ```
>> case JSONTYPE_JSON:
>>      /* JSON and JSONB output will already be escaped */
>>      outputstr = OidOutputFunctionCall(outfuncoid, val);
>>      appendStringInfoString(result, outputstr);
>>      pfree(outputstr);
>>      break;
>> ```
>>
>> Therefore I intended to document it as below:
>>
>>    <refsect2 id="sql-copy-json-format" xreflabel="JSON Format">
>>     <title>JSON Format</title>
>>      <para>
>>        When the <literal>json</literal> format is used, data is
>> exported with one JSON object per line,
>>        where each line corresponds to a single record.
>>        The <literal>json</literal> format has no standard way to
>> distinguish between an SQL <literal>NULL</literal> and a JSON
>> <literal>null</literal> literal.
>>        In the examples that follow, the following table containing JSON
>> data will be used:
>> <programlisting>
>> CREATE TABLE my_test (a jsonb, b int);
>> INSERT INTO my_test VALUES ('null', 1), (NULL, 1);
>> </programlisting>
>>
>>      When exporting this table using the <literal>json</literal> format:
>> <programlisting>
>> COPY my_test TO STDOUT (FORMAT JSON);
>> </programlisting>
>>      In the resulting output, both the SQL <literal>NULL</literal> and
>> the JSON <literal>null</literal> are rendered identically:
>> <screen>
>> {"a":null,"b":1}
>> {"a":null,"b":1}
>> </screen>
>>     </para>
>>    </refsect2>
>>
>>
>>
>> what do you think?
>>
>>
>>
> 
> I can live with that, if others can.

+1
WFM

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