Re: Emitting JSON to file using COPY TO

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