Re: Emitting JSON to file using COPY TO

On 3/20/26 08:41, Andrew Dunstan wrote:
>
> On 2026-03-19 Th 12:06 PM, Joe Conway wrote:
>> 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(at)manitou-mail(dot)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
>>
>
> pushed with that addition.

Awesome -- thanks for carrying that over the top!

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