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(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

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