Re: Emitting JSON to file using COPY TO

[Andrew Dunstan]

On 2026-03-18 We 9:58 PM, jian he wrote:

On Wed, Mar 18, 2026 at 10:37 PM Daniel Verite <dan...@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.


cheers


andrew


--
Andrew Dunstan
EDB: https://www.enterprisedb.com