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help / color / mirror / Atom feedFrom: Joe Conway <[email protected]>
To: Andrew Dunstan <[email protected]>
To: jian he <[email protected]>
To: Daniel Verite <[email protected]>
Cc: Masahiko Sawada <[email protected]>
Cc: Junwang Zhao <[email protected]>
Cc: Florents Tselai <[email protected]>
Cc: Andrey M. Borodin <[email protected]>
Cc: Dean Rasheed <[email protected]>
Cc: Davin Shearer <[email protected]>
Cc: PostgreSQL development <[email protected]>
Subject: Re: Emitting JSON to file using COPY TO
Date: Fri, 20 Mar 2026 08:58:45 -0400
Message-ID: <[email protected]> (raw)
In-Reply-To: <[email protected]>
References: <[email protected]>
<[email protected]>
<CACJufxGAQk5cr3zcqX-PfN-9S+qnJgLL+OmNGhKeG_iG5mvddw@mail.gmail.com>
<[email protected]>
<[email protected]>
<[email protected]>
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
>>>> <[email protected]> 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
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Subject: Re: Emitting JSON to file using COPY TO
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