Re: Emitting JSON to file using COPY TO

On 12/5/23 15:55, Andrew Dunstan wrote:
>
> On 2023-12-05 Tu 14:50, Davin Shearer wrote:
>> Hi Joe,
>>
>> In reviewing the 005 patch, I think that when used with FORCE ARRAY,
>> we should also _imply_ FORCE ROW DELIMITER.  I can't envision a use
>> case where someone would want to use FORCE ARRAY without also using
>> FORCE ROW DELIMITER.  I can, however, envision a use case where
>> someone would want FORCE ROW DELIMITER without FORCE ARRAY, like maybe
>> including into a larger array.  I definitely appreciate these options
>> and the flexibility that they afford from a user perspective.
>>
>> In the test output, will you also show the different variations with
>> FORCE ARRAY and FORCE ROW DELIMITER => {(false, false), (true, false),
>> (false, true), (true, true)}?  Technically you've already shown me the
>> (false, false) case as those are the defaults.
>>
>>
>
> I don't understand the point of FORCE_ROW_DELIMITER at all. There is
> only one legal delimiter of array items in JSON, and that's a comma.
> There's no alternative and it's not optional. So in the array case you
> MUST have commas and in any other case (e.g. LINES) I can't see why you
> would have them.

The current patch already *does* imply row delimiters in the array case.
It says so here:
8<---------------------------
+ <varlistentry>
+ <term><literal>FORCE_ARRAY</literal></term>
+ <listitem>
+ <para>
+ Force output of array decorations at the beginning and end of
output.
+ This option implies the <literal>FORCE_ROW_DELIMITER</literal>
+ option. It is allowed only in <command>COPY TO</command>, and only
+ when using <literal>JSON</literal> format.
+ The default is <literal>false</literal>.
+ </para>
8<---------------------------

and it does so here:
8<---------------------------
+ if (opts_out->force_array)
+ opts_out->force_row_delimiter = true;
8<---------------------------

and it shows that here:
8<---------------------------
+ copy copytest to stdout (format json, force_array);
+ [
+ {"style":"DOS","test":"abc\r\ndef","filler":1}
+ ,{"style":"Unix","test":"abc\ndef","filler":2}
+ ,{"style":"Mac","test":"abc\rdef","filler":3}
+ ,{"style":"esc\\ape","test":"a\\r\\\r\\\n\\nb","filler":4}
+ ]
8<---------------------------

It also does not allow explicitly setting row delimiters false while
force_array is true here:
8<---------------------------

+ if (opts_out->force_array &&
+ force_row_delimiter_specified &&
+ !opts_out->force_row_delimiter)
+ ereport(ERROR,
+ (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ errmsg("cannot specify FORCE_ROW_DELIMITER false with
FORCE_ARRAY true")));
8<---------------------------

Am I understanding something incorrectly?

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