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From: Ayush Tiwari <ayushtiwari.slg01@gmail.com>
Date: Wed, 6 May 2026 13:12:38 +0530
Message-ID: 
 <CAJTYsWUdfOCo=m6dWpX=uxXFKiTUO751ju3mVJidd9p5VqApFg@mail.gmail.com>
Subject: Re: COPY JSON: use trailing commas in FORCE_ARRAY output
To: Chao Li <li.evan.chao@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
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Hi,


On Wed, 6 May 2026 at 12:11, Chao Li <li.evan.chao@gmail.com> wrote:

> Hi,
>
> Another issue I found with COPY TO (FORMAT json, FORCE_ARRAY) is that it
> places the delimiter comma at the beginning of the next line, like this:
> ```
> evantest=# copy test_json_copy to stdout with (format json, force_array);
> [
>
> {"id":1,"name":"Alice","is_active":true,"tags":["dev","db"],"created_at":"2026-05-01"}
>
> ,{"id":2,"name":"Bob","is_active":false,"tags":["manager"],"created_at":"2026-05-02"}
> ,{"id":3,"name":"Charlie","is_active":null,"tags":null,"created_at":null}
> ,{"id":4,"name":"Special Case:
> \"Quotes\"","is_active":true,"tags":["a","b"],"created_at":"2026-05-04"}
> ]
> ```
>
> I was surprised by the comma placement. It is valid JSON, but it looks
> quite uncommon.
>
> For comparison, the existing json_agg() places commas at the end of the
> line:
> ```
> evantest=# select json_agg(t) from (select id, name from test_json_copy) t;
>                    json_agg
> ----------------------------------------------
>  [{"id":1,"name":"Alice"},                   +
>   {"id":2,"name":"Bob"},                     +
>   {"id":3,"name":"Charlie"},                 +
>   {"id":4,"name":"Special Case: \"Quotes\""}]
> (1 row)
> ```
>
> If this feature had already been released, I would not think it worth
> changing just for formatting. But since "FORMAT json" is a new PG19 feature
> and has not been released yet, I think it is better to make the output to
> follow the more common style.
>
> This patch changes the output to place the comma at the end of the
> previous line instead. The fix only adjusts how commas and newlines are
> emitted. It does not buffer the whole result, so it should not have any
> performance impact.
>

The patch looks good to me.  I applied and the focused 'copy'
regression test too passed.

I agree that since FORMAT json is new for PG19, it is reasonable to adjust
the formatting before release.  The implementation still streams the output
and only changes where the separator/newline are emitted.

Regards,
Ayush

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<div dir=3D"ltr"><div dir=3D"ltr">Hi,<div><br></div></div><br><div class=3D=
"gmail_quote gmail_quote_container"><div dir=3D"ltr" class=3D"gmail_attr">O=
n Wed, 6 May 2026 at 12:11, Chao Li &lt;<a href=3D"mailto:li.evan.chao@gmai=
l.com">li.evan.chao@gmail.com</a>&gt; wrote:<br></div><blockquote class=3D"=
gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(20=
4,204,204);padding-left:1ex">Hi,<br>
<br>
Another issue I found with COPY TO (FORMAT json, FORCE_ARRAY) is that it pl=
aces the delimiter comma at the beginning of the next line, like this:<br>
```<br>
evantest=3D# copy test_json_copy to stdout with (format json, force_array);=
<br>
[<br>
{&quot;id&quot;:1,&quot;name&quot;:&quot;Alice&quot;,&quot;is_active&quot;:=
true,&quot;tags&quot;:[&quot;dev&quot;,&quot;db&quot;],&quot;created_at&quo=
t;:&quot;2026-05-01&quot;}<br>
,{&quot;id&quot;:2,&quot;name&quot;:&quot;Bob&quot;,&quot;is_active&quot;:f=
alse,&quot;tags&quot;:[&quot;manager&quot;],&quot;created_at&quot;:&quot;20=
26-05-02&quot;}<br>
,{&quot;id&quot;:3,&quot;name&quot;:&quot;Charlie&quot;,&quot;is_active&quo=
t;:null,&quot;tags&quot;:null,&quot;created_at&quot;:null}<br>
,{&quot;id&quot;:4,&quot;name&quot;:&quot;Special Case: \&quot;Quotes\&quot=
;&quot;,&quot;is_active&quot;:true,&quot;tags&quot;:[&quot;a&quot;,&quot;b&=
quot;],&quot;created_at&quot;:&quot;2026-05-04&quot;}<br>
]<br>
```<br>
<br>
I was surprised by the comma placement. It is valid JSON, but it looks quit=
e uncommon.<br>
<br>
For comparison, the existing json_agg() places commas at the end of the lin=
e:<br>
```<br>
evantest=3D# select json_agg(t) from (select id, name from test_json_copy) =
t;<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0json_a=
gg<br>
----------------------------------------------<br>
=C2=A0[{&quot;id&quot;:1,&quot;name&quot;:&quot;Alice&quot;},=C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0+<br>
=C2=A0 {&quot;id&quot;:2,&quot;name&quot;:&quot;Bob&quot;},=C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0+<br>
=C2=A0 {&quot;id&quot;:3,&quot;name&quot;:&quot;Charlie&quot;},=C2=A0 =C2=
=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0+<br>
=C2=A0 {&quot;id&quot;:4,&quot;name&quot;:&quot;Special Case: \&quot;Quotes=
\&quot;&quot;}]<br>
(1 row)<br>
```<br>
<br>
If this feature had already been released, I would not think it worth chang=
ing just for formatting. But since &quot;FORMAT json&quot; is a new PG19 fe=
ature and has not been released yet, I think it is better to make the outpu=
t to follow the more common style.<br>
<br>
This patch changes the output to place the comma at the end of the previous=
 line instead. The fix only adjusts how commas and newlines are emitted. It=
 does not buffer the whole result, so it should not have any performance im=
pact.<br></blockquote><div><br></div>The patch looks good to me.=C2=A0 I ap=
plied and=C2=A0the=C2=A0focused &#39;copy&#39;</div><div class=3D"gmail_quo=
te gmail_quote_container">regression test too passed.<br><br>I agree that s=
ince FORMAT json is new for PG19, it is reasonable to adjust<br>the formatt=
ing before release.=C2=A0 The implementation still streams the output<br><d=
iv>and only changes where the separator/newline are emitted.</div><div><br>=
</div><div>Regards,<br>Ayush</div></div></div>

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