From: Tom Lane <tgl@sss.pgh.pa.us>
To: Daniel Gustafsson <daniel@yesql.se>
cc: Chao Li <li.evan.chao@gmail.com>,
        PostgreSQL Hackers <pgsql-hackers@postgresql.org>
Subject: Re: Fix GROUP BY ALL handling of ORDER BY operator semantics
In-reply-to: <A3C289BE-8916-4DC6-962D-624F35E02B44@yesql.se>
References: <5243308F-8E5C-45AA-828C-FAD96C4F34DA@gmail.com>
 <A0B7C417-5710-47B3-8FA2-C0DDA518DA07@yesql.se>
 <3278800.1783989856@sss.pgh.pa.us>
 <A3C289BE-8916-4DC6-962D-624F35E02B44@yesql.se>
Comments: In-reply-to Daniel Gustafsson <daniel@yesql.se>
	message dated "Tue, 14 Jul 2026 16:01:34 +0200"
MIME-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-ID: <3338662.1784045300.1@sss.pgh.pa.us>
Date: Tue, 14 Jul 2026 12:08:20 -0400
Message-ID: <3338663.1784045300@sss.pgh.pa.us>
Archived-At: 
 <https://www.postgresql.org/message-id/3338663.1784045300%40sss.pgh.pa.us>
Precedence: bulk

Daniel Gustafsson <daniel@yesql.se> writes:
> Ack. Another case I am a bit confused by is:

> postgres=# create table t (a integer);
> CREATE TABLE
> postgres=# select 1 as x from t group by ();
>  x
> ---
>  1
> (1 row)

> postgres=# select 1 as x from t group by all;
>  x
> ---
> (0 rows)

> Shouldn't those two queries yield the same result, or am I missing something
> obvious?

I might be undercaffeinated still, but I think those are both correct.
"GROUP BY ()" has similar effects to use of an aggregate or HAVING
clause: it forces the table scan's results to be combined into a
single grouped row.  But you get a grouped row even if the table is
empty.  The second case is equivalent to "select 1 as x from t group
by x", and this is different because it will produce a grouped row
only if the table isn't empty.  (The fact that the grouping expression
is a constant doesn't change the rule.)  Not one of SQL's more
consistent behaviors perhaps, but I believe it's all per spec.

			regards, tom lane