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From: David G. Johnston <[email protected]>
To: Peter Eisentraut <[email protected]>
Cc: Tom Lane <[email protected]>
Cc: pgsql-hackers <[email protected]>
Cc: David Christensen <[email protected]>
Cc: Jelte Fennema-Nio <[email protected]>
Subject: Re: [PATCH] GROUP BY ALL
Date: Tue, 14 Apr 2026 05:57:27 -0700
Message-ID: <CAKFQuwZ2MoUqWv0oKX4M0d85k92HugdF=1qJzhoC1pO20ywWYA@mail.gmail.com> (raw)
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On Tuesday, April 14, 2026, Peter Eisentraut <[email protected]> wrote:
>
>
> I don't see any mention of using GROUP BY with window functions in our
> relevant documentation, for example
>
> https://www.postgresql.org/docs/devel/sql-expressions.html#
> SYNTAX-WINDOW-FUNCTIONS
> https://www.postgresql.org/docs/devel/functions-window.html


The select reference page covers this.  But the window clause could get
better treatment, it’s buried in step 5.

https://www.postgresql.org/docs/current/sql-select.html


> Commit ef38a4d9756 added a regression test
>
> EXPLAIN (COSTS OFF) SELECT a, COUNT(a) OVER (PARTITION BY a) FROM t1 GROUP
> BY ALL;
>
> but the test table contains no data, so I don't know if this kind of query
> produces interesting information.
>

Each non-null value of “a” would have an output of 1, while a null valued
“a” would have an output of 0.  “A” is grouped since all expressions
involving “a” are non-aggregated.  The equivalent rewrite is:

Select a, count(a_expr) over … from ( — step 5
    select a, a as a_expr from tbl group by all. — step 4
);

For purposes of group by all one would erase/ignore the actual window
wrapper while leaving the expressions it operates over in place.  This
extends from “window expressions are processed after group by/having”
documented in SELECT.  IOW, group by all resolves during processing step 4
with intermediate results for the expressions within the window functions,
then step 5 removes the intermediate expressions that don’t appear in the
final output while adding in the results of processing the window functions.

David J.

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