Re: group by can use alias from select list

On Wed, Sep  6, 2023 at 08:01:30PM -0400, Tom Lane wrote:
> Bruce Momjian <bruce@momjian.us> writes:
> > On Tue, Jul 11, 2023 at 07:31:26AM -0700, David G. Johnston wrote:
> >> I think the complaint is that someone seeing the behavior in the wild comes to
> >> this order-of-operations and doesn't see that the observed behavior is
> >> documented.
> 
> > How is the attached patch?
> 
> Maybe better "Although query output columns are nominally computed in the
> next step, they can also be referenced (by name or by ordinal number)
> as <literal>GROUP BY</literal> elements".

I modified the text slightly in the attached patch.

> You could go further and add "Such references cannot be parts of
> <literal>GROUP BY</literal> expressions, however."  Not sure if we
> cover that explicitly anywhere else.

We do farther down the docs in the GROUP BY section:

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

    GROUP BY will condense into a single row all selected rows that share
    the same values for the grouped expressions. An expression used inside a
    grouping_element can be an input column name, or the name or ordinal
    number of an output column (SELECT list item), or an arbitrary
    expression formed from input-column values. In case of ambiguity, a
                      ------------------------
    GROUP BY name will be interpreted as an input-column name rather than an
    output column name.

It says

*  input column name
*  output column name or ordinal number
*  expression formed from input-column values

Is this too subtle?

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Only you can decide what is important to you.