The following documentation comment has been logged on the website:
Page: https://www.postgresql.org/docs/16/queries-table-expressions.html
Description:
>> In the second query, we could not have written SELECT * FROM test1 GROUP
BY x, because there is no single value for the column y that could be
associated with each group. The grouped-by columns can be referenced in the
select list since they have a single value in each group.