wrong field in example

On Tuesday, July 23, 2024, PG Doc comments form <noreply@postgresql.org>
wrote:

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.

I think you mis-copied the query - the one on the page has “select x”, not
“select *”.

David J.

On Thu, 25 Jul 2024, 12:57 am David G. Johnston, <david.g.johnston@gmail.com>
wrote:

I think you mis-copied the query - the one on the page has “select x”, not
“select *”.

That text exists as it was quoted. What the report is lacking is an
indication of what is wrong with the text.

David

On Wednesday, July 24, 2024, David Rowley <dgrowleyml@gmail.com> wrote:

On Thu, 25 Jul 2024, 12:57 am David G. Johnston, <
david.g.johnston@gmail.com> wrote:

I think you mis-copied the query - the one on the page has “select x”,
not “select *”.

That text exists as it was quoted. What the report is lacking is an
indication of what is wrong with the text.

Ok, I was just looking at the examples, thinking the report indicated a
broken example. I agree nothing seems wrong with the text and the report
has zero comment.

David J.

Just noticed you replied to me only. Send replies to the list. You can
leave individuals cc'd. The convention here is also to inline your replies
- top-posting is undesirable.

In any case, in order to write "select *" you have to group on both x and y
at the same time. If you group on either column only, the query will
fail. So the choice of x versus y is immaterial. Having grouped on x the
text is explaining that the failure to also group on y is a problem.

I don't know that the existing wording is the most clear, but it is correct.

David J.

On Wed, Jul 24, 2024 at 8:02 AM Yaroslav Saburov <y.saburov@gmail.com>
wrote:

"David G. Johnston" <david.g.johnston@gmail.com> writes:

I don't know that the existing wording is the most clear, but it is correct.

Perhaps it'd be better to write "... we could not have written
SELECT x, y FROM test1 GROUP BY x, because ..." ? The first
half of the example uses "SELECT *", and this bit was meant
to be consistent with that; but if the reader is confused about
how "y" got into the discussion then writing it out explicitly
should help.

regards, tom lane

first query
=> SELECT * FROM test1;
x | y
---+---
a | 3
c | 2
b | 5
a | 1
(4 rows)

second query
=> SELECT x FROM test1 GROUP BY x;
x
---
a
b
c
(3 rows)
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.