Subject: [QUESTION] Window function with partition by and order by

From: Ankit Kumar Pandey <[email-masked]>
Date: Sun, 27 Nov 2022 21:38:33 +0530

Hello,

While looking at aggregates in window function, I found something 
unusual and would be glad I could get some clarification.

Consider following table (mytable):

id, name

1, A

1, A

2, B

3, A

1, A

select *, avg(id) over (partition by name, order by id) from mytable;

Output:

id, name, avg

1, A, 1

1, A, 1

1, A, 1

3, A, 1.5

2, B, 2

Question is: Average of id for partition name (A) should be 6/4 = 1.5 
for all rows in that partition but this result is seen only at the last 
one row in partition (A). Am I missing here something?

Thanks

-- 
Regards,
Ankit Kumar Pandey

---

From: William Alves Da Silva <[email-masked]>
Date: Sun, 27 Nov 2022 13:21:44 -0300

Hello Ankit.

This behavior is correct. This is because you are using ORDER BY in your aggregation function.

Looking at the documentation you will find the following quote:
"You can also control the order in which rows are processed by window functions by using ORDER BY within OVER. (The window ORDER BY doesn't even have to match the order in which the rows are produced)."

So, if you use ORDER BY you are controlling how the row are processed.

If you don't use, the result is like this:

postgres=# select *, avg(id) over (partition by name) from my_teste ;
 id | name | avg
----+------+--------------------
 1 | A | 1.5000000000000000
 1 | A | 1.5000000000000000
 3 | A | 1.5000000000000000
 1 | A | 1.5000000000000000
 2 | B | 2.0000000000000000
(5 rows)

--
Regards,
William Alves

---

From: Samed YILDIRIM <[email-masked]>
Date: Sun, 27 Nov 2022 18:23:38 +0200

Hello Ankit,

It is absolutely expected behaviour of a window function with ORDER BY
clause. The default frame clause of window definition is *RANGE BETWEEN
UNBOUNDED PRECEDING AND CURRENT ROW*. If you add an ORDER BY clause in a
window definition, PostgreSQL takes the current row and all rows before it
within the partition into calculation. If you don't add, it means all rows
within the partition are peers, and PostgreSQL uses all rows for
calculation. I'm putting the related part from the documentation and its
link below.

The default framing option is RANGE UNBOUNDED PRECEDING, which is the same
> as RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW; it sets the frame to
> be all rows from the partition start up through the current row's last peer
> (a row that the window's ORDER BY clause considers equivalent to the
> current row; all rows are peers if there is no ORDER BY).

https://www.postgresql.org/docs/15/sql-select.html#SQL-WINDOW

Best regards.
Samed YILDIRIM

---

From: Tom Lane <[email-masked]>
Date: Sun, 27 Nov 2022 11:28:04 -0500

> one row in partition (A). Am I missing here something?

https://www.postgresql.org/docs/current/tutorial-window.html

particularly:

    By default, if ORDER BY is supplied then the frame consists of all
    rows from the start of the partition up through the current row, plus
    any following rows that are equal to the current row according to the
    ORDER BY clause.

			regards, tom lane

---

From: Ankit Kumar Pandey <[email-masked]>
Date: Sun, 27 Nov 2022 22:24:51 +0530

>      any following rows that are equal to the current row according to the
Thanks, I understand the behavior now.

-- 
Regards,
Ankit Kumar Pandey

---

From: Ankit Kumar Pandey <[email-masked]>
Date: Sun, 27 Nov 2022 22:24:58 +0530

On 27/11/22 21:53, Samed YILDIRIM wrote:
> clause. The default frame clause of window definition is *RANGE 
> BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW*. If you add an ORDER BY 
> clause in a window definition, PostgreSQL takes the current row and 
> all rows before it within the partition into calculation. If you don't 
> add, it means all rows within the partition are peers, and PostgreSQL 
> uses all rows for calculation. I'm putting the related part from the 
> documentation and its link below.
>
>     The default framing option is RANGE UNBOUNDED PRECEDING, which is
>     the same as RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW; it
>     sets the frame to be all rows from the partition start up through
>     the current row's last peer (a row that the window's ORDER BY
>     clause considers equivalent to the current row; all rows are peers
>     if there is no ORDER BY).
>
> On Sun, 27 Nov 2022 at 18:08, Ankit Kumar Pandey 
> <[email-masked]> wrote:
>
>     Hello,
>
Thanks, this makes sense.

-- 
Regards,
Ankit Kumar Pandey

---

From: Ankit Kumar Pandey <[email-masked]>
Date: Sun, 27 Nov 2022 22:25:05 +0530

>     one row in partition (A). Am I missing here something?
Okay, I understand this now.

-- 
Regards,
Ankit Kumar Pandey