Thread

1. [QUESTION] Window function with partition by and order by

   Ankit Kumar Pandey <itsankitkp@gmail.com> — 2022-11-27T16:08:33Z

   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
   
   
   
   
   

2. Re: [QUESTION] Window function with partition by and order by

   William Alves Da Silva <william_silva@unochapeco.edu.br> — 2022-11-27T16:21:44Z

   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
   On 27 Nov 2022 13:08 -0300, Ankit Kumar Pandey <itsankitkp@gmail.com>, wrote:
   > 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
   >
   >
   >
   

3. Re: [QUESTION] Window function with partition by and order by

   Samed YILDIRIM <samed@reddoc.net> — 2022-11-27T16:23:38Z

   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
   
   
   On Sun, 27 Nov 2022 at 18:08, Ankit Kumar Pandey <itsankitkp@gmail.com>
   wrote:
   
   > 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
   >
   >
   >
   >
   

4. Re: [QUESTION] Window function with partition by and order by

   Tom Lane <tgl@sss.pgh.pa.us> — 2022-11-27T16:28:04Z

   Ankit Kumar Pandey <itsankitkp@gmail.com> writes:
   > 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?
   
   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
   
   
   
   

5. Re: [QUESTION] Window function with partition by and order by

   Ankit Kumar Pandey <itsankitkp@gmail.com> — 2022-11-27T16:54:51Z

   On 27/11/22 21:58, Tom Lane wrote:
   > Ankit Kumar Pandey <itsankitkp@gmail.com> writes:
   >> 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?
   > 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
   Thanks, I understand the behavior now.
   
   -- 
   Regards,
   Ankit Kumar Pandey
   
   
   
   
   

6. Re: [QUESTION] Window function with partition by and order by

   Ankit Kumar Pandey <itsankitkp@gmail.com> — 2022-11-27T16:54:58Z

   On 27/11/22 21:53, Samed YILDIRIM wrote:
   > 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
   >
   >
   > On Sun, 27 Nov 2022 at 18:08, Ankit Kumar Pandey 
   > <itsankitkp@gmail.com> wrote:
   >
   >     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
   >
   >
   >
   Thanks, this makes sense.
   
   -- 
   Regards,
   Ankit Kumar Pandey
   

7. Re: [QUESTION] Window function with partition by and order by

   Ankit Kumar Pandey <itsankitkp@gmail.com> — 2022-11-27T16:55:05Z

   On 27/11/22 21:53, Samed YILDIRIM wrote:
   > 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
   >
   >
   > On Sun, 27 Nov 2022 at 18:08, Ankit Kumar Pandey 
   > <itsankitkp@gmail.com> wrote:
   >
   >     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
   >
   >
   Okay, I understand this now.
   
   -- 
   Regards,
   Ankit Kumar Pandey