Re: Documentation, window functions

On Wed, Sep 22, 2010 at 6:03 AM, Dennis Björklund <db@zigo.dhs.org> wrote:
> In
>
>  http://www.postgresql.org/docs/9.0/static/tutorial-window.html
>
> it say
>
> "Although avg will produce the same result no matter what order it
> processes the partition's rows in, this is not true of all window
> functions. When needed, you can control that order using ORDER BY within
> OVER."
>
> While it's true that avg() produce the same result no matter what order. A
> ORDER BY clause will affect what rows are included in the computation and
> thus change the result (the default window frame is
> RANGE BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW). So one can not in
> general add an ORDER BY to the example in the tutorial and get the same
> result as without an ORDER BY.
>
> Maybe we can find some better wording of the above?

Yeah, that doesn't seem right.

rhaas=# create table foo (a integer);
CREATE TABLE
rhaas=# insert into foo values (1);
INSERT 0 1
rhaas=# insert into foo values (2);
INSERT 0 1
rhaas=# insert into foo values (3);
INSERT 0 1
rhaas=# select a, avg(a) over () from foo;a |        avg
---+--------------------1 | 2.00000000000000002 | 2.00000000000000003 | 2.0000000000000000
(3 rows)

rhaas=# select a, avg(a) over (order by a) from foo;a |          avg
---+------------------------1 | 1.000000000000000000002 |     1.50000000000000003 |     2.0000000000000000
(3 rows)

But I confess that I'm sort of murky on how ORDER affects the window
frame, or how to rephrase this more sensibly.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise Postgres Company