Re: BUG #14020: row_number() over(partition by order by) - weird behavior

On Mon, Mar 14, 2016 at 1:43 PM, <b.yordanov2@gmail.com> wrote:

>
> db=3D# update offers_past_data a set position =3D b.position from (select=
 id,
> feed, row_number() over(partition by product order by grossprice asc) as
> position from offers_testing) b where a.id =3D b.id and a.feed =3D b.feed=
 and
> a.position <> b.position;
> UPDATE 0
>

=E2=80=8BUpdating offers_past_data
=E2=80=8B


>
> This should update every row in offers_past_data when its =E2=80=9Cpositi=
on=E2=80=9D
> changes. In the example above no changes were introduced since the last r=
un
> so nothing is updated (expected).
>
> db=3D# select count(*) from offers_testing where product =3D 2;
>  count
> -------
>     99
> (1 row)
>
So there are 99 offers for product 2.
>

=E2=80=8BCounting offers_testing=E2=80=8B


> Getting a single offer:
>
> db=3D# select id,grossprice from offers_testing where product =3D 2 limit=
 1;
>    id    | grossprice
> ---------+------------
>  4127918 |    5000.00
> (1 row)
>
>
=E2=80=8BCounting offers_testing=E2=80=8B

Updating its grossprice:
>
> db=3D# update offers_testing set grossprice =3D 20 where id =3D 4127918;
> UPDATE 1
>
>
=E2=80=8BUpdating offers_testing=E2=80=8B

Now when executing the first query again I expect that no more than 99 rows
> get updated in offers_past_data since this is the maximum amount of
> positions that would be affected by offer 4127918 grossprice change.
>

You haven't proven to us that a single row in offers_testing cannot match
more than one row in offers_past_data.  Assuming a 1-to-many situation the
update count for offers_past_data can definitely be more than the number of
rows returned by the sub-query.
=E2=80=8B=E2=80=8B


> db=3D# update offers_past_data a set position =3D b.position from (select=
 id,
> feed, row_number() over(partition by product order by grossprice asc) as
> position from offers_testing) b where a.id =3D b.id and a.feed =3D b.feed=
 and
> a.position <> b.position;
> UPDATE 104
>
> 104 rows get updated.
>
> Executing the same query again a few minutes later (no changes meanwhile =
in
> either table):
>
> db=3D# update offers_past_data a set position =3D b.position from (select=
 id,
> feed, row_number() over(partition by product order by grossprice asc) as
> position from offers_testing) b where a.id =3D b.id and a.feed =3D b.feed=
 and
> a.position <> b.position;
> UPDATE 28058
>
> This time it updates 28058 rows.
>
> This is a test environment and nothing reads or writes to these tables.
>
> Is this a bug or am I missing something obvious?


=E2=80=8BIts likely data related, not a bug.

Using the "UPDATE ... RETURNING *" form should provide good insight.
Specifically, look for all rows having the same (id, feed) pair.

Also, "ORDER BY grossprice" seems inadequate.  The potential for duplicates
here - which would then make the assignment of row numbers within the
product partition random - is non-zero and is a quite likely source of your
problem - along with the probable one-to-many relationship between
offers_testing and offers_past_data.

David J.
=E2=80=8B