Re: Lag and lead window functions order by weirdness

2009/10/10 Thom Brown <thombrown(at)gmail(dot)com>

> I've had a look at examples of lag and lead window functions with order by
> inside the OVER clause, and I'm confused as to why it influences the overall
> order in the output.
> For example (for a table called category with incrementing id numbers up to
> 26):
>
> SELECT id, lag(id) OVER (ORDER BY id DESC) FROM category;
>
> This would yield:
>
> id lag
> 26 NULL
> 25 26
> 24 25
>
> This is the equivalent of what we would get with:
> SELECT id, lead(id) OVER (ORDER BY id ASC) FROM category ORDER BY id DESC;
>
> I would expect the row order in the results not to be influenced by the
> OVER clause's ORDER BY as I thought that was just to determine how the
> aggregate window function's values would be output. It's almost as if the
> main part of the query has inherited it's order from the OVER clause.
>
> Am I missing something here?
>
> Thanks
>
> Thom
>

Having a look around, it looks as if Postgres might be misbehaving.
According to this page,
http://my.safaribooksonline.com/0596004818/sqlnut2-CHP-4-SECT-3, the ORDER
BY in the window function's clause shouldn't be having this ordering effect:

"Furthermore, the order within these groups is defined by an ordering
clause, but that order only affects function evaluation, and has no effect
on the order in which rows are returned by the query."

The behaviour is unexpected from my perspective, but obviously there are
workarounds. Is anyone able to confirm any of this?

Thom