On Fri, Feb 22, 2013 at 8:26 AM, Clodoaldo Neto
<clodoaldo.pinto.neto@gmail.com> wrote:
> Suppose there is the need to get the average of a value v over a 6 hours
> time window starting 7 hours before the current row's time.
>
> create table t (ts timestamp, v integer);
> insert into t (ts, v) values
> ('2013-01-01 00:46', 2),
> ('2013-01-01 03:54', 4),
> ('2013-01-01 06:28', 4),
> ('2013-01-01 11:19', 2),
> ('2013-01-01 14:44', 1),
> ('2013-01-01 15:56', 5),
> ('2013-01-01 18:01', 4),
> ('2013-01-01 19:40', 0),
> ('2013-01-01 20:38', 5),
> ('2013-01-01 21:22', 0);
>
> I can do it with a correlated subquery:
>
> select ts, v,
> (
> select avg(v)
> from t s
> where ts between
> t.ts - interval '7 hours'
> and t.ts - interval '1 hour'
> ) average
> from t
> order by ts
> ;
> ts | v | average
> ---------------------+---+--------------------
> 2013-01-01 00:46:00 | 2 |
> 2013-01-01 03:54:00 | 4 | 2.0000000000000000
> 2013-01-01 06:28:00 | 4 | 3.0000000000000000
> 2013-01-01 11:19:00 | 2 | 4.0000000000000000
> 2013-01-01 14:44:00 | 1 | 2.0000000000000000
> 2013-01-01 15:56:00 | 5 | 1.5000000000000000
> 2013-01-01 18:01:00 | 4 | 2.6666666666666667
> 2013-01-01 19:40:00 | 0 | 3.3333333333333333
> 2013-01-01 20:38:00 | 5 | 3.3333333333333333
> 2013-01-01 21:22:00 | 0 | 2.5000000000000000
>
> But if I could access a window's frame_end row as a record from a window
> function:
>
> select ts,
> avg(case when ts between
> frame_end.ts - interval '7 hours'
> and frame_end.ts - interval '1 hour'
> then v else null end
> ) over(order by ts)
> from t
> order by ts
>
> I'm naively posting this as I have no idea how complex would it be to add
> this feature. Would it perform better than the correlated subquery?
Well, correlated subquery is about the bottom of the barrel in
performance terms, so anything would be an improvement.
merlin
