Re: short-cutting if sum()>constant

On Wed, 23 Dec 2009 00:00:31 +0100
Ivan Sergio Borgonovo <mail(at)webthatworks(dot)it> wrote:

> On Tue, 22 Dec 2009 20:47:18 +0100
> Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> wrote:
>
> > Hello
> >
> > I found one ugly trick. You can multiply lines and SUM > cons
> > could be replaced limit clause:
>
> The trick is really smart (and fun), kudos, really, it's always a
> pleasure to read your solutions, thanks.
>
> But as expected:

as unexpected...

> test=# create or replace function tano(a int, out b int)
^^^ should be anything but a
> returns int as
> $$
> declare
> row record;
> begin
> b :=0;
> for row in select a as _a from data

where a>0

> loop
> b := row._a + b;
> if (b>=a) then
> return;
> end if;
> end loop;
> return;
> end;
> $$ language plpgsql;

Making it longer to better appreciate the difference: 1M rows where
a [0,2]

select * from tano((1000000)::int);
b
---------
1000001
(1 row)

Time: 1235.243 ms

select sum(x) from (select 1 as x,(a =
generate_series(1,a))::int from data limit 1000000) s;
sum
---------
1000000
(1 row)

Time: 1309.441 ms

Being fair once you add the where clause to the generate_series
version the difference in performance is negligible and saying that
the plpgsql version is faster would require some more serious
benchmarking.

Surprised! If the generate_series can compete with the plpgsql for
loop... why is the plpgsql version so "slow"?

--
Ivan Sergio Borgonovo
http://www.webthatworks.it