Re: [HACKERS] proposal: window function - change_number

[Rémi Cura]

Hey, sorry I what I say is obvious for you .

If I understood your problem correctly, it is strictly equivalent to this
one :
http://postgresql.1045698.n5.nabble.com/Count-of-records-in-a-row-td5775363.html

there is a postgres trick to solve this problem :
what you want is essentially generate a unique group_id,
but one that depends of an order of row not defined in the group.

The solution
is to generate a row number by the order you want , then a row number by
the group ,
then a subtraction of the 2 row number gives you an unique id per group.

The cost is that you have to use 2 windows function., hence 2 scans I guess.

Cheers,
Rémi-C

2014-09-21 17:51 GMT+02:00 Andrew Gierth :

> > "Pavel" == Pavel Stehule  writes:
>
>  Pavel> Hi
>  Pavel> I tried to solve following task:
>
>  Pavel> I have a table
>
>  Pavel> start, reason, km
>  Pavel> =
>  Pavel>  2014-01-01 08:00:00, private, 10
>  Pavel>  2014-01-01 09:00:00, commerc, 20
>  Pavel>  2014-01-01 10:00:00, commerc, 20
>  Pavel>  2014-01-01 11:00:00, private, 8
>
>  Pavel> and I would reduce these rows to
>
>  Pavel>  2014-01-01 08:00:00, private, 10
>  Pavel>  2014-01-01 09:00:00, commerc, 20 + 20 = 40
>  Pavel>  2014-01-01 11:00:00, private, 8
>
>  Pavel> It is relative hard to it now with SQL only.
>
> Only relatively. My standard solution is something like this:
>
> select start_time, reason, sum(km) as km
>   from (select max(label_time) over (order by start) as start_time,
>reason, km
>   from (select start, reason, km,
>case when reason
>  is distinct from
>  lag(reason) over (order by start)
> then start
>end as label_time
>   from yourtable
>) s2
>) s1
>  group by start_time, reason
>  order by start_time;
>
> (Your change_number idea is essentially equivalent to doing
> sum(case when x is distinct from lag(x) over w then 1 end) over w,
> except that since window functions can't be nested, that expression
> requires a subquery.)
>
> --
> Andrew (irc:RhodiumToad)
>
>
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[HACKERS] python modul pre-import to avoid importing each time

[Rémi Cura]

Hey List,
this is a repost from the general list where it get no responses (5 days)

I use plpython with postgis and 2 python modules (numpy and shapely).
Sadly importing such module in the plpython function is very slow (about
half a second).

I also don't know if this overhead is applied each time the function is
called in the same session.

Is there a way to pre-import those modules once and for all (at server
start up for example),
such that the python function are accelerated?

Thanks,

Cheers,
Rémi-C