Re: proposal: window function - change_number

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 <andrew(at)tao11(dot)riddles(dot)org(dot)uk>:

> >>>>> "Pavel" == Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com> 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)
>
>
> --
> Sent via pgsql-hackers mailing list (pgsql-hackers(at)postgresql(dot)org)
> To make changes to your subscription:
> http://www.postgresql.org/mailpref/pgsql-hackers
>