Thursday, September 09 2010
PostgreSQL, YeSQL

Window Functions example

So, when 8.4 came out there was all those comments about how getting window functions was an awesome addition. Now, it seems that a lot of people seeking for help in #postgresql just don't know what kind of problem this feature helps solving. I've already been using them in some cases here in this blog, for getting some nice overview about Partitioning: relation size per “group”.

That's another way to count change

Now, another example use case rose on IRC today. I'll quote directly our user here:

hey there, how can i count the number of (value) changes in one column?

example: a table with a column weight. let's say we have 5 rows, having the following values for weight: 5, 10, 7, 7, 7. the number of changes of weight would be 2 here (from 5 to 10 and 10 to 7). any idea how I could do that in SQL using PGSQL 8.4.4? GROUP BY or count(distinct weight) obviously does not work. thx in advance

Now, several of us began talking about window functions and about the fact that you need some other column to identify the ordering of those weights, obviously, because that's the only way to define what a change is in this context. Let's have a first try at it.

=# select o, w, 
          case when lag(w) over(order by o) is distinct from w then 1 end as change
     from (values (1, 5), (2, 10), (3, 7), (4, 7), (5, 7)) as data(o, w);
 o | w  | change 
 1 |  5 |      1
 2 | 10 |      1
 3 |  7 |      1
 4 |  7 |       
 5 |  7 |       
(5 rows)

Not too bad, but of course we are seeing a false change on the first line, as for any window of rows you define the previous one, given by lag() over(), will be NULL. The easiest way to accommodate is the following:

=# select sum(change) -1 as changes 
     from (select case when lag(w) over(order by o) is distinct from w
                       then 1
                   end as change
             from (values (1, 5),
                          (2, 10),
                          (3, 7),
                          (4, 7),
                          (5, 7)) as t(o, w)) as x;
(1 row)

So don't be shy and go read about window functions in SQL expressions and window function processing in the query table expressions. That's a very nice tool to have and my guess is that you will soon enough realize the only reason why you could think you don't have a need for them is that you didn't know it existed, and what you can do with it. Sharpen your saw! :)