PostgreSQL calculate difference between rows

I tried to calculate difference between rows in a field using a query:

Illustrations:
input:year,month,fixes
output:increase

     year | month | fixes    | increase
    ------+-------+----------+-----------
     2006 | 04    |       1  | 0
     2006 | 05    |       4  | 3
     2006 | 06    |       3  | -1

Increase column as output by difference between adjacent rows in fixes.

Solution 1:

This is what window functions are for:

select year, 
       month,
       fixes,
       fixes - lag(fixes) over (order by year, month) as increase,
from the_table;

For more details please see the manual:
http://www.postgresql.org/docs/current/static/tutorial-window.html