Question

I have a rating table. It boils down to:

rating_value  created
+2            april 3rd
-5            april 20th

So, every time someone gets rated, I track that rating event in the database.

I want to generate a rating history/time graph where the rating is the sum of all ratings up to that point in time on a graph.

I.E. A person’s rating on April 5th might be select sum(rating_value) from ratings where created <= april 5th

The only problem with this approach is I have to run this day by day across the interval I’m interested in. Is there some trick to generating a running total using this sort of data?

Otherwise, I’m thinking the best approach is to create a denormalized “rating history” table alongside the individual ratings.

Answer 1

If you have postgresql 8.4, you can use a window-aggregate function to calculate a running sum:

steve@steve@[local] =# select rating_value, created,
                              sum(rating_value) over(order by created)
                       from rating;
 rating_value |  created   | sum 
--------------+------------+-----
            2 | 2010-04-03 |   2
           -5 | 2010-04-20 |  -3
(2 rows)

See http://www.postgresql.org/docs/current/static/sql-expressions.html#SYNTAX-WINDOW-FUNCTIONS