On Feb 16, 2012, at 20:01, vpapavas <vicky.papavas@gmail.com> wrote:
> Hello all,
>
> I am trying to use this query in a toy database with customers and orders in
> order to understand the capabilities of partitioning. In plain english what
> I want to do is to select the orders of each customer and return only 3 of
> those orders.
>
> The query I am using is this:
> select c_custkey, o_orderkey, o_orderpriority, id from (
> select c_custkey, o_orderkey, o_orderpriority, o_totalprice, row_number()
> over(PARTITION BY c_custkey ROWS between UNBOUNDED PRECEDING and 3
> FOLLOWING) as id
> from customers left outer join orders on c_custkey = o_custkey) as temp
>
> Although I am using the frame clause ROWS between UNBOUNDED PRECEDING and 3
> FOLLOWING which in my understanding should return the first row of the
> partition and the three following, this query returns all rows in the
> partition. Am I doing something wrong? Or have I understood wrong the
> semantics of the frame clause? I am using Postgresql v9.1
>
> I rewrote the query like this in order to make it work:
> select c_custkey, o_orderkey, o_orderpriority, id from (
> select c_custkey, o_orderkey, o_orderpriority, row_number()
> over(PARTITION BY c_custkey) as id
> from customers left outer join orders on c_custkey = o_custkey ) as temp
> where id <= 3
>
> but the problem is that I would like to not have to compute the entire join
> since I am interested in only 3 orders for each customer.
>
> Thank you,
> Vicky
>
>
Put the window function on the order table, perform the where-limit, then join customer to the result.
Also, you are numbering rows but not imposing any kind of order before doing so.
Row_number doesn't make sense with a frame clause...frame is more useful for stuff like calculating rolling
sums/averagesand the like - where you evaluate fields in the surrounding frame as part of the aggregate.
Window functions do not affect the number of rows returned.
David J.