Window function frame clause

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


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