Everlasting SQL query

Hi everyone,

I have a customer table (17518 records) and an orders table (88393 records). One of the columns of orders is
customerid,containing the customerid (what else, but it is not a foreign key as this table is imported from a database
thatdid not support foreign keys).  

If I do this query (with pgadmin III):

select customer.id, customer.name, orders.id
from customers, orders
order by customer.id, orders.id
limit 25

The query runs forever (the longest I let it run is 500 seconds).

Explain gives me this (why 7 rows?):

                                     QUERY PLAN
-------------------------------------------------------------------------------------
 Limit  (cost=722506879.16..722506879.22 rows=25 width=44)
   ->  Sort  (cost=722506879.16..726378050.59 rows=1548468574 width=44)
         Sort Key: klt_alg.klantnummer, orders.ordernummer
         ->  Nested Loop  (cost=9408.93..36288661.59 rows=1548468574 width=44)
               ->  Seq Scan on klt_alg  (cost=0.00..1927.18 rows=17518 width=40)
               ->  Materialize  (cost=9408.93..10595.86 rows=88393 width=4)
                     ->  Seq Scan on orders  (cost=0.00..9105.93 rows=88393 width=4)
(7 rows)

If I only order by customer.id or by orders.is the query return within a second.

Can anyone give me a reason why this all happens?

Groeten,

Joost Kraaijeveld
Askesis B.V.
Molukkenstraat 14
6524NB Nijmegen
tel: 024-3888063 / 06-51855277
fax: 024-3608416
e-mail: J.Kraaijeveld@Askesis.nl
web: www.askesis.nl