A question about indexes...

Hello,
I have the  following tables in my db
Painter (id integer,  uri varchar(256)) and
paints (id1 integer, id2 integer)
I  want to optimize the question  select id from Painter where uri = 'xxxxx';What kind of index (Btree or Hash) is more
efficientto create on field
 
uri  since it's a string?
I also want  to optimize the join between the ables Painter and paints 
on the fields id and id1 respectively?
I can either define the field id as a Primary Key or create an Btree index
on it. What is more effient?? 
From  my test I see that creating  Btree index is  a bit faster!!. 
Would the performance (of the join) be  improved if  I created indexes
both on field id and id1 or it's sufficient  to create one of the two
indexes ?
As far as I can see the performance is improved if I have a Primary Key on
Painter.id and a BTree index on paints.id1. However when I create a 
Btree index on Painter.id and a BTree index on paints.id1 performance
gets worst.


thank you in advance for your help
Sofia Alexaki
alexaki@ics.forth.gr