How to auto-increment?

Hi,

 

I have a table like this:

 

id_product

id_increment

 

and I need to increment values in id_increment like this

 

prod_1

1

 

prod_1

2

 

prod_1

3

 

prod_2

1

 

Wich is the best way to do this? Using a trigger? Where can I find examples of plpgsql doing this?

 

Best Regards, André.

 

Sorry for my bad english.