Redshift comma separated string to column

Question

I have data with the following structure in redshift:

select 1 as id, 'a,b,c' as str
union all select 2, 'c,d'

I would like to convert str to a flat column, obtaining the following data:

1,a
1,b
1,c
2,c
2,d

How can I do that in redshift?

Thanks!