I just read Ann's article. It clarified many things for me.
It just remided me of one thing I always wanted to ask here:

Tables: Products (millions of records) and Categories (10 records)
have Cat_id column (=PK for Categories). Should I also create index
on
Products.Cat_id even if there will be only 10 possible values among
millions of records?
I will use Cat_id in WHERE part.
Thanks,
Nick