I am trying to use indexes on my tables to improve the performance.
The fields I have are: id, merchantname, Category, productName, Brand

I currently have the primary key as the id.
The merchant name is always the same. The category varies, and brand is sometimes empty.
1) For the indexes should I only use the category, or should I have 2 seperate indexes one for brand and one for category.
Should I set the length to 10 as the field is 255?

2) Is it okay to have a index with Cardinality: NULL?