Mysql varchar unique column varchar(255) vs. varchar(50)

Question

I will always be entering things of 20 characters into a particular column in my table.

I need this column to be unique.

Will there be any speed difference in SELECT queries if I set this column to be a varchar(255) instead of varchar(20)?

(data entered will always be 20 characters)

Answer 1

if data entered will always be 20 characters than why not consider using char(20). using varchar(20) will use 20 bytes for storing character and 1 byte for storing length. so if there are 1 million records, 1 million bytes will be wasted.

as far as speed is concerned between varchar(20) and varchar(255), then I dont think it might be very hard to pick one of them, both of them will be using 21 bytes, I dont see any significant performance benefit or loss of one over other.

Answer 2

If it can only be 20, why would you want to specify 255? If it is always 20, even char(20) would be better.