[ADMIN] performance cost for varchar(20), varchar(255), and text

I am tuning a database created by someone else.

I noticed that some column lengths were defined longer than needed.

For example, an Id column is holding a stand length of 20 characters but was defined as varchar(255).

On some other columns, for example, a Description column is supposed to hold less than 100 characters but defined as text.

I am trying to understand the performance impact if a column is over defined in the following cases:

1. char(20) vs varchar(20)

2. varchar(20) vs varchar(255)

3. varchar(255) vs text


thanks,
Jessica