[GENERAL] alter multiple tables

[Kodok Marton]

Hello,

I have in every table columns like:
username character varying(20) NOT NULL

I want to extend the length of varchar in all tables. 

Since I have a lot of tables and mirrored backups, I am wondering if there is a 
way to alter automatically all tables where colname matches 'username'

Is there a way to do this?

Marton