Best way to add a new column to a large table? (MySQL - MyISAM)

Question

Here's more info on the table that needs to be altered:

• 15GB size MyISAM table with Index around 5GB
• Over 100 million rows
• MySQL 5.1 on RHEL 5 with 4GB RAM
• Currently live online

What's the best way to add a couple of new column (minimum 2 columns) to a large table like this? With minimum downtime. Just using direct ALTER table commands would take hours if not a day or two to finish.

Additional question: Will it affect the length of time needed to add new columns if the new column default value are set to NULL instead of having a 'pre-defined' value?

Answer 1

To make online changes to a table you can use pt-online-schema-change

    pt-online-schema-change --alter "ADD COLUMN c1 INT" D=sakila,t=actor

The table will be blocked for a few seconds when it switches new and old tables. The table though must have a primary keys and not have triggers.