Add new mandatory column without killing concurrent transactions

Question

I know of several approaches to this problem, but as the table in question is key to my organisation, I'd quite like to get this right first time!
I need to add a 1 character column to a large (approx 1 million rows, approx 100 cols) table which also happens to be central to our database (contains client details). The table is involved in the majority of transactions on our database, 24/7 so I need to find the method with the least impact on concurrent transactions.
We use mod_plsql for our applications, and a previous attempt (by another developer) caused all web pages to become unresponsive, followed by a database shutdown.
Any advice?

Answer 1

Just a note that I ended up writing a pl/sql script which used a select for update cursor to update the non-locked rows, and stored any locked rowids in an associative array. It then looped round the array trying to update these rows until they were all done. Did the trick. DBA wasn't keen or the redefinition package when he saw what it did!
thanks,
M