Adam,

<< I have realized that the table is PUBLISHED. It also has links to other tables, and triggers ... so I am guessing that I ought to probably switch off the triggers and publishing during the ALTER, then refresh triggered values afterwards with a single statement. >>

In addition to disabling triggers, you can also use the new statement/procedure caching in 2.28 to optimize triggers and other executable SQL that may get executed during such long operations.  Triggers can be called recursively, so EDB has to constantly create/destroy any objects (statements, cursors) referenced in the triggers, and this can take its toll in the same way that executing 1 million inserts using dynamic SQL is much slower than executing 1 million inserts using a manually-prepared statement with parameters: it's incurring a lot of overhead involved with constantly opening/closing other tables.

Tim Young
Elevate Software
www.elevatesoft.com