Michel,
Having just added a 6-char field to an ancient SQL Server table: IME it can be very quick to add a NULL field, but if there's a default value and the table is large, we (and more especially longstanding customers) have been caught out by the many hours needed to ALTER the large table, and the buckets of space needed for the database log or the alter will churn for hours before failing and rolling back. It was worse at a customer who needed to prove any changes in test, but their test system didn't have enough space for the log. Boy did we wish we'd added an extra table that would have been almost instant! Just something to think about.
"... They ne'er cared for us
yet: suffer us to famish, and their store-houses
crammed with grain; make edicts for usury, to
support usurers; repeal daily any wholesome act
established against the rich, and provide more
piercing statutes daily, to chain up and restrain
the poor. If the wars eat us not up, they will; and
there's all the love they bear us."
-- Shakespeare: Coriolanus, Act 1, scene 1
