In-Place ALTER TABLE Costs

For certain conditions, the database server uses an in-place alter algorithm to modify each row when you execute an ALTER TABLE statement (rather than during the alter table operation). After the alter table operation, the database server inserts rows using the latest definition.

If your query accesses rows that are not yet converted to the new table definition, you might notice a slight degradation in the performance of your individual query because the database server reformats each row in memory before it is returned.

For more information on the conditions and performance advantages when an in-place alter occurs, see Altering a Table Definition.