Locks and Concurrency
For an ALTER TABLE statement, a WRITE lock is placed on the base table being altered. Teradata Database also places a table‑level lock on DBC.AccessRights, but removes it before the physical row altering phase of the process to enhance ALTER TABLE performance.
Because of the overhead required to alter a base table for dropped or added columns, you should establish a final form for the table before it contains very many rows. When a very large table is altered, it can be unavailable to other users for a long time.