Once the database exists and other users have access to it, follow these guidelines when you need to perform additional data definition operations, such as adding or modifying tables or indexes.
You may schedule data definition operations for times when database usage is low. All other users are blocked from reading or writing to a table while you are creating or altering that table, although for a brief time only. If the table is part of a join index, users cannot read or write to any of the tables in the join index until the data definition operation is complete. For more information on concurrency rules during data definition, see “Locks for DDL operations”.