database-migration is the process of transferring data between storage types, formats, or computer systems. Also refers to migrating the database from one vendor to another, or to upgrading the version of the database software.

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Updates also become especially challenging if you want the existing web site to stay ‘live’ while you change the database, i.e. the old software still works with the new database while you are updating its schema and data.

The primary purpose of table-level locks is to block reads and/or writes when changes to the underlying table structure are made during DDL commands such as ALTER TABLE. However, not all DDL commands need to block reads or writes, some only block each other.