Partly because MySQL requires a full table lock and full table copy to implement those operations. Much of the nosql movement originated from implementing schem

Partly because MySQL requires a full table lock and full table copy to implement those operations. Much of the nosql movement originated from implementing schema changes on production MySQL databases. In many if not most cases, NoSQL is really NoMySQL. Many other engines, PostgreSQL for example, can add a new column without constraints as a nearly instant metadata change only. Data type changes that do not require validation (expanding a VARCHAR vs CHAR to INT) are also rapid.