Most DDL operations, except for those listed in the section, "Share DDL Locks" require
exclusive DDL locks for a resource to prevent destructive interference with other DDL
operations that might modify or reference the same schema object. For example, a
DROP TABLE operation is not allowed to drop a table while an ALTER TABLE
operation is adding a column to it, and vice versa.
During the acquisition of an exclusive DDL lock, if another DDL lock is already held
on the schema object by another operation, the acquisition waits until the older DDL
lock is released and then proceeds.
DDL operations also acquire DML locks (data locks) on the schema object to be
1. 大多数的DDL操作需要获得排他DDL锁 , 来保护方案对象
2. 在获取DDL锁的过程中 , 如果同一方案对象上的DDL锁被获得 , 则需要等待DDL锁释放
vice versa : 反之亦然[@more@]
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