如果超过了最大读锁数量（由实现定义），pthread_rwlock_rdlock 可能会失败。
pthread_rwlock_rdlock may fail if the maximum number of read locks (implementation defined) for the lock has been exceeded.
当存在一个读锁时，很多读操作均可使用这个锁。
When a read lock exists, many read operations may use this lock.
通过使用乐观并发控制，数据库管理器在完成读操作之后可以立即释放行或页锁。
With optimistic concurrency control, the database manager releases the row or page locks immediately after a read operation.
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