Logical Standby Databases为何能处于write mode

etl2007

按照 oracle文档的说法
http://docs.oracle.com/cd/E11882_01/server.112/e25608/standby.htm#i50975

A logical standby database is initially created as an identical copy of the primary database, but it later can be altered to have a different structure. The logical standby database is updated by executing SQL statements. This allows users to access the standby database for queries and reporting at any time. Thus, the logical standby database can be used concurrently for data protection and reporting operations.

Data Guard automatically applies information from the archived redo log file or standby redo log file to the logical standby database by transforming the data in the log files into SQL statements and then executing the SQL statements on the logical standby database. Because the logical standby database is updated using SQL statements, it must remain open. Although the logical standby database is opened in read/write mode, its target tables for the regenerated SQL are available only for read-only operations. While those tables are being updated, they can be used simultaneously for other tasks such as reporting, summations, and queries. Moreover, these tasks can be optimized by creating additional indexes and materialized views on the maintained tables.


我还是有个疑问 logical standby database 怎么可以处于 write mode呢？
有这样一种情况

例如主库现在创建A表，现在同步到备库，那么备库（logical standby database）也有了A表，因为现在备库处于write mode
我现在执行 drop table A
过会我在主库的A表里面插入一条数据，结果同步到备库过来的时候发现A表都不存在了？这可如何同步啊？
不是都玩不转了吗？