Difference between logical & physical standby database?

Question:  What is the difference between logical standby and physical standby in Oracle Data Guard?

Answer: In  Oracle Data Guard you, Oracle transfers data from the main database to a standby database, and in case of failures, Oracle will switch over to the standby database.  We have two ways to create a standby database, logical standby and physical standby:

Physical standby differs from logical standby:

• Physical standby schema matches exactly the source database.
  

• Archived redo logs and FTP'ed directly to the standby database which is always running in "recover" mode.  Upon arrival, the archived redo logs are applied directly to the standby database.

Logical standby is different from physical standby:

• Logical standby database does not have to match the schema structure of the source database.
   

• Logical standby uses LogMiner techniques to transform the archived redo logs into native DML statements (insert, update, delete).  This DML is transported and applied to the standby database.
   

• Logical standby tables can be open for SQL queries (read only), and all other standby tables can be open for updates.
   

• Logical standby database can have additional materialized views and indexes added for faster performance.

Installing Physical standbys offers these benefits:

• An identical physical copy of the primary database

• Disaster recovery and high availability

• High Data protection

• Reduction in primary database workload

• Performance Faster

Installing Logical standbys offer:

• Simultaneous use for reporting, summations and queries

• Efficient use of standby hardware resources

• Reduction in primary database workload

• Some limitations on the use of certain datatypes

For more complete differences between logical and physical standby databases, see the book "Oracle Data Guard", by Bipul Kumar.

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