Logical Standby

1. What is a logical Standby? A logical standby database is a type of standby database that maintains a logical copy of the primary database's data. It uses SQL Apply to apply changes from the primary database in the form of SQL statements, making it suitable for reporting, testing, and data manipulation tasks without interrupting the primary database's operations.

2. What are the disadvantages and limitations that are associated with the Logical Standby systems? While logical standby databases have some excellent benefits, they do have some definite drawbacks as well, including restrictions on unsupported objects and data types, limited support for DDL and DCL commands.

3. Unsupported Objects.

* A logical standby database cannot accept updates for some objects:

* Any tables or sequences owned by SYS

* Any tables that use table compression

* Any tables that underlie a materialized view

* Global temporary tables (GTTs)

4. Unsupported Data Types. Any tables that are comprised of or columns contain the some unsupported data types cannot be updated in a logical standby:

* Datatypes BFILE, ROWID, and UROWID

* Any user-defined TYPEs

* Multimedia data types like Oracle Spatial, ORDDICOM, and Oracle Text

* Collections (e.g. nested tables, VARRAYs)

* SecureFile LOBs

* OBJECT RELATIONAL XMLTypes

* BINARY XML

5. Difference between physical and logical standby databases?

* For physical standby databases, Data Guard uses Redo Apply technology, which applies redo data on the standby database using standard recovery techniques of an Oracle database.

* For logical standby databases, Data Guard uses SQL Apply technology, which first transforms the received redo data into SQL statements and then executes the generated SQL statements on the logical standby database.