Expert Oracle Exadata - Parent Directory

CHAPTER 13 MIGRATING TO EXADATAThere are a lot of steps in configuring and instantiating a logical standby and we won’t go into all thedetails here. But to give you an idea of how a logical standby is set up, here are the high-level steps:1. Create a physical standby database (we’ll talk about this in the “PhysicalStandby” section).2. Stop redo apply on the physical standby database.3. Configure Log Miner in the Primary database.4. Convert the physical standby database to a logical standby.5. Open the logical standby database and restart redo apply.6. Verify that the logical standby database is applying changes from the source.Logical standby uses SQL Apply technology to replicate changes from the source database to thetarget. As mentioned earlier, changes in the source database are converted to SQL statements andshipped to the target for execution. Although this may sound fairly straightforward, there are someinteresting implications because of how it is implemented. Here are a few things to keep in mind.Considerations for DML Statements:• Batch updates are executed one row at a time.• Direct path inserts are performed in conventional manner.• Parallel DML is not executed in parallel.Considerations for DDL Statements:• Parallel DDL is not executed in parallel.• CTAS is executed as create table, insert, insert, insert, …When to Use Logical StandbyLogical standby has a very narrow list of benefits that set it apart from other, more likely, migrationmethods. It provides full database replication and allows read access to the standby database duringreplication. As such, the standby may be used to offload resource-intensive reporting operations fromthe production database until the time you are ready to make the final switch. Tables and materializedviews may be created on the standby database, and indexes may be created or dropped to improveperformance.What to Watch Out for with Logical StandbyNOLOGGING operations do not generate redo, which means they will not be replicated to the standbydatabase. Be sure to turn off NOLOGGING operations using the alter database force logging command inSQL*Plus. Considering its lack of support for modifying storage of target tables, compression, andpartitioning, you might opt to use the physical standby approach instead. If you don’t require fulldatabase replication, Streams and Golden Gate are excellent alternatives providing support for many ofthe capabilities lacking in a logical standby database.449