Database Reference
In-Depth Information
Another big difference between a physical and logical standby database is the fact that a logical standby database
is open read-write while it still receives and applies changes from production. A logical standby database is unlikely
to be used for disaster recovery purposes. Its main purpose is to provide an environment in which reporting can be
offloaded from production as changes from the live system are fed into the database. This provides a high degree of
data accuracy. The fact that the logical standby database is open read-write means that additional data structures
such as indexes and materialized views can be created to speed up queries that would otherwise be too expensive to
maintain on the primary database.
Finally, logical standby databases can be used as part of the process to upgrade primary databases to newer
releases or to apply patch sets to a system with almost no downtime. This little used technique is referred to as a
rolling upgrade in the Oracle documentation. The transient logical standby database discussed in the next section is
what you really want to use in cases where you want to apply a rolling upgrade in your own environment.
For operational reasons you would not expect a logical standby database to be part of the standard offering of a
hosting service. The management of the logical standby is more complex, and it contradicts the ideas of saving cost
and allowing lesser-skilled operators to manage the environment.
The transient logical standby database
Oracle has recognized that few businesses are willing to set up a logical standby database only for the rolling upgrade
of an Oracle database. Setting up a logical standby database is not a trivial task, and maintaining a logical standby
database requires close monitoring to check that all transactions have been applied. For this reason, Oracle 11g
Release 1 provides the capability to transiently convert a physical standby database into a logical standby database for
the duration of a rolling upgrade. After the upgrade, the logical standby database is converted back to its original role
as a physical standby database.
This type of standby database is not listed under the name transient logical standby in the documentation;
however, it is mentioned in the Maximum Availability Architecture (MAA) series of documents as well as in Chapter
13 of the Data Guard Concepts and Administration chapter, “Using SQL Apply to Upgrade the Oracle Database.”
The use of a transient logical standby is not trivial and requires in-depth knowledge of the Oracle database as well
as Flashback Database and general backup and recovery skills in addition to what you need to know and understand
for the database upgrade. A transient logical standby database remains a logical standby database with its limitations
towards unsupported data types and close monitoring requirements. For that reason (complexity!) it might not be
the best choice for migrations on a hosted platform. This is even more relevant with the introduction of Pluggable
Databases which can potentially be unplugged and plugged into databases with different patch levels
The Active Data Guard Option
The majority of standby databases are probably physical standby databases running in remote disaster recovery
data centers waiting to be activated. Many users of such configurations have remarked that this isn't the best use of
resources. While it is possible to use the standby database as a source for backups, the tapes must be shipped from the
DR site to the primary site if something occurs. The other option, using the standby database for reporting and ad-hoc
queries that couldn't possibly be executed on the primary database, also has a downside: while the database is open
for read-only mode, no archived logs are applied, which causes the primary and the standby databases to go out of
sync and the data to become stale.
Beginning with 11g Release 1, Oracle addressed these concerns with the Active Data Guard option, which needs
to be licensed on top of Enterprise Edition. When purchased, this option offers the following benefits in Oracle 12c
Enterprise Edition over and above what Data Guard already provides:
Physical Standby databases in read-only mode can have managed recovery enabled. This
allows users to offload their queries to current production data, thus combining the best of
both worlds. Oracle refers to this feature as Real Time Query .
This option also allows the use of block change tracking for faster incremental backups on the
standby database
 
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