The DBMS_ROLLING package facilitates a rolling upgrade process across a Data Guard configuration. The START PLAN procedure in particular handles several critical actions, including:

Creating a guaranteed restore point on the standby databases (B): This ensures that the standby databases can be reverted to their state before the rolling upgrade process in case of any issues.

Building a LogMiner dictionary on the primary database instance (C): This is necessary for logical standby databases to interpret redo data during the SQL Apply process.

Creating a guaranteed restore point on the primary database (D): Similar to the standby databases, this ensures that the primary database can be reverted to a known good state if necessary. Reference:

Oracle Database PL/SQL Packages and Types Reference

Oracle Data Guard Concepts and Administration Guide

Snapshot standby databases are a feature of Oracle Data Guard that allows a physical standby database to be temporarily converted into a read-write database for testing or other purposes. The true statements about snapshot standby databases are:

FLASHBACK DATABASE is enabled automatically on a snapshot standby database after converting it from a physical standby database if not already enabled (B): When a physical standby is converted to a snapshot standby, FLASHBACK DATABASE is automatically enabled to allow the database to be easily reverted back to its original state.

FLASHBACK DATABASE is enabled automatically on a physical standby database as part of the conversion into a snapshot standby database, if not already enabled (C): As part of the conversion process, FLASHBACK DATABASE is turned on to ensure that changes made while the database is in snapshot standby mode can be undone.

A snapshot standby database may be opened read-write (E): Once a physical standby is converted to a snapshot standby, it can be opened for read-write operations, allowing for testing and other tasks that require a writable database.

Oracle Data Guard Concepts and Administration

Oracle Database Backup and Recovery User's Guide

Based on Oracle Database 19c: Data Guard Administration documents, the three measures that can prevent clients from connecting to the wrong database instances during switchovers, failovers, or regular operations in a Data Guard environment are:

B) The standby database services must be defined statically with the Listeners running on the standby database hosts.

D) The client applications must use the correct TNS entries when requesting connections to the database instances.

E) Client TNS entries for the databases use the correct service names for the intended service.

In an Oracle Data Guard configuration, correctly configuring Oracle Net Services (including TNS entries and listeners) is crucial for ensuring that clients connect to the appropriate database instance, whether it's the primary or standby. Defining services on the standby database and associating them with listeners ensures that client applications can connect to the standby when needed, especially useful in a role transition or when the standby is open for read-only access or real-time query. It's essential that TNS entries used by client applications specify the correct service names that correspond to the intended database roles, such as primary or standby. This setup facilitates seamless connectivity to the appropriate instance based on the role, especially critical during switchovers and failovers when the roles of the databases change.

Reference: Oracle's Data Guard concepts and administration guide provides extensive information on configuring network services for Data Guard environments, ensuring that applications connect to the correct database instance based on the current role of the databases in the Data Guard configuration.

Flashback Database is a feature that allows reverting a database to a previous point in time, which is extremely useful in various Data Guard configurations:

It may be used to flash back a physical standby that receives redo from a far sync instance (C): Flashback Database can be used on a physical standby database to revert it to a past point in time, even when it is receiving redo data from a far sync instance. This can be particularly useful to recover from logical corruptions or unwanted changes.

You can use it when real-time apply is enabled in case the physical standby suffers from logical corruption (D): Even when real-time apply is enabled, which allows redo data to be applied to the standby database as soon as it is received, Flashback Database can be used to revert the physical standby database to a point in time before the logical corruption occurred.

It may not be used to flash back a primary database after a failover to a physical standby (E): After a failover has occurred from a primary to a physical standby database, making the standby the new primary, Flashback Database cannot be used to revert the old primary database to a state before the failover because the failover operation makes irreversible changes to the database role and configuration. Reference:

Oracle Database Backup and Recovery User's Guide

Oracle Data Guard Concepts and Administration

Logical standby databases are a key feature of Oracle Data Guard and offer several distinct advantages, especially in terms of flexibility for reporting, upgrades, and disaster recovery:

Disaster-recovery solution with switchover and failover options (A): Logical standby databases provide a robust disaster-recovery solution, ensuring that any data updated on the primary database can be recovered. They support both switchover and failover operations, allowing for smooth role transitions between the primary and standby databases.

Used for reporting workloads requiring additional indexes or materialized views (B): Logical standby databases can be opened for read-write operations and can have additional indexes or materialized views that are not present in the primary database. This makes them ideal for offloading reporting and querying workloads from the primary database.

Database rolling release upgrades (D): Logical standby databases can be used to perform rolling upgrades of the Oracle Database software. This allows the database to be upgraded with minimal downtime, as the standby database is upgraded first, followed by a switchover to make it the new primary. Reference:

Oracle Data Guard Concepts and Administration Guide

Oracle Database High Availability Overview