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ORACLE DBA Interview Questions with Answers

> What is difference between Restoring and Recovery of database ?
Restoring means copying the database object from the backup media to the destination where actually it is required where as recovery means to apply the database object copied earlier (roll forward) in order to bring the database into consistent state.

> What is the difference between complete and incomplete recovery ?
An incomplete database recovery is a recovery that it does not reach to the point of failure. The recovery can be either point of time or particular SCN or Particular archive log specially incase of missing archive log or redolog failure where as a complete recovery recovers to the point of failure possibly when having all archive log backup.

> What is the benefit of running the DB in archivelog mode over no archivelog mode ?
When a database is in no archivelog mode whenever log switch happens there will be a loss of some redoes log information in order to avoid this, redo logs must be archived. This can be achieved by configuring the database in archivelog mode.

> If an oracle database is crashed? How would you recover that transaction which is not in backup ?
If the database is in archivelog we can recover that transaction otherwise we cannot recover that transaction which is not in backup.

> What is the difference between HOTBACKUP and RMAN backup ?
For hotbackup we have to put database in begin backup mode, then take backup where as RMAN would not put database in begin backup mode. RMAN is faster can perform incremental (changes only) backup, and does not place tablespace in hotbackup mode.

> Can we use Same target database as Catalog database ?
No, the recovery catalog should not reside in the target database (database to be backed up) because the database can not be recovered in the mounted state.

> Why RMAN incremental backup fails even though full backup exists ?
If you have taken the RMAN full backup using the command 'Backup database', where as a level 0 backup is physically identical to a full backup. The only difference is that the level 0 backup is recorded as an incremental backup in the RMAN repository so it can be used as the parent for a level 1 backup. Simply the 'full backup without level 0' can not be considered as a parent backup from which you can take level 1 backup.

> Can we perform RMAN level 1 backup without level 0 ?
If no level 0 is available, then the behavior depends upon the compatibility mode setting (oracle version).
If the compatibility mode less than 10.0.0, RMAN generates a level 0 backup of files contents at the time of backup.
If the compatibility is greater than 10.0.0, RMAN copies all block changes since the file was created, and stores the results as level 1 backup.

> How to put Manual/User managed backup in RMAN ?
In case of recovery catalog, you can put by using catalog command:
RMAN> CATALOG START WITH '/oracle/backup.ctl';

> How to check RMAN version in oracle ?
If you want to check RMAN catalog version then use the below query from SQL*plus
SQL> Select * from rcver;

> What happens actually in case of instance Recovery ?
While Oracle instance fails, Oracle performs an Instance Recovery when the associated database is being re-started. Instance recovery occurs in 2 steps:
Cache recovery: Changes being made to a database are recorded in the database buffer cache as well as redo log files simultaneously. When there are enough data in the database buffer cache, they are written to data files. If an Oracle instance fails before these data are written to data files, Oracle uses online redo log files to recover the lost data when the associated database is re-started. This process is called cache recovery.
Transaction recovery: When a transaction modifies data in a database (the before image of the modified data is stored in an undo segment which is used to restore the original values in case the transaction is rolled back). At the time of an instance failure, the database may have uncommitted transactions. It is possible that changes made by these uncommitted transactions have gotten saved in data files. To maintain read consistency, Oracle rolls back all uncommitted transactions when the associated database is re-started. Oracle uses the undo data stored in undo segments to accomplish this. This process is called transaction recovery.

> When you moved oracle binary files from one ORACLE_HOME server to another server then which oracle utility will be used to make this new ORACLE_HOME usable ?
Relink all.

> In which months oracle release CPU patches ?

> When we applying single Patch, can you use opatch utility ?
Yes, you can use Opatch incase of single patch. The only type of patch that cannot be used with OPatch is a patchset.

> Is it possible to apply OPATCH without downtime ?
As you know for apply patch your database and listener must be down. When you apply OPTACH it will update your current ORACLE_HOME. Thus coming to your question to the point in fact it is not possible without or zero downtime in case of single instance but in RAC you can Apply Opatch without downtime as there will be more separate ORACLE_HOME and more separate instances (running once instance on each ORACLE_HOME).

> You have collection of patch (nearly 100 patches) or patchset. How can you apply only one patch from it ?
With Napply itself (by providing patch location and specific patch id) you can apply only one patch from a collection of extracted patch. For more information check the opatch util NApply –help. It will give you clear picture.
For Example:
opatch util napply <patch_location> -id 9 -skip_subset -skip_duplicate
This will apply only the patch id 9 from the patch location and will skip duplicate and subset of patch installed in your ORACLE_HOME.

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