Oracle DBA Troubleshooting Interview Questions & Answers

Question 1. Explain The Difference Between A Hot Backup And A Cold Backup And The Benefits Associated With Each?

Answer :

A hot backup is basically taking a backup of the database while it is still up and running and it must be in archive log mode. A cold backup is taking a backup of the database while it is shut down and does not require being in archive log mode. The benefit of taking a hot backup is that the database is still available for use while the backup is occurring and you can recover the database to any point in time. The benefit of taking a cold backup is that it is typically easier to administer the backup and recovery process. In addition, since you are taking cold backups the database does not require being in archive log mode and thus there will be a slight performance gain as the database is not cutting archive logs to disk.

Question 2. You Have Just Had To Restore From Backup And Do Not Have Any Control Files. How Would You Go About Bringing Up This Database?

Answer :

I would create a text based backup control file, stipulating where on disk all the data files where and then issue the recover command with the using backup control file clause.

Question 3. How Do You Switch From An Init.ora File To A Spfile?

Answer :

Issue the create spfile from pfile command.

Question 4. Explain The Difference Between A Data Block, An Extent And A Segment.?

Answer :

A data block is the smallest unit of logical storage for a database object. As objects grow they take chunks of additional storage that are composed of contiguous data blocks. These groupings of contiguous data blocks are called extents. All the extents that an object takes when grouped together are considered the segment of the database object.

Question 5. Give Two Examples Of How You Might Determine The Structure Of The Table Dept?

Answer :

Use the describe command or use the dbms_metadata.get_ddl package.

Question 6. Where Would You Look For Errors From The Database Engine?

Answer :

In the alert log.

Question 7. Compare And Contrast Truncate And Delete For A Table?

Answer :

Both the truncate and delete command have the desired outcome of getting rid of all the rows in a table. The difference between the two is that the truncate command is a DDL operation and just moves the high water mark and produces a now rollback. The delete command, on the other hand, is a DML operation, which will produce a rollback and thus take longer to complete.

Question 8. Give The Reasoning Behind Using An Index?

Answer :

Faster access to data blocks in a table.

Question 9. Give The Two Types Of Tables Involved In Producing A Star Schema And The Type Of Data They Hold?

Answer :

Fact tables and dimension tables. A fact table contains measurements while dimension tables will contain data that will help describe the fact tables.

Question 10. What Type Of Index Should You Use On A Fact Table?

Answer :

Bitmap index.

Question 11. Give Two Examples Of Referential Integrity Constraints?

Answer :

primary key and a foreign key.

Question 12. A Table Is Classified As A Parent Table And You Want To Drop And Re-create It. How Would You Do This Without Affecting The Children Tables?

Answer :

Disable the foreign key constraint to the parent, drop the table, re-create the table, enable the foreign key constraint.

Question 13. Explain The Difference Between Archivelog Mode And Noarchivelog Mode And The Benefits And Disadvantages To Each?

Answer :

ARCHIVELOG mode is a mode that you can put the database in for creating a backup of all transactions that have occurred in the database so that you can recover to any point in time. NOARCHIVELOG mode is basically the absence of ARCHIVELOG mode and has the disadvantage of not being able to recover to any point in time.
NOARCHIVELOG mode does have the advantage of not having to write transactions to an archive log and thus increases the performance of the database slightly.

Question 14. What Command Would You Use To Create A Backup Control File?

Answer :

Alter database backup control file to trace.

Question 15. Give The Stages Of Instance Start Up To A Usable State Where Normal Users May Access It?

Answer :

• STARTUP NOMOUNT – Instance startup
• STARTUP MOUNT – The database is mounted
• STARTUP OPEN – The database is opened

Question 16. What Column Differentiates The V$ Views To The Gv$ Views And How?

Answer :

The INST_ID column which indicates the instance in a RAC environment the information came from.

Question 17. How Would You Go About Generating An Explain Plan?

Answer :

• Create a plan table with utlxplan.sql.
• Use the explain plan set statement_id = ‘tst1’ into plan_table for a SQL statement
• Look at the explain plan with utlxplp.sql or utlxpls.sql

Question 18. How Would You Go About Increasing The Buffer Cache Hit Ratio?

Answer :

Use the buffer cache advisory over a given workload and then query the v$db_cache_advice table. If a change was necessary then I would use the alter system set db_cache_size command.

Question 19. Explain An Ora-01555?

Answer :

You get this error when you get a snapshot too old within rollback. It can usually be solved by increasing the undo retention or increasing the size of rollbacks. You should also look at the logic involved in the application getting the error message.

Question 20. Explain The Difference Between $oracle_home And $oracle_base?

Answer :

ORACLE_BASE is the root directory for oracle. ORACLE_HOME located beneath ORACLE_BASE is where the oracle products reside.

Question 21. How Would You Determine The Time Zone Under Which A Database Was Operating?

Answer :

select DBTIMEZONE from dual;

Question 22. Explain The Use Of Setting Global_names Equal To True?

Answer :

Setting GLOBAL_NAMES dictates how you might connect to a database. This variable is either TRUE or FALSE and if it is set to TRUE it enforces database links to have the same name as the remote database to which they are linking.

Question 23. What Command Would You Use To Encrypt A Pl/sql Application?

Answer :

WRAP

Question 24. Explain The Difference Between A Function, Procedure And Package?

Answer :

A function and procedure are the same in that they are intended to be a collection of PL/SQL code that carries a single task. While a procedure does not have to return any values to the calling application, a function will return a single value. A package on the other hand is a collection of functions and procedures that are grouped together based on their commonality to a business function or application.

Question 25. Explain The Use Of Table Functions?

Answer :

Table functions are designed to return a set of rows through PL/SQL logic but are intended to be used as a normal table or view in a SQL statement. They are also used to pipeline information in an ETL process.

Question 26. Name Three Advisory Statistics You Can Collect?

Answer :

• Buffer Cache Advice
• Segment Level Statistics
• Timed Statistics

Question 27. Where In The Oracle Directory Tree Structure Are Audit Traces Placed?

Answer :

In unix $ORACLE_HOME/rdbms/audit, in Windows the event viewer

Question 28. Explain Materialized Views And How They Are Used?

Answer :

Materialized views are objects that are reduced sets of information that have been summarized, grouped, or aggregated from base tables. They are typically used in data warehouse or decision support systems.

Question 29. When A User Process Fails, What Background Process Cleans Up After It?

Answer :

PMON

Question 30. What Background Process Refreshes Materialized Views?

Answer :

The Job Queue Processes.

Question 31. How Would You Determine What Sessions Are Connected And What Resources They Are Waiting For?

Answer :

Use of V$SESSION and V$SESSION_WAIT

Question 32. Describe What Redo Logs Are?

Answer :

Redo logs are logical and physical structures that are designed to hold all the changes made to a database and are intended to aid in the recovery of a database.

Question 33. How Would You Force A Log Switch?

Answer :

ALTER SYSTEM SWITCH LOGFILE;

Question 34. Give Two Methods You Could Use To Determine What Ddl Changes Have Been Made?

Answer :

You could use Logminer or Streams

Question 35. What Does Coalescing A Tablespace Do?

Answer :

Coalescing is only valid for dictionary-managed tablespaces and de-fragments space by combining neighboring free extents into large single extents.

Question 36. What Is The Difference Between A Temporary Tablespace And A Permanent Tablespace?

Answer :

A temporary tablespace is used for temporary objects such as sort structures while permanent tablespaces are used to store those objects meant to be used as the true objects of the database.

Question 37. Name A Tablespace Automatically Created When You Create A Database?

Answer :

The SYSTEM tablespace.

Question 38. When Creating A User, What Permissions Must You Grant To Allow Them To Connect To The Database?

Answer :

Grant the CONNECT to the user.

Question 39. How Do You Add A Data File To A Tablespace?

Answer :

ALTER TABLESPACE ADD DATAFILE SIZE

Question 40. How Do You Resize A Data File?

Answer :

ALTER DATABASE DATAFILE RESIZE ;

Question 41. What View Would You Use To Look At The Size Of A Data File?

Answer :

DBA_DATA_FILES

Question 42. What View Would You Use To Determine Free Space In A Tablespace?

Answer :

DBA_FREE_SPACE

Question 43. How Would You Determine Who Has Added A Row To A Table?

Answer :

Turn on fine grain auditing for the table.

Question 44. How Can You Rebuild An Index?

Answer :

ALTER INDEX REBUILD;

Question 45. Explain What Partitioning Is And What Its Benefit Is?

Answer :

Partitioning is a method of taking large tables and indexes and splitting them into smaller, more manageable pieces.

Question 46. You Have Just Compile A Pl/sql Package But Got Errors, How Would You View The Errors?

Answer :

SHOW ERRORS

Question 47. How Can You Gather Statistics On A Table?

Answer :

The ANALYZE command.

Question 48. How Can You Enable A Trace For A Session?

Answer :

Use the DBMS_SESSION.SET_SQL_TRACE or Use ALTER SESSION SET SQL_TRACE = TRUE;

Question 49. What Is The Difference Between The Sql*loader And Import Utilities?

Answer :

These two Oracle utilities are used for loading data into the database. The difference is that the import utility relies on the data being produced by another Oracle utility EXPORT while the SQL*Loader utility allows data to be loaded that has been produced by other utilities from different data sources just so long as it conforms to ASCII formatted or delimited files.

Question 50. Name Two Files Used For Network Connection To A Database?

Answer :

TNSNAMES.ORA and SQLNET.ORA

Question 51. What Are Dynamic Performance Views?

Answer :

Dynamic performance views are also called V$ views. These views provided information about the sessions.

Any Oracle user can get information from dynamic performance views if the user has the select any table privilege. This privilege is generally granted through the SELECT_CATALOG_ROLE etc.

Question 52. What Is The Statspack Tool?

Answer :

• The STATSPACK tool is an ORACLE supplied tool to monitor database performance.
• It can be used to diagnose instance wide problems both proactively and reactively.

Question 53. What Is Db File Sequential Read Wait Event?

Answer :

The db file sequential read wait event performs single block read operations against indexes, tables, control files, rollback segments and data file headers.

It has three parameters:

file#, firstblock# and block count.

Question 54. Explain Wait Events?

Answer :

• Wait event occurs when a user process is kept waiting because of some problem, such as an I/O bottleneck or a busy CPU.
• The information about wait event is available in the V$SYSTEM_WAIT and V$SESSION_WAIT dynamic performance views.

Question 55. What Is Cost Based Optimizer?

Answer :

• Cost based optimizer is the optimizer component of the Oracle, which is recommended and supported by Oracle.
• It determines query plans based on overall cost of usage of each resource to get the best possible plan with respect to the resource usage cost.
• It uses internal statistics to determine the best execution plan for the statement.

Question 56. Which Parameters Affect The Behaviour Of Merge Join?

Answer :

The behavior of merge join is influenced by the initialization parameters:

• sort_area_size and
• db_file_multiblock_read_count

Question 57. Why Union All Faster Than Union?

Answer :

The UNION operation removes redundancy while UNION ALL does not; therefore, the UNION operation needs to perform sort. As a result, UNION ALL performs better as it does not need to perform any sort.

Question 58. How Can You Get More Details About The Blocking Session?

Answer :

You can use the v$session or gv$session view in Real Application Clusters (RAC) environment to get the session information.

Question 59. What Is The Ora-01555: Snapshot Too Old Error? How Can It Be Avoided?

Answer :

The ORA-01555: snapshot too old error indicates that the query cannot find the snapshot it is looking for in the rollback segment.

Rollback segment is designed to hold data blocks that are being changed. It is required to hold old snapshot until the transaction is committed. However, it holds the data until the space is required for other transactions.

Question 60. How Do You Handle Ora – 01403: No Data Found Error?

Answer :

You can handle ORA- 01403: no data found error by terminating the processing for the SELECT statement.

Question 61. What Are Different Types Of Locks?

Answer :

There are two different types of locks, which are given as follows:

• System locks – Held for a very brief period of time and controlled by Oracle.
• User locks – Created and managed using dbms_lock package.
  
  Different types of user locks are given as follows:
• The UL lock – Defined with the dbms_lock package.
• The TX lock – Acquired once for every transaction. It is a row transaction lock.
• The TM lock – Acquired once for each object, which is being changed. It is a DML lock. The IDI column identifies the object being modified.

Question 62. What Is The Significance Of Latches With Respect To Performance Tuning?

Answer :

• An Oracle process must acquire relevant latch in order to get resource allocated.
• A latch is required for a very short amount of time to ensure that the resource is allocated.

Whenever there is a contention for latch, it indicates that there is a performance issue, which may be due to either of the two following reasons:

• Lack of availability of resource.
• Poor application programming resulting in high number of requests for resource.

Question 63. What Is The Use Of Alert Log File?

Answer :

The ALERT log is a log file that records database wide events. The information in the ALERT log file is generally used for trouble shooting.

Following events are recorded in the ALERT log file:

• Database shutdown and startup information
• All non default parameters
• Oracle internal (ORA – 600) errors
• Information about a modified control file
• At log switch
• The location of ALERT log file is specified in the BACKGROUND_DUMP_DEST parameter.

Question 64. What Is A Lock?

Answer :

Lock is a mechanism provided by Oracle to reserve a database object so that different sessions do not interfere in each other's work.

Locking helps in ensuring data consistency and maintaining database objects in usable state in a multi user environment. However, it can cause one session to block another.

Question 65. What Is A Latch? How It Is Used In Oracle?

Answer :

• A latch is a semaphore or an on/off switch in Oracle database that a process must access in order to conduct certain type of activities. Latches govern the usage of Oracle's internal resources by its processes.
• They enforce serial access to the resources and limit the amount of time for which a single process can use a resource.
• There are over 80 latches available in Oracle.

Question 66. What Are The Background Trace Files?

Answer :

Background trace files are associated with background processes and are generated when certain background process experiences an error.

The information in background trace files is generally used for trouble shooting.

Question 67. What Is The Difference Between Latches And Enqueues?

Answer :

• Enqueue is used to queue request for lock on any db object that can't be served immediately and session is ready to wait.
• Latches are internal locking mechanism of Oracle to provide short term exclusive access to Oracle's internal objects like library cache etc.
• Another difference is that enqueues follow first in first out (FIFO) algorithm while latches do not follow any such algorithm.

Question 68. What Is A User Process Trace File?

Answer :

A user process trace file is a trace file that is produced by user session. However, this is an optional file, which is generated if the user wants to generate the file. This file is generated when the value of SQL_TRACE parameter is set to TRUE for a session.

1. This parameter can be set at database, instance, or session level.
2. If it is instance level, trace file will be generated for all the connected sessions.
3. If it is set at session level, trace file will be generated only for the specified session.
4. The location of user process trace file is specified in the USER_DUMP_DEST parameter.
5. The information in user process trace file is generally used for trouble shooting.

Question 69. Which Tools Are Available To Monitor Performance?

Answer :

• Oracle offers Oracle Enterprise Manager (OEM) to monitor performance.
• OEM can also be used to startup and shutdown the instance. In addition, it can be used to manage database in general.

Question 70. What Is An Event?

Answer :

An event is an Oracle is an occurrence that substantially alters the way your database executes or performs.

There are two types of events in Oracle:

• wait events 
• OEM defined events.

Question 71. What Is The Main Reason For Block Corruption?

Answer :

Block corruption or physical corruption occurs when a block on a physical disk becomes unreadable or inconsistent to the state that the data is unusable. Block corruption can be caused by many different sources; and therefore, it is difficult to find the exact reason of block corruption.

However, it is mostly due to human error with the use of software (patches) firmware, or hardware bugs.

You can avoid this by testing all the hardware and software patches thoroughly in the test environment. In addition, you can use mirrored disk to protect your data.

Question 72. What Do You Understand By Db File Scattered Read?

Answer :

• Db file scattered read indicates a scatter read into multiple discontinuous locations.
• It generally indicates full table scan and that multiple blocks are being read into memory.
• Such reads are called scattered read calls, because the blocks are scattered throughout memory.

Question 73. What Do The Db_file_sequential_read And Db_file_scattered_read Events Indicate?

Answer :

• The db_file_sequential_read event generally indicates index usage and shows an access by rowid while the db_file_scattered_read event indicates full table scan.
• A single block is read at one time in the db_file_sequential_read event while multiple blocks are read parallel in the db_file_scattered_read event.

Question 74. Which Trace File Is Used For Performance Tuning And Why?

Answer :

User process trace file is used for performance tuning because it contains information about execution plan and resource consumption. This information can be used for performance tuning.

These files are located in the directory specified in the BACKGROUND_DUMP_DIRECTORY parameter.

Question 75. How Can You Monitor Performance Of The Database Proactively?

Answer :

• Several tools, such as Oracle Enterprise Manager, and utilities from third party are available for monitoring database performance.
• However, all these tools or utilities depend on statistics gathered by Oracle, which are available through dynamic performance views.

Question 76. Describe The Oracle Wait Interface?

Answer :

• The Oracle Wait Interface is the set of data dictionary tables that store information about wait events.
• Oracle offers multiple views to give information about wait events, such as v$system_event and v$session_event.
• You can get the information about the wait events for the database or a specific session from these views and find out the event, which seems too high.

Question 77. Name A Few Places You Will Look To Get More Details On A Performance Issue?

Answer :

Oracle records the information about different kind of errors and the processes in the files, such as ALERT log, user process trace files and background process trace files.

Question 78. What Are The Different Types Of Locking Modes?

Answer :

Lock modes vary from 0 to 6 in order of increasing exclusively, given as follows:

• No lock (0)
• Shared Row Exclusive lock (3)
• Shared lock (4)
• Exclusive lock (6)

You can get the information on modes of TM and TX locks by using the LMODE and REQUEST columns. Both the locks use the same numbering for lock modes.

• DML requires a Shared Row Exclusive lock, that is, TM-3.
• DDL requires an Exclusive lock, that is, TM-6.

Question 79. How Do You Handle Ora-01403: No Data Found Error?

Answer :

You can handle ORA- 01403: no data found error by terminating the processing for the select statement.

Question 80. How Can You Detect Block Corruption?

Answer :

There are four methods for detecting block corruption, which are given as follows:

1. The ANALYZE_TABLE <table_name> VALIDATE STRUCTURE command
2. The Oracle DBVERIFY utility against the offline datafiles
3. The init.ora parameter DB_BLOCK_CHECKING, whihc checks data and index_blocks each time they are created or modified.
4. THE DBMS_REPAIR packages used against a table, index or partition.