3+Years Oracle DBA Interview Questions

February 9, 2011 Mario Alcaide Leave a comment Go to comments

In response for a reader of this blog who asked me for some interview questions for a 3+Year experienced DBA, I have written this series of questions that you can use in your interviews. Either if you are the interviewer, or the interviewer. Enjoy!

1. Basic (Every DBA should answer correctly ALL these questions. This knowledge is just basic for a 3+ year experienced DBA)1.1 Q- Which are the default passwords of SYSTEM/SYS?A-  MANAGER / CHANGE_ON_INSTALL1.2 Q- How can you execute a script file in SQLPLUS?A- To execute a script file in SQLPlus, type @ and then the file name.1.3 Q- Where can you find official Oracle documentation?A- tahiti.oracle.com1.4 Q- What is the address of the Official Oracle Support?A- metalink.oracle.com or support.oracle.com1.5 Q- What file will you use to establish Oracle connections from a remote client?A- tnsnames.ora1.6 Q- How can you check if the database is accepting connections?A- lsnrctl status or lsnrctl services1.7 Q- Which log would you check if a database has a problem?A- Alert log1.8 Q- Name three clients to connect with Oracle, for example, SQL Developer:A- SQL Developer, SQL-Plus, TOAD, dbvisualizer, PL/SQL Developer… There are several, but an experienced dba should know at least three clients.1.9 Q- How can you check the structure of a table from sqlplus?A- DESCRIBE or DESC1.10 Q- What command will you start to run the installation of Oracle software on Linux?A- runInstaller

2. Moderate (Standard knoledge for a daily-work of every DBA. He could fail one or two questions, but not more)2.1 Q- What should you do if you encounter an ORA-600?A- Contact Oracle Support2.2 Q- Explain the differences between PFILE and SPFILEA- A PFILE is a Static, text file that initialices the database parameter in the moment that it’s started. If you want to modify parameters in PFILE, you have to restart the database.A SPFILE is a dynamic, binary file that allows you to overwrite parameters while the database is already started (with some exceptions)2.3 Q- In which Oracle version was Data Pump introduced?A- Oracle 10g2.4 Q- Say two examples of DML, two of DCL and two of DDLA- DML: SELECT, INSERT, UPDATE, DELETE, MERGE, CALL, EXPLAIN PLAN, LOCK TABLE

DDL: CREATE, ALTER, DROP, TRUNCATE, COMMENT, RENAMEDCL: GRANT, REVOKE2.5 Q- You want to save the output of an Oracle script from sqlplus. How would you do it?A- spool script_name.txtselect * from your_oracle_operations;spool off;2.6 Q- What is the most important requirement in order to use RMAN to make consistent hot backups?A- Your database has to be in ARCHIVELOG mode.2.7 Q- Can you connect to a local database without a listener?A- Yes, you can.2.8 Q- In which view can you find information about every view and table of oracle dictionary?A- DICT or DICTIONARY2.9 Q- How can you view all the users account in the database?A- SELECT USERNAME FROM DBA_USERS;2.10 Q- In linux, how can we change which databases are started during a reboot?A- Edit /etc/oratab

3. Advanced (A 3+ year experienced DBA should have enough knowledge to answer these questions. However, depending on the work he has done, he could still fail up to 4 questions)3.1 Q- When a user process fails, what Oracle background process will clean after it?A- PMON3.2 Q- How can you reduce the space of TEMP datafile?A- Prior to Oracle 11g, you had to recreate the datafile. In Oracle 11g a new feature was introduced, and you can shrink the TEMP tablespace.3.3 Q- How can you view all the current users connected in your database in this moment?A- SELECT COUNT(*),USERNAME FROM V$SESSION GROUP BY USERNAME;3.4 Q- Explain the differences between SHUTDOWN, SHUTDOWN NORMAL, SHUTDOWN IMMEDIATE AND SHUTDOWN ABORTA- SHUTOWN NORMAL = SHUTDOWN : It waits for all sessions to end, without allowing new connections.SHUTDOWN IMMEDIATE : Rollback current transactions and terminates every session.SHUTDOWN ABORT : Aborts all the sessions, leaving the database in an inconsistent state. It’s the fastest method, but can lead to database corruption.3.5 Q- Is it possible to backup your database without the use of an RMAN database to store the catalog?A- Yes, but the catalog would be stored in the controlfile.3.6 Q- Which are the main components of Oracle Grid Control?A- OMR (Oracle Management Repository), OMS (Oracle Management Server) and OMA (Oracle Management Agent).3.7 Q- What command will you use to navigate through ASM files?A- asmcmd3.8 Q- What is the difference between a view and a materialized view?A- A view is a select that is executed each time an user accesses to it. A materialized view stores the result of this query in memory for faster access purposes.3.9 Q- Which one is faster: DELETE or TRUNCATE?

A- TRUNCATE3.10 Q- Are passwords in oracle case sensitive?A- Only since Oracle 11g.

4. RAC (Only intended for RAC-specific DBAs, with varied difficultied questions)4.1 Q- What is the recommended method to make backups of a RAC environment?A- RMAN to make backups of the database, dd to backup your voting disk and hard copies of the OCR file.4.2 Q- What command would you use to check the availability of the RAC system?A- crs_stat -t -v (-t -v are optional)4.3 Q- What is the minimum number of instances you need to have in order to create a RAC?A- 1. You can create a RAC with just one server.4.4 Q- Name two specific RAC background processesA- RAC processes are: LMON, LMDx, LMSn, LKCx and DIAG.4.5 Q- Can you have many database versions in the same RAC?A- Yes, but Clusterware version must be greater than the greater database version.4.6 Q- What was RAC previous name before it was called RAC?A- OPS: Oracle Parallel Server4.7 Q- What RAC component is used for communication between instances?A- Private Interconnect.4.8 Q- What is the difference between normal views and RAC views?A- RAC views has the prefix ‘G’. For example, GV$SESSION instead of V$SESSION4.9 Q- Which command will we use to manage (stop, start…) RAC services in command-line mode?A- srvctl4.10 Q- How many alert logs exist in a RAC environment?A- One for each instance.

5. Master (A 3+ year experienced DBA would probably fail these questions, they are very specifid and specially difficult. Be glad if he’s able to answer some of them)5.1 Q- How can you difference a usual parameter and an undocumented parameter?A- Undocumented parameters have the prefix ‘_’. For example, _allow_resetlogs_corruption5.2 Q- What is BBED?A- An undocumented Oracle tool used for foresnic purposes. Stans for Block Browser and EDitor.5.3 Q- The result of the logical comparison (NULL = NULL) will be… And in the case of (NULL != NULL)A- False in both cases.5.4 Q- Explain Oracle memory structureThe Oracle RDBMS creates and uses storage on the computer hard disk and in random access memory (RAM). The portion in the computer s RAM is called memory structure. Oracle has two memory structures in the computer s RAM. The two structures are the Program Global Area (PGA) and the System Global Area (SGA).

The PGA contains data and control information for a single user process. The SGA is the memory segment that stores data that the user has retrieved from the database or data that the

user wants to place into the database.5.5 Q- Will RMAN take backups of read-only tablespaces?A- No5.6 Q- Will a user be able to modify a table with SELECT only privilege?A- He won’t be able to UPDATE/INSERT into that table, but for some reason, he will still be able to lock a certain table.5.7 Q- What Oracle tool will you use to transform datafiles into text files?A- Trick question: you can’t do that, at least with any Oracle tool. A very experienced DBA should perfectly know this.5.8 Q- SQL> SELECT * FROM MY_SCHEMA.MY_TABLE;SP2-0678: Column or attribute type can not be displayed by SQL*PlusWhy I’m getting this error?A- The table has a BLOB column.5.9 Q- What parameter will you use to force the starting of your database with a corrupted resetlog?A- _ALLOW_RESETLOGS_CORRUPTION5.10 Q- Name the seven types of Oracle tablesA- Heap Organized Tables, Index Organized Tables, Index Clustered Tables, Hash Clustered Tables, Nested Tables, Global Temporary Tables, Object Tables.

DBA   Tasks

December 3, 2010 Mario Alcaide 1 comment

We can separate the dba tasks between daily, weekly and monthly procedures:

Daily- Verify instance status- Check alerts- Check configured metrics- Check RMAN backups- Check storage- Check CPU contention- Check waiting times- Check memory usage- Check network load- Check iostat

Weekly

- Invalid objects- Tunning: indexes and execution plans- Top SQL- Environments consistence- Review of ressource policy- Trends and peaks- Cleaning of alert logs- Review of RMAN

Monthly

- Recovery tests- Analyze the data increment trend- Tunning- Review I/O- Fragmentation- Row chaining- High Availability Analysis- Scalability- Schedule monthly downtime

Default passwords in   Oracle

November 26, 2010 Mario Alcaide Leave a comment

Have you ever asked yourself which are the default passwords in Oracle?

Here is the list:

SYS: CHANGE_ON_INSTALLSYSTEM: MANAGERSCOTT: TIGERADAMS: WOODJONES: STEELBLAKE: PAPERFORD: CARKING: GOLD

\

What is the data   dictionary?

July 8, 2010 Mario Alcaide Leave a comment

The scope of this article is to clarify what exactly is the data dictionary. Probably all of you have heard of it, and you know it’s a bunch of metadata that Oracle uses to work, but if you want to know more about this Data Dictionary, then I hope this article teaches you something new.

The data dictionary is a repository of metadata (Information about information), about all the information inside the database. This repository is owned by SYS, and is stored principally in the SYSTEM tablespace, though some components are stored in the SYSAUX tablespace (in Oracle 10+).

The data dictionary is composed of tables and views. Some of these tables are inside of the Oracle kernel, so you would never work directly with them unless you are working for Oracle support or performing a disaster recovery scenario. But instead you can access to the views in order to know the “information about the information”.

For example, a possible usage of this data dictionary would be to know all the tables owned by a single user, or the list or relationships between all the tables of the database.

The main view of the data dictionary is the view DICT (or DICTIONARY):

SQL> DESC DICTNombre                                    ?Nulo?   Tipo—————————————– ——– —————————-TABLE_NAME                                         VARCHAR2(30)COMMENTS                                           VARCHAR2(4000)

Through the DICT view, you can access to all the data dictionary views that could provide you the information that you need.

For example, if you are looking for information related to db_links, but you don’t know where to look for, then query the DICT view:

SQL>  SELECT TABLE_NAME FROM DICT WHERE TABLE_NAME LIKE ‘%LINK%’;

TABLE_NAME——————————USER_DB_LINKSALL_DB_LINKSDBA_DB_LINKS

V$DBLINKGV$DBLINK

Now, you have just to query one of these views to find the data you are looking for. GV$ views are very useful when you are working with RAC, and V$ views are instance related.

Remember that the data dictionary provides critical information of the database, and it should be restricted to users. However, if a user really needs to query the data dictionary, you can use the following sentence:

GRANT SELECT_CATALOG_ROLE TO user_name;

How would you determine the time zone under which a database was operating?

select DBTIMEZONE from dual;

Related Articles

Oracle Technical Interview Questions Answered - Part1 Oracle Technical Interview

22.  Explain the use of setting GLOBAL_NAMES equal to TRUE.

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.

23.  What command would you use to encrypt a PL/SQL application?

WRAP

24.  Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.

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.

25.  Explain the use of table functions.

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.

26.  Name three advisory statistics you can collect.

Buffer Cache Advice, Segment Level Statistics, & Timed Statistics

27.  Where in the Oracle directory tree structure are audit traces placed?

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

28.  Explain materialized views and how they are used.

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.

29.  When a user process fails, what background process cleans up after it?

PMON

30.  What background process refreshes materialized views?

The Job Queue Processes.

31.  How would you determine what sessions are connected and what resources they are waiting for?

Use of V$SESSION and V$SESSION_WAIT

32.  Describe what redo logs are.

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.

33.  How would you force a log switch?

ALTER SYSTEM SWITCH LOGFILE;

34.  Give two methods you could use to determine what DDL changes have been made.

You could use Logminer or Streams

35.  What does coalescing a tablespace do?

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

36.  What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace?

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.

37.  Name a tablespace automatically created when you create a database.

The SYSTEM tablespace.

38.  When creating a user, what permissions must you grant to allow them to connect to the database?

Grant the CONNECT to the user.

39.  How do you add a data file to a tablespace?

ALTER TABLESPACE <tablespace_name> ADD DATAFILE <datafile_name> SIZE <size>

40.  How do you resize a data file?

ALTER DATABASE DATAFILE <datafile_name> RESIZE <new_size>;

41.  What view would you use to look at the size of a data file?

DBA_DATA_FILES

42.  What view would you use to determine free space in a tablespace?

DBA_FREE_SPACE

43.  How would you determine who has added a row to a table?

Turn on fine grain auditing for the table.

44.  How can you rebuild an index?

ALTER INDEX <index_name> REBUILD;

45.  Explain what partitioning is and what its benefit is.

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

46.  You have just compiled a PL/SQL package but got errors, how would you view the errors?

SHOW ERRORS

47.  How can you gather statistics on a table?

The ANALYZE command.

48.  How can you enable a trace for a session?

Use the DBMS_SESSION.SET_SQL_TRACE or

Use ALTER SESSION SET SQL_TRACE = TRUE;

49.  What is the difference between the SQL*Loader and IMPORT utilities?

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.

50.  Name two files used for network connection to a database.

TNSNAMES.ORA and SQLNET.ORA

Technical - UNIX

Every DBA should know something about the operating system that the database will be running on. The questions here are related to UNIX but you should equally be able to answer questions related to common Windows environments.

1.  How do you list the files in an UNIX directory while also showing hidden files?

ls -ltra

2.  How do you execute a UNIX command in the background?

Use the "&"

3.  What UNIX command will control the default file permissions when files are created?

Umask

4.  Explain the read, write, and execute permissions on a UNIX directory.

Read allows you to see and list the directory contents.

Write allows you to create, edit and delete files and subdirectories in the directory.

Execute gives you the previous read/write permissions plus allows you to change into the directory and execute programs or shells from the directory.

5.  the difference between a soft link and a hard link?

A symbolic (soft) linked file and the targeted file can be located on the same or different file system while for a hard link they must be located on the same file system.

6.  Give the command to display space usage on the UNIX file system.

df -lk

7.  Explain iostat, vmstat and netstat.

Iostat reports on terminal, disk and tape I/O activity.

Vmstat reports on virtual memory statistics for processes, disk, tape and CPU activity.

Netstat reports on the contents of network data structures.

8.  How would you change all occurrences of a value using VI?

Use :%s/<old>/<new>/g

9.  Give two UNIX kernel parameters that effect an Oracle install

SHMMAX & SHMMNI

10.  Briefly, how do you install Oracle software on UNIX.

Basically, set up disks, kernel parameters, and run orainst.

I hope that these interview questions were not too hard. Remember these are "core" DBA questions and not necessarily related to the Oracle options that you may encounter in some interviews. Take a close look at the requirements for any job and try to extract questions that interviewers may ask from manuals and real life experiences. For instance, if they are looking for a DBA to run their databases in RAC environments, you should try to determine what hardware and software they are using BEFORE you get to the interview. This would allow you to brush up on particular environments and not be caught off-guard. Good luck!

Oracle Technical Interview Questions Answered - Part2james koopmann | Nov 3, 2005 | Comments (40)

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1

I promised the second part of this a couple of weeks ago. Well here it is. Again this is an article I put together a while back but still get quite a bit of email in its regard. So thought it might be of interest to the readers at ITtoolbox.

The Oracle Technical Interview can be quite daunting. You never quite know what to study for and how to prepare. I am fully aware of this, as I have received many emails since my original article on interview questions was released. While these questions are only guidelines as to what should and more than likely will be asked, I hope that you find some comfort in the review of them. As always, do not just memorize the answers, as there are jewels to be found in the quest of figuring out the answer from the question. As always, remember that as you go through the article, it is not enough to know the answer to a particular question; you must try to put yourself in an interview situation and experience answering the question for yourself. Therefore, after you have gone through the questions and answers read the question again and then answer it with your own words. As always, good luck, and cheers.Technical - Oracle

Last time, we answered questions 1 thru 20 of the technical part of the interview. Here are the next 30 in this section. Depending on the mood of the interview and your ability to elaborate on the answer, try to give some insight that you know more than just the simple answer to some of these questions. Also, be sensitive to the interviewer getting tired of you talking too much. Well here they are.

21. How would you determine the time zone under which a database was operating?

select DBTIMEZONE from dual;

22. Explain the use of setting GLOBAL_NAMES equal to TRUE.

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.

23. What command would you use to encrypt a PL/SQL application?

WRAP

24. Explain the difference between a FUNCTION, PROCEDURE and PACKAGE.

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.

25. Explain the use of table functions.

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.

26. Name three advisory statistics you can collect.

Buffer Cache Advice, Segment Level Statistics, & Timed Statistics

27. Where in the Oracle directory tree structure are audit traces placed?

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

28. Explain materialized views and how they are used.

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.

29. When a user process fails, what background process cleans up after it?

PMON

30. What background process refreshes materialized views?

The Job Queue Processes.

31. How would you determine what sessions are connected and what resources they are waiting for?

Use of V$SESSION and V$SESSION_WAIT

32. Describe what redo logs are.

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.

33. How would you force a log switch?

ALTER SYSTEM SWITCH LOGFILE;

34. Give two methods you could use to determine what DDL changes have been made.

You could use Logminer or Streams

35. What does coalescing a tablespace do?

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

36. What is the difference between a TEMPORARY tablespace and a PERMANENT tablespace?

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.

37. Name a tablespace automatically created when you create a database.

The SYSTEM tablespace.

38. When creating a user, what permissions must you grant to allow them to connect to the database?

Grant the CONNECT to the user.

39. How do you add a data file to a tablespace?

ALTER TABLESPACE ADD DATAFILE SIZE

40. How do you resize a data file?

ALTER DATABASE DATAFILE RESIZE ;

41. What view would you use to look at the size of a data file?

DBA_DATA_FILES

42. What view would you use to determine free space in a tablespace?

DBA_FREE_SPACE

43. How would you determine who has added a row to a table?

Turn on fine grain auditing for the table.

44. How can you rebuild an index?

ALTER INDEX REBUILD;

45. Explain what partitioning is and what its benefit is.

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

46. You have just compiled a PL/SQL package but got errors, how would you view the errors?

SHOW ERRORS

47. How can you gather statistics on a table?

The ANALYZE command.

48. How can you enable a trace for a session?

Use the DBMS_SESSION.SET_SQL_TRACE or

Use ALTER SESSION SET SQL_TRACE = TRUE;

49. What is the difference between the SQL*Loader and IMPORT utilities?

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.

50. Name two files used for network connection to a database.

TNSNAMES.ORA and SQLNET.ORATechnical - UNIX

Every DBA should know something about the operating system that the database will be running on. The questions here are related to UNIX but you should equally be able to answer questions related to common Windows environments.

1. How do you list the files in an UNIX directory while also showing hidden files?

ls -ltra

2. How do you execute a UNIX command in the background?

Use the "&"

3. What UNIX command will control the default file permissions when files are created?

Umask

4. Explain the read, write, and execute permissions on a UNIX directory.

Read allows you to see and list the directory contents.

Write allows you to create, edit and delete files and subdirectories in the directory.

Execute gives you the previous read/write permissions plus allows you to change into the directory and execute programs or shells from the directory.

5. the difference between a soft link and a hard link?

A symbolic (soft) linked file and the targeted file can be located on the same or different file system while for a hard link they must be located on the same file system.

6. Give the command to display space usage on the UNIX file system.

df -lk

7. Explain iostat, vmstat and netstat.

Iostat reports on terminal, disk and tape I/O activity.

Vmstat reports on virtual memory statistics for processes, disk, tape and CPU activity.

Netstat reports on the contents of network data structures.

8. How would you change all occurrences of a value using VI?

Use :%s///g

9. Give two UNIX kernel parameters that effect an Oracle install

SHMMAX & SHMMNI

10. Briefly, how do you install Oracle software on UNIX.

Basically, set up disks, kernel parameters, and run orainst.

I hope that these interview questions were not too hard. Remember these are "core" DBA questions and not necessarily related to the Oracle options that you may encounter in some interviews. Take a close look at the requirements for any job and try to extract questions that interviewers may ask from manuals and real life experiences. For instance, if they are looking for a DBA to run their databases in RAC environments, you should try to determine what hardware and software they are using BEFORE you get to the interview. This would allow you to brush up on particular environments and not be caught off-guard. Good luck!