ORACLE DBA INTERVIEW QUESTIONS

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 file.

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: A Bitmap index.

Question 11. Give two examples of referential integrity constraints.??
Answer:A 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, and 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 CONTROLFILE TO TRACE

Question 15. Give the stages of instance startup to a usable state where normal users may access
it.??
Answer: STARTUP NOMOUNT - Instance startup
ALTER DATABASE MOUNT - The database is mounted
ALTER DATABASE 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 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 18. 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 19. 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 20. How would you determine the time zone under which a database was operating?
Answer: select DBTIMEZONE from dual;

Question 21. 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 22. What command would you use to encrypt a PL/SQL application?
Answer: WRAP

Question 23. 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 24. 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 25. Name three advisory statistics you can collect.??
Answer: Buffer Cache Advice, Segment Level Statistics, & Timed Statistics

Question 26. Where in the Oracle directory tree structure are audit traces placed?
Answer: In unix $ORACLE_HOME/rdbms/audit,
in Windows the event viewer

Question 27. 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 28. When a user process fails, what background process cleans up after it?
Answer: PMON

Question 29. What background process refreshes materialized views?
Answer: The Job Queue Processes.