Oracle Database 9i Interview Questions

Intention of this post is to get as much interview question for Oracle DBA as possible to help you get the idea about the type of questions you can expect in interviews and exams. This will help you in increasing the knowledge about oracle database 9i as some of the questions may be new to you. This post has questions related to oracle 9i DBA only.
1) Explain the difference between a hot backup and a cold backup and the benefits associated with each.

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.

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?

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.

3) How do you switch from an init.ora file to a spfile?

Issue the create spfile from pfile command.

4) Explain the difference between a data block, an extent and a segment.

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. Extent may not be continuous.

5) Give two examples of how you might determine the structure of the table DEPT.

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

6) Where would you look for errors from the database engine?

In the alert log.

7) Compare and contrast TRUNCATE and DELETE for a table.

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.

8) Give the reasoning behind using an index.

Faster access to data blocks in a table.

9) Give the two types of tables involved in producing a star schema and the type of data they hold.

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

10) What type of index should you use on a fact table?

A Bitmap index.

11) Give two examples of referential integrity constraints.

A primary key and a foreign key.

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?

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

13) Explain the difference between ARCHIVELOG mode and NOARCHIVELOG mode and the benefits and disadvantages to each.

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.

14) What command would you use to create a backup control file?

Alter database backup control file to trace.

15) Give the stages of instance startup to a usable state where normal users may access it.

STARTUP NOMOUNT – Instance startup. Control File is read here.

STARTUP MOUNT – The database is mounted. Datafiles are read for the status and checked with control file.

STARTUP OPEN – The database is opened. Normal users can access.

16) What column differentiates the V$ views to the GV$ views and how?

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

17) How would you go about generating an EXPLAIN plan?

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

18) How would you go about increasing the buffer cache hit ratio?

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.

19) Explain an ORA-01555

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.

20) Explain the difference between $ORACLE_HOME and $ORACLE_BASE.

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

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) 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.

24) 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.

25) Name three advisory statistics you can collect.

Buffer Cache Advice, Segment Level Statistics, & Timed Statistics

26) Where in the Oracle directory tree structure are audit traces placed?

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

27) 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.

28) When a user process fails, what background process cleans up after it?


29) What background process refreshes materialized views?

The Job Queue Processes.

30) What is the Difference between OLTP and OLAP

OLTP is nothing but OnLine Transaction Processing ,which contains a normalised tables and online data,which have frequent insert/updates/delete.

But OLAP(Online Analtical Programming) contains the history of OLTP data, which is, non-volatile ,acts as a Decisions Support System and is used for creating forecasting reports.

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


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?


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?


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


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?


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?


47) How can you gather statistics on a table?

The ANALYZE command.

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



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.



The above questions and answers are taken from



Oracle Database 9i – Basic Architecture


This post is regarding the basic database architecture for 9i. Its been very late to upload this basic stuff, but I realized that it would be an incomplete blog without having even a brief architecture. So here it is.

Below is the figure which gives a overview of “Inside Oracle”.

An Oracle database is a combination of oracle Instance and data files on the file system.

Oracle Database = Oracle Instance + Datafiles

Again Oracle Instance is nothing but Memory architecture and Background processes. Lets start the discussion with Memory architecture first.

Memory Architecture

Oracle database uses memory for its operation. The total memory allocated to the Oracle database can be broadly categorized into SGA (System Global Area) and PGA (Program Global Area).

SGA Contains following data structure

  • Database buffer cache
  • Redo log buffer
  • Shared pool
  • Java pool
  • Large pool (optional)
  • Data dictionary cache
  • Other miscellaneous information

We can also categorized SGA into fixed SGA and variable SGA. When asked about Fixed SGA, AskTom says that “fixed SGA is a component of the SGA that varies in size from platform to platform and  release to release.  It is “compiled” into the database.  The fixed SGA contains a set of variables that point to the other components of the SGA and variables that contain the values of various parameters.  The size of the fixed SGA is something over which we have no control and it is generally very small.  Think of this area as a “bootstrap” section of the SGA, something Oracle uses internally to find the other bits and pieces of the

Variable SGA contains 4 main components as listed above, those are “Database Buffer Cache”, “Redo Log Buffer”, “Shared Pool” and “Large Pool”. We call it variable SGA because we can alter the size of each of these components manually using ALTER SYSTEM command. The size of each of the components of variable SGA is determined by INIT.ORA parameters. Following are the INIT.ORA parameter for each of the component.

  • Database Buffer Cache – db_block_buffers
  • Redo Log Buffer – log_buffer
  • Shared Pool – shared_pool_size
  • Large Pool – Large_pool_size

We cannot however alter the size of Fixed SGA.

Database Buffer Cache – This is used to hold the data into the memory. When ever a user access the data, it gets fetched into database buffer cache and it will be managed according to LRU (Least recently used) algorithm. Advantages – If a user is requesting data, which gets fetched into the buffer cache, then next time if he ask for same data with in a short period of time, the data will be read from buffer cache and Oracle process does not have to fetch data again from disk. Reading data from buffer cache is a faster operation. Another advantage is that if a user is modifying the data, it can be modified in the buffer cache which is a faster operation then modifying the data directly on the disk.

Redo Log Buffer – This memory block hold the data which is going to be written to redo log file. Why do we need this data? To rollback the changes if the need be. But instead of writing the data directly to the redo log files, it is first written to log buffer which improves performance and then with the occurrence of certain event it will be written to redo log file.

Shared Pool – This contains 2 memory section, 1) Library Cache 2) Dictionary Cache. Library cache hold the parsed SQL statement and execution plans and parsed PLSQL codes. Dictionary cache hold the information about user privileges, tables and column definitions, passwords etc. These 2 memory components are included in the size of shared pool.

Large Pool – If defined then used for heavy operations such as bulk copy during backup or during restore operation.

The total size of SGA is determined by a parameter SGA_MAX_SIZE. Below is the simple calculation of memory sizes.

SQL> show sga

Total System Global Area  577574308 bytes
Fixed Size                   452004 bytes
Variable Size             402653184 bytes
Database Buffers          163840000 bytes
Redo Buffers               10629120 bytes

This will show fixed and variable size SGA. Fixed size SGA, as I said is not in our control. However we can verify the size of variable SGA and other memory values shown above.

Database Buffers          163840000 bytes

SQL> show parameters db_block_buffer

NAME                                 TYPE        VALUE
———————————— ———– ——————————
db_block_buffers                     integer     20000

This value is in terms of blocks. we can find the size of a block using DB_BLOCK_SIZE parameter

SQL> show parameters db_block_size

NAME                                 TYPE        VALUE
———————————— ———– ——————————
db_block_size                        integer     8192

So Database Buffers = db_block_buffers X db_block_size = 20000 X 8192 = 163840000 bytes

Also Variable size = “Shared Pool Size” + “Large Pool Size” + “Java Pool size” (some times defined)

SQL> SELECT pool, sum(bytes) from v$sgastat group by pool;

———– ———-
java pool     50331648
shared pool  352321536
Variable size = 352321536 + 50331648 = 402653184 bytes

Program Global Area

PGA contains information about bind variables, sort areas, and other aspect of cursor handling. This is not a shared area and every user has its own PGA. But why PGA is required for every user? The reason being that even though the parse information for SQL or PLSQL may be available in library cache of shared pool, the value upon which the user want to execute the select or update statement cannot be shared. These values are stored in PGA. This is also called Private Global Area.

Going still deeper into the memory structure…

Database buffer cache is again divided into 3 different types of cache.

  1. Default Cache
  2. Keep Cache
  3. Recycle Cache

If we define the cache size using DB_CACHE_SIZE (or DB_BLOCK_BUFFER and specify the block size) then this will be default cache. The cache has a limited size, so not all the data on disk can fit in the cache. When the cache is full, subsequent cache misses cause Oracle to write dirty data already in the cache to disk to make room for the new data.

You can configure the database buffer cache with separate buffer pools that either keep data in the buffer cache or make the buffers available for new data immediately after using the data blocks.

  • The KEEP buffer pool retains the schema object’s data blocks in memory. This is defined using the INIT.ORA parameter DB_KEEP_CACHE_SIZE
  • The RECYCLE buffer pool eliminates data blocks from memory as soon as they are no longer needed. This is defined using the INIT.ORA parameter DB_RECYCLE_CACHE_SIZE

You can also define multiple DB block sizes using following parameters. Example if you have defined standard default block size of 4K, then following parameters can be used to define a size of 2K, 8K, 16K and 32K.


Note that you can define the Keep and Recycle cache only on standard block size and buffer cache size is the sum of sizes of each of these pools.

Shared Pool Reserved Size

Shared Pool, as we have seen previously contains the parsed SQL statements and execution plans. With continuous use of database, after a period of time the shared pool will get fragmented. New parsed SQL and execution plans comes and old one gets aged out and hence overwritten. This will also lead to larger packages being aged out with new entries going into shared pool. Hence access to such larger packages will take time to parse and create execution plan. This might cause performance issues.

To avoid such situation, you can define a parameter SHARED_POOL_RESERVED_SIZE. This will reserve some additional space other then shared_pool_size. If an object (either parsed SQL statement or execution plan) is stored in reserved shared pool area then it will not age out.

For large allocations, the order in which Oracle attempts to allocate space in the shared pool is the following:

  1. From the unreserved part of the shared pool.
  2. If there is not enough space in the unreserved part of the shared pool, and if the allocation is large, then Oracle checks whether the reserved pool has enough space.
  3. If there is not enough space in the unreserved and reserved parts of the shared pool, then Oracle attempts to free enough memory for the allocation. It then retries the unreserved and reserved parts of the shared pool.

Process Architecture

Oracle has several process running in the background for proper functioning of database. Following are the main categories of process.

  1. Server Process
  2. Background Process

Server Process – to handle the requests of user processes connected to the instance. Server processes (or the server portion of combined user/server processes) created on behalf of each user’s application can perform one or more of the following:

  • Parse and execute SQL statements issued through the application
  • Read necessary data blocks from datafiles on disk into the shared database buffers of the SGA, if the blocks are not already present in the SGA
  • Return results in such a way that the application can process the information

Background Process – An Oracle instance can have many background processes; not all are always present. The background processes in an Oracle instance include the following:

  • Database Writer (DBW0 or DBWn)
  • Log Writer (LGWR)
  • Checkpoint (CKPT)
  • System Monitor (SMON)
  • Process Monitor (PMON)
  • Archiver (ARCn)
  • Recoverer (RECO)
  • Lock Manager Server (LMS) – Real Application Clusters only
  • Queue Monitor (QMNn)
  • Dispatcher (Dnnn)
  • Server (Snnn)

On many operating systems, background processes are created automatically when an instance is started.

Database writer (DBWn) – The database writer process (DBWn) writes the contents of buffers to datafiles. The DBWn processes are responsible for writing modified (dirty) buffers in the database buffer cache to disk. Although one database writer process (DBW0) is adequate for most systems, you can configure additional processes (DBW1 through DBW9) to improve write performance if your system modifies data heavily. These additional DBWn processes are not useful on uniprocessor systems.

Log Writer (LGWR) – The log writer process (LGWR) is responsible for redo log buffer management–writing the redo log buffer to a redo log file on disk. LGWR writes all redo entries that have been copied into the buffer since the last time it wrote.

Checkpoint (CKPT) – When a checkpoint occurs, Oracle must update the headers of all datafiles to record the details of the checkpoint. This is done by the CKPT process. The CKPT process does not write blocks to disk; DBWn always performs that work.

System Monitor (SMON)The system monitor process (SMON) performs crash recovery, if necessary, at instance startup. SMON is also responsible for cleaning up temporary segments that are no longer in use and for coalescing contiguous free extents within dictionary-managed tablespaces. If any dead transactions were skipped during crash and instance recovery because of file-read or offline errors, SMON recovers them when the tablespace or file is brought back online. SMON wakes up regularly to check whether it is needed.

Process Monitor (PMON)

The process monitor (PMON) performs process recovery when a user process fails. PMON is responsible for cleaning up the database buffer cache and freeing resources that the user process was using. For example, it resets the status of the active transaction table, releases locks, and removes the process ID from the list of active processes.

PMON periodically checks the status of dispatcher and server processes, and restarts any that have died (but not any that Oracle has terminated intentionally). PMON also registers information about the instance and dispatcher processes with the network listener.

Archiver Process (ARCn)

The archiver process (ARCn) copies online redo log files to a designated storage device after a log switch has occurred. ARCn processes are present only when the database is in ARCHIVELOG mode, and automatic archiving is enabled.

An Oracle instance can have up to 10 ARCn processes (ARC0 to ARC9). The LGWR process starts a new ARCn process whenever the current number of ARCn processes is insufficient to handle the workload. The ALERT file keeps a record of when LGWR starts a new ARCn process.

Recoverer (RECO) – The recoverer process (RECO) is a background process used with the distributed database configuration that automatically resolves failures involving distributed transactions. The RECO process of a node automatically connects to other databases involved in an in-doubt distributed transaction. When the RECO process reestablishes a connection between involved database servers, it automatically resolves all in-doubt transactions, removing from each database’s pending transaction table any rows that correspond to the resolved in-doubt transactions.

The RECO process is present only if the instance permits distributed transactions and if the DISTRIBUTED_TRANSACTIONS parameter is greater than zero. If this initialization parameter is zero, RECO is not created during instance startup.

Lock Manager Server (LMS) – In Oracle9i Real Application Clusters, a Lock Manager Server process (LMS) provides inter-instance resource management.

Queue Monitor (QMNn) – The queue monitor process is an optional background process for Oracle Advanced Queuing, which monitors the message queues. You can configure up to 10 queue monitor processes. These processes, like the Jnnn processes, are different from other Oracle background processes in that process failure does not cause the instance to fail.

The above once explained are the mail background processes. Please refer to the Oracle documentation for detailed Oracle 9i Architecture.

Hope this helps !!


Oracle9i Database Online Documentation (Release 9.0.1)

Tom Kyte