oracle dba interview q&a

8/13/2019 Oracle DBA Interview Q&A

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Prepared by: Nasser 1 | P a g e

Oracle DBA Q&A:

Part 1:

1.1 Q- Which are the default passwords of SYSTEM/SYS?

A- MANAGER / CHANGE_ON_INSTALL

1.2 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.com

1.4 Q- What is the address of the Official Oracle Support?

A- metalink.oracle.com or support.oracle.com

1.5 Q- What file will you use to establish Oracle connections from a remote client?

A- tnsnames.ora

1.6 Q- How can you check if the database is accepting connections?

A- lsnrctl status or lsnrctl services

1.7 Q- Which log would you check if a database has a problem?

A- Alert log

1.8 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 DESC

1.10 Q- What command will you start to run the installation of Oracle software on Linux?

A- runInstaller

2.1 Q- What should you do if you encounter an ORA-600?

A- Contact Oracle Support

2.2 Q- Explain the differences between PFILE and SPFILE

A- A PFILE is a Static, text file that initializes the database parameter in the moment that its 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 10g

2.4 Q- Say two examples of DML, two of DCL and two of DDL

A- DML: SELECT, INSERT, UPDATE, DELETE, MERGE, CALL, EXPLAIN PLAN, LOCK TABLE

DDL: CREATE, ALTER, DROP, TRUNCATE, COMMENT, RENAME

DCL: GRANT, REVOKE


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2.5 Q- You want to save the output of an Oracle script from sqlplus. How would you do it?

A- spool script_name.txt

select * 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 DICTIONARY

2.9 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

3.1 Q- When a user process fails, what Oracle background process will clean after it?

A- PMON

3.2 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 ABORT.A- 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. Its 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- asmcmd

3.8 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- TRUNCATE


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3.10 Q- Are passwords in oracle case sensitive?

A- Only since Oracle 11g.

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 processes

A- 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 Server

4.7 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$SESSION instead of V$SESSION

4.9 Q- Which command will we use to manage (stop, start) RAC services in command-line mode?

A- srvctl

4.10 Q- How many alert logs exist in a RAC environment?

A- One for each instance.

5.1 Q- How can you difference a usual parameter and an undocumented parameter?

A- Undocumented parameters have the prefix _. For example, _allow_resetlogs_corruption

5.2 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 structure

The 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 takes backups of read-only tablespaces?

A- No


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5.6 Q- Will a user be able to modify a table with SELECT only privilege?

A- He wont 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 cant 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 cannot be displayed by SQL*Plus

Why Im 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_CORRUPTION

5.10 Q- Name the seven types of Oracle tables

A- Heap Organized Tables, Index Organized Tables, Index Clustered Tables, Hash Clustered Tables, Nested Tables,

Global Temporary Tables, Object Tables.


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Part 2:

1.What is an Oracle Instance?

An Oracle database server consists of an Oracle database and an Oracle instance. Every time a database is started,

a system global area (SGA) is allocated and Oracle background processes are started. The combination of the

background processes and memory buffers is called an Oracle instance. We can run multiple instances on the sameOracle Database Server, where each instance connects to its database.

Oracle instance includes:

SGA- System or Shared Global Area

Components of SGA:

DBBC - Database Buffer Cache SP - Shared Pool; divided into Library Cache (LC) and Data Dictionary Cache (DDC) or Row Cache. RLB - Redo Log Buffer

Background Process (10/11g database):

Mandatory Processes

SMON - System Monitor PMON - Process Monitor DBWR - Database writer LGWR - Log Writer CKPT - Check point RECO - Recoverer DIAG - Diagnosability (new in 11g) VKTM - Virtual keeper of time (keeps "SGA Time" variable in current, new in 11g)

Optional Process

ARCN - Archiver MMAN - Memory Manager - ASMM MMON - Memory Monitor MMNL - Memory Monitor Light - AWR

and few more...

List of running processes of a single instance (11g) on Linux:

[oracle@hostname ~]$ top -n 1 -U oracle -c

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND

9181 oracle 15 0 745m 111m 109m S 6.9 7.5 1:11.15 ora_j000_DB1_SID

9163 oracle 16 0 745m 68m 65m S 5.3 4.6 0:11.95 ora_j001_DB1_SID

10420 oracle 18 0 744m 12m 11m R 3.0 0.8 0:00.09 ora_j002_DB1_SID

6773 oracle 16 0 748m 40m 34m S 0.7 2.7 0:03.16 ora_dbw0_DB1_SID

6775 oracle 16 0 759m 34m 33m S 0.7 2.4 0:10.74 ora_lgwr_DB1_SID

6767 oracle 15 0 744m 13m 11m S 0.3 0.9 0:02.17 ora_psp0_DB1_SID

6785 oracle 15 0 744m 19m 18m S 0.3 1.3 0:02.66 ora_mmnl_DB1_SID

6560 oracle 18 0 42048 9348 6788 S 0.0 0.6 0:00.86 tnslsnr LISTENER -inherit

6755 oracle 15 0 744m 16m 14m S 0.0 1.1 0:02.50 ora_pmon_DB1_SID

6757 oracle -2 0 744m 13m 11m S 0.0 0.9 0:04.31 ora_vktm_DB1_SID

6761 oracle 15 0 744m 13m 11m S 0.0 0.9 0:00.34 ora_gen0_DB1_SID

6763 oracle 18 0 744m 12m 11m S 0.0 0.9 0:00.53 ora_diag_DB1_SID

6765 oracle 15 0 744m 19m 18m S 0.0 1.3 0:00.59 ora_dbrm_DB1_SID

6769 oracle 18 0 744m 16m 14m S 0.0 1.1 0:07.11 ora_dia0_DB1_SID

6771 oracle 18 0 744m 17m 16m S 0.0 1.2 0:11.13 ora_mman_DB1_SID

6777 oracle 16 0 744m 16m 14m S 0.0 1.1 0:08.51 ora_ckpt_DB1_SID


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6779 oracle 15 0 748m 87m 84m S 0.0 5.9 0:04.61 ora_smon_DB1_SID

6781 oracle 18 0 744m 18m 17m R 0.0 1.3 0:00.52 ora_reco_DB1_SID

6783 oracle 15 0 748m 56m 51m S 0.0 3.8 0:06.01 ora_mmon_DB1_SID

6787 oracle 15 0 744m 13m 11m S 0.0 0.9 0:00.35 ora_d000_DB1_SID

6789 oracle 15 0 744m 12m 11m S 0.0 0.8 0:00.31 ora_s000_DB1_SID

6852 oracle 18 0 744m 14m 13m S 0.0 1.0 0:00.40 ora_qmnc_DB1_SID

6859 oracle 15 0 744m 25m 23m S 0.0 1.7 0:00.53 ora_q000_DB1_SID

6864 oracle 18 0 744m 15m 14m S 0.0 1.0 0:00.21 ora_q001_DB1_SID

6983 oracle 15 0 748m 54m 48m S 0.0 3.7 0:05.40 ora_cjq0_DB1_SID

7141 oracle 15 0 744m 13m 12m S 0.0 0.9 0:00.26 ora_smco_DB1_SID

7722 oracle 16 0 753m 58m 55m S 0.0 4.0 0:07.64 oracleDB1_SID (LOCAL=NO)

10254 oracle 15 0 744m 14m 12m S 0.0 1.0 0:00.10 ora_w000_DB1_SID

2. What information is stored in Control File?

Oracle Database must have at least one control file.

It's a binary file contains some of the following information:

The database name and unique ID The timestamp of database creation The names and locations of associated datafiles and redo log files Tablespace information Datafile offline ranges Archived log information and history Backup set and backup piece information Backup datafile and redo log information Datafile copy information Log records: sequence numbers, SCN range in each log RMAN Catalog Database block corruption information

The location of the control files is specified through the control_filesinitparam:

SYS@DB1_SID SQL>show parameter control_file;

NAME TYPE VALUE

------------------------------------ ----------- ------------------------------control_file_record_keep_time integer 7

control_files string /u01/app/oracle/oradata/DB1_SID

/control01.ctl, /u01/app/oracl

e/flash_recovery_area/DB1_SID/c

ontrol02.ctl

3. When you start an Oracle DB which file is accessed first?

Oracle first opens and reads the initialization parameter file (init.ora)[oracle@hostname ~]$ls -la $ORACLE_HOME/dbs/initDB1_SID.ora

-rw-r--r-- 1 oracle oinstall 1023 May 10 19:27 /u01/app/oracle/product/11.2.0/dbs/initDB1_SID.ora

4. What is the job of SMON and PMON processes?

SMON- System Monitor Process - Performs recovery after instance failure, monitors temporary segments and

extents; cleans temp segments, coalesces free space (mandatory process for DB and starts by default)


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PMON- Process Monitor - Recovers failed process resources. In Shared Server architecture, monitors and restarts

any failed dispatcher or server processes (mandatory process for DB and starts by default)

[oracle@hostname ~]$ps -ef |grep -e pmon -e smon |grep -v grep

oracle 6755 1 0 12:59 ? 00:00:05 ora_pmon_DB1_SID

oracle 6779 1 0 12:59 ? 00:00:06 ora_smon_DB1_SID

5. What is Instance Recovery?

While Oracle instance fails, Oracle performs an Instance Recovery when the associated database is being re-

started.

Instance recovery occurs in two steps:

Cache recovery:

Changes being made to a database are recorded in the database buffer cache.

These changes are also recorded in online 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 the data in the database buffer cache are written to data files, Oracle uses the

data recorded in the 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.

The data stored in the undo segment is used to restore the original values in case a 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.

6. What is being written into the Redo Log Files?

Redo log records all changes made in datafiles.

In the Oracle database, redo logs comprise files in a proprietary format which log a history of all changes made to

the database. Each redo log file consists of redo records. A redo record, also called a redo entry, holds a group of

change-vectors, each of which describes or represents a change made to a single block in the database.

Let's get into this topic a little bit dipper:

Log writer (LGWR) writes redo log buffer contents Into Redo Log FIles. LGWR does this every three seconds, when

the redo log buffer is 1/3 full and immediately before the Database Writer (DBWn) writes its changed buffers into

the datafile. The redo log of a database consists of two or more redo log files. The database requires a minimum of

two files to guarantee that one is always available for writing while the

other is being archived (if the DB is in ARCHIVELOG mode). LGWR writes to redo log files in a circular fashion.When the current redo log file fills, LGWR begins writing to the next available redo log file. When the last available

redo log file is filled, LGWR returns to the first redo log file and writes to it, starting the cycle again.

Filled redo log files are available to LGWR for reuse depending on whether archiving is enabled.

If archiving is disabled (the database is in NOARCHIVELOG mode), a filled redo log file is available after the changes

recorded in it have been written to the datafiles.


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If archiving is enabled (the database is in ARCHIVELOG mode), a filled redo log file is available to LGWR after the

changes recorded in it have been written to the datafiles and the file has been archived.

Oracle Database uses only one redo log files at a time to store redo records written from the redo log buffer. The

redo log file that LGWR is actively writing to is called the current redo log file. Redo log files that are required for

instance recovery are called active redo log files. Redo log files that are no longer required for instance recovery

are called inactive redo log files.If the database is in ARCHIVELOG mode it cannot reuse or overwrite an active online log file until one of the

archiver background processes (ARCn) has archived its contents.

If archiving is disabled (DB is in NOARCHIVELOG mode), then when the last redo log file is full, LGWR continues by

overwriting the first available active file.

A log switch is the point at which the database stops writing to one redo log file and begins writing to another.

Normally, a log switch occurs when the current redo log file is completely filled and writing must continue to the

next redo log file. However, you can configure log switches to occur at regular intervals, regardless of whether the

current redo log file is completely filled. You can also force log switches manually.

Oracle Database assigns each redo log file a new log sequence number every time a log switch occurs and LGWR

begins writing to it.

When the database archives redo log files, the archived log retains its log sequence number.

7. How do you control number of Datafiles one can have in an Oracle database?

The db_files parameter is a "soft limit " parameter that controls the maximum number of physical OS files that can

map to an Oracle instance.

The maxdatafiles parameter is a different - "hard limit" parameter.

When issuing a "create database" command, the value specified for maxdatafiles is stored in Oracle control files

and default value is 32.

The maximum number of database files can be set with the init parameter db_files.

8. How many Maximum Datafiles can there be in Oracle Database?

Regardless of the setting of this paramter, maximum per database: 65533 (May be less on some operatingsystems)

Maximum number of datafiles per tablespace: OS dependent = usually 1022

Limited also by size of database blocks and by the DB_FILES initialization parameter for a particular instance

Bigfile tablespaces can contain only one file, but that file can have up to 4G blocks.

9. What is a Tablespace?

A tablespace is a logical storage unit within an Oracle database.

Tablespace is not visible in the file system of the machine on which the database resides.

A tablespace, in turn, consists of at least one datafile which, in turn, are physically located in the file system of the

server.

A datafilebelongs to exactly one tablespace. Each table, index and so on that is stored in an Oracle database

belongs to a tablespace.

The tablespace builds the bridge between the Oracle database and the filesystem in which the table's or ind ex'

data is stored.

There are three types of tablespaces in Oracle:

Permanent tablespaces Undo tablespaces Temporary tablespaces


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10. What is the purpose of Redo Log files?

Before Oracle changes data in a datafile it writes these changes to the redo log.

If something happens to one of the datafiles, a backed up datafile can be restored and the redo, that was written

since, replied, which brings the datafile to the state it had before it became unavailable.

11. Which default Database roles are created when you create a Database?

CONNECT , RESOURCE and DBA are three default roles. The DBA_ROLES data dictionary view can be used to list all

roles of a database and the authentication used for each role.

The following query lists all the roles in the database:

SELECT * FROM DBA_ROLES;

ROLE PASSWORD

---------------- --------

CONNECT NO

RESOURCE NO

DBA NO

SECURITY_ADMIN YES

12. What is a Checkpoint?

A checkpoint occurs when the DBWR (database writer) process writes all modified buffers in the SGA buffer cache

to the database data files.

Data file headers are also updated with the latest checkpoint SCN, even if the file had no changed blocks.

Checkpoints occur AFTER (not during) every redo log switch and also at intervals specified by initialization

parameters.

Set parameter LOG_CHECKPOINTS_TO_ALERT=TRUE to observe checkpoint start and end times in the database

alert log.

Checkpoints can be forced with the ALTER SYSTEM CHECKPOINT; command.

SCNcan refer to:

System Change Number- A number, internal to Oracle that is incremented over time as change vectors are

generated, applied, and written to the Redo log.

System Commit Number - A number, internal to Oracle that is incremented with each database COMMIT.

Note: System Commit Numbers and System Change Numbers share the same internal sequence generator.

13. Which Process reads data from Datafiles?

Server Process- There is no background process which reads data from datafile or database buffer.

Oracle creates server processes to handle requests from connected user processes. A server process

communicates with the user process and interacts with Oracle to carry out requests from the associated user

process. For example, if a user queries some data not already in the database buffers of the SGA, then the

associated server process reads the proper data blocks from the datafiles into the SGA.

Oracle can be configured to vary the number of user processes for each server process.

In a dedicated server configuration, a server process handles requests for a single user process.A shared server configuration lets many user processes share a small number of server processes, minimizing the

number of server processes and maximizing the use of available system resources.

14. Which Process writes data in Datafiles?

Database Writer background process DBWn (20 possible) writes dirty buffers from the buffer cache to the data

files.

In other words, this process writes modified blocks permanently to disk.


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15. Can you make a Datafile auto extendible? If yes how?

YES. A Datafile can be auto extendible.

Here's how to enable auto extend on a Datafile:

SQL>alter database datafile '/u01/app/oracle/product/10.2.0/oradata/DBSID/EXAMPLE01.DBF' autoextend on;Note: For tablespaces defined with multiple data files (and partitioned table files), only the "last" data file needs

the autoextend option.

SQL>spool runts.sqlSQL>select 'alter database datafile '|| file_name|| ' '|| ' autoextend on;' from dba_data_files;SQL>@runts

16. What is a Shared Pool?

The shared pool portion of the SGA contains the library cache, the dictionary cache, buffers for parallel execution

messages, and control structures. The total size of the shared pool is determined by the initialization parameter

SHARED_POOL_SIZE.The default value of this parameter is 8MB on 32-bit platforms and 64MB on 64-bit platforms.

Increasing the value of this parameter increases the amount of memory reserved for the shared pool.

17. What is kept in the Database Buffer Cache?

The database buffer cache is the portion of the SGA that holds copies of data blocks read from datafiles.

All user processes concurrently connected to the instance share access to the database buffer cache.

18. How many maximum Redo Logfiles one can have in a Database?

Maximum number of logfiles is limited by value of MAXLOGFILES parameter in the CREATE DATABASE statement.

Control file can be resized to allow more entries; ultimately an operating system limit. Maximum number of

logfiles per group - Unlimited

Consider the parameters that can limit the number of redo log files before setting up or altering the configuration

of an instance redo log.

The following parameters limit the number of redo log files that you can add to a database: MAXLOGFILES &

MAXLOGMEMBERS.

The MAXLOGFILES parameter used in the CREATE DATABASE statement determines the maximum number of

groups of redo log files for each database. Group values can range from 1 to MAXLOGFILES.

When the compatibility level is set earlier than 10.2.0, the only way to override this upper limit is to re-create the

database or its control file. Therefore, it is important to consider this limit before creating a database.

When compatibility is set to 10.2.0 or later, you can exceed the MAXLOGFILES limit, and the control files expand as

needed.

If MAXLOGFILES is not specified for the CREATE DATABASE statement, then the database uses an operating system

specific default value.The MAXLOGMEMBERS parameter used in the CREATE DATABASE statement determines the maximum number of

members for each group. As with MAXLOGFILES, the only way to override this upper limit is to re-create the

database or control file. Therefore, it is important to consider this limit before creating a database.

If no MAXLOGMEMBERS parameter is specified for the CREATE DATABASE statement, then the database uses an

operating system default value.

19. What is difference between PFile and SPFile?


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A PFILEis a static, text file located in $ORACLE_HOME/dbs - UNIX

An SPFILE(Server Parameter File) is a persistent server-side binary file that can only be modified with the "ALTER

SYSTEM SET" command.

20. What is PGA_AGGREGATE_TARGET parameter?

PGA_AGGREGATE_TARGET: specifies the target aggregate PGA memory available to all server processes attachedto the instance.

21. Large Pool is used for what?

The large pool is an optional memory area and provides large memory allocations for:

Session memory for the shared server and the Oracle XA interface (used where transactions interact withmore than one database)

I/O server processes, buffer area Oracle backup and restore operations (RMAN) User Global Area (UGA) for shared servers

22. What is PCT Increase setting?

PCTINCREASErefers to the percentage by which each next extent (beginning with the third extend) will grow.

The size of each subsequent extent is equal to the size of the previous extent plus this percentage increase.

Preventing tablespace fragmentation

Try to set PCTINCREASE to 0 or 100. Bizarre values for PCTINCREASE will contribute to fragmentation.

For example if you set PCTINCREASE to 1 you will see that your extents are going to have weird and wacky sizes:

100K, 100K, 101K, 102K, etc. Such extents of bizarre size are rarely re-used in their entirety.

PCTINCREASE of 0 or 100 gives you nice round extent sizes that can easily be reused. Eg.100K, 100K, 200K, 400K,

etc.

Locally Managed tablespaces (available from Oracle 8i onwards) with uniform extent sizes virtually eliminates any

tablespace fragmentation.

Note that the number of extents per segment does not cause any performance issue anymore, unless they run into

thousands and thousands where additional I/O may be required to fetch the additional blocks where extent mapsof the segment are stored.

23. What is PCTFREE and PCTUSED Setting?

PCTFREEis a block storage parameter used to specify how much space should be left in a database block for future

updates.

For example, for PCTFREE=10, Oracle will keep on adding new rows to a block until it is 90% full. This leaves 10%

for future updates (row expansion).

When using Oracle Advanced Compression, Oracle will trigger block compression when the PCTFREE is reached.

This eliminates holes created by row deletions and maximizes contiguous free space in blocks.

See the PCTFREE setting for a table:

SQL>SELECT pct_free FROM user_tables WHERE table_name = 'EMP';PCT_FREE

----------

10

PCTUSEDis a block storage parameter used to specify when Oracle should consider a database block to be empty

enough to be added to the freelist. Oracle will only insert new rows in blocks that isenqueued on the freelist.


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For example, if PCTUSED=40, Oracle will not add new rows to the block unless sufficient rows are deleted from the

block so that it falls below 40% empty.

24. What is Row Migration and Row Chaining?

Row Migration refers to rows that were moved to another blocks due to an update making them too large to fit

into their original blocks.Oracle will leave a forwarding pointer in the original block so indexes will still be able to "find" the row. Note that

Oracle does not discriminate between chained and migrated rows, even though they have different causes. A

chained row is a row that is too large to fit into a single database data block.

For example, if you use a 4KB blocksize for your database, and you need to insert a row of 8KB into it, Oracle will

use 3 blocks and store the row in pieces.

Some conditions that will cause row chaining are:

Tables whose row size exceeds the blocksize Tables with long and long raw columns are prone to having chained rows Tables with more than 255 columns will have chained rows as Oracle break wide tables up into pieces.

Detecting row chaining:

This query will show how many chained (and migrated) rows each table has:SQL>SELECT owner, table_name, chain_cnt FROM dba_tables WHERE chain_cnt> 0;

To see which rows are chained:SQL>ANALYZE TABLE tablename LIST CHAINED ROWS;

This will put the rows into the INVALID_ROWS table which is created by the utlvalid.sql script (located in

$ORACLE_HOME/rdbms/admin).

25. What is ORA-01555 - Snapshot Too Old error and how do you avoid it?

The ORA-01555is caused by Oracle read consistency mechanism. If you have a long running SQL that starts at

11:30 AM, Oracle ensures that all rows are as they appeared at 11:30 AM, even if the query runs until noon!

Oracles does this by reading the "before image" of changed rows from the online undo segments. If you have lots

of updates, long running SQL and too small UNDO, the ORA-01555 error will appear. ORA-01555 error relates to

insufficient undo storage or a too small value for the undo_retention parameter:

ORA-01555:snapshot too old: rollback segment number string with name "string" too small

Cause:Rollback records needed by a reader for consistent read are overwritten by other writers.

Action:If in Automatic Undo Management mode, increase the setting of UNDO_RETENTION. Otherwise, use larger

rollback segments.

You can get an ORA-01555 error with a too-small undo_retention, even with a large undo tables.

However, you can set a super-high value for undo_retention and still get an ORA-01555 error.

The ORA-01555 snapshot too old error can be addressed by several remedies:

Re-schedule long-running queries when the system has less DML load Increasing the size of your rollback segment (undo) size The ORA-01555 snapshot too old also relates to your setting for automatic undo retention Don't fetch between commits

26. What is a Locally Managed Tablespace?


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Locally Managed Tablespace is a tablespace that record extent allocation in the tablespace header.

Each tablespace manages its own free and used space within a bitmap structure stored in one of the tablespace's

data files.

Advantages of Locally Managed Tablespaces:

Eliminates the need for recursive SQL operations against the data dictionary (UET$ and FET$ tables) Reduce contention on data dictionary tables (single ST enqueue) Locally managed tablespaces eliminate the need to periodically coalesce free space (automatically tracks

adjacent free space)

Changes to the extent bitmaps do not generate rollback information27. Can you audit SELECT statements?

YES. But beware; you will need a storage mechanism to hold your SQL SELECT audits, a high data volume that can

exceed the size of your whole database, every day.

SQL SELECT auditing can be accomplished in several ways:

Oracle audit table command: audit SELECT table by FRED by access; Oracle Fined-grained Auditing

In a busy database, the volume of the SELECT audit trail could easily exceed the size of the database every data.

Plus, all data in the audit trail must also be audited to see who has selected data from the audit trail.

28. What does DBMS_FGA package do?

The DBMS_FGA package provides fine-grained security functions. DBMS_FGA is a PL/SQL package used to define

Fine Grain Auditing on objects.

DBMS_FGA Package Subprograms:

ADD_POLICYProcedure - Creates an audit policy using the supplied predicate as the audit condition DISABLE_POLICYProcedure - Disables an audit policy DROP_POLICYProcedure - Drops an audit policy ENABLE_POLICYProcedure - Enables an audit policy

29. What is Cost Based Optimization?

The Oracle Cost Based Optimizer (CBO) is a SQL Query optimizer that uses data statistics to identify the query plan

with lowest cost before execution. The cost is based on the number of rows in a table, index efficiency, etc.

All applications should be converted to use the Cost Based Optimizer as the Rule Based Optimizer is not be

supported in Oracle 10g and above releases.

30. How often you should collect statistics for a table?

Analyse if it's necessary!

- Refresh STALE statistics before the batch processes run but only for tables involved in batch run,

- Don't do it if you don't have to.

- Oracle databse has default, scheduled job "gather_stats_job" that analyses stats on a daily basis during the

maintenance window time.

31. How do you collect statistics for a table, schema and Database?

Using DBMS_STATS package to gather Oracle dictionary statistics.


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32. Can you make collection of Statistics for tables automatically?

YES. Oracle databse has default, scheduled job "gather_stats_job" that analyses stats on a daily basis during the

maintenance window time.

There are two scheduled activities related to the collection of Oracle "statistics":

AWR statistics: Oracle has an automatic method to collect AWR "snapshots" of data that is used to createelapsed-time performance reports. Optimizer statistics: Oracle has an automatic job to collect statistics to help the optimizer make intelligent

decisions about the best access method to fetch the desired rows.

This job can be disabled with this command: exec dbms_scheduler.disable(SYS.GATHER_STATS_JOB);

Oracle collects optimizer statistics for SQL via the default of autostats_target = auto.

33. On which columns you should create Indexes?

In general, you should create an index on a column in any of the following situations:

The column is queried frequently A referential integrity constraint exists on the column A UNIQUE key integrity constraint exists on the column

The following list gives guidelines in choosing columns to index:

You should create indexes on columns that are used frequently in WHERE clauses Are used frequently to join tables Are used frequently in ORDER BY clauses On columns that have few of the same values or unique values in the table

34. What type of indexes are available in Oracle?

There are many index types within Oracle:

B*Tree Indexes- common indexes in Oracle. They are similar construct to a binary tree; they provide fast access by

key, to an individual row or range of rows, normally requiring very few reads to find the correct row.The B*Tree index has several subtypes:

Index Organised Tables - A table stored in a B*Tree structure B*Tree Cluster Indexes - They are used to index the cluster keys Reverse Key Indexes - The bytes in the key are reversed. This is used to stop sequential keys being on the

same block like 999001, 999002, 999003 would be reversed to 100999, 200999, 300999 thus these would

be located on different blocks.

Descending Indexes - They allow data to be sorted from big to small (descending) instead of small to big(ascending).

Bitmap Indexes- With a bitmap index, a single index entry uses a bitmap to point to many rows simultaneously,

they are used with low data that is mostly read-only. Schould be avoided in OLTP systems.

Function Based Indexes- These are B*Tree or bitmap indexes that store the computed result of a function on a

row(s) (for example sorted results)- not the column data itself.

Application Domain Indexes- These are indexes you build and store yourself, either in Oracle or outside of Oracle

interMedia Text Indexes - This is a specialized index built into Oracle to allow for keyword searching of large bodies

of text.

35. What is B-Tree Index?


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A B-Treeindex is a data structure in the form of a tree, but it is a tree of database blocks, not rows.

Note: "B" is not for binary; it's balanced.

36. A table is having few rows, should you create indexes on this table

Small tables do not require indexes; if a query is taking too long, then the table might have grown from small to

large.You can create an index on any column; however, if the column is not used in any of these situations, creating an

index on the column does not increase performance and the index takes up resources unnecessarily.

37. A Column is having many repeated values which type of index you should create on this column, if you have

to?

For example, assume there is a motor vehicle database with numerous low-cardinality columns such as car_color,

car_make, car_model, and car_year. Each column contains less than 100 distinct values by themselves, and a b-

tree index would be fairly useless in a database of 20 million vehicles.

38. When should you rebuilt indexes?

In 90% cases - NEVER.

When the data in index is sparse (lots of holes in index, due to deletes or updates) and your query is usually range

based.

Also index blevel is one of the key indicators of performance of sql queries doing Index range scans.

39. Can you built indexes online?

YES. You can create and rebuild indexes online.

This enables you to update base tables at the same time you are building or rebuilding indexes on that table.

You can perform DML operations while the index build is taking place, but DDL operations are not allowed.

Parallel execution is not supported when creating or rebuilding an index online.

The following statements illustrate online index build operations:

CREATE INDEX emp_name ON emp (mgr, emp1, emp2, emp3) ONLINE;

40. Can you see Execution Plan of a statement?

YES. In many ways, for example from GUI based tools like TOAD, Oracle SQL Developer.

Configuring AUTOTRACE, a SQL*Plus facility

AUTOTRACE is a facility within SQL*Plus to show us the explain plan of the queries we've executed, and the

resources they used.

Once the PLAN_TABLE has been installed in the database, You can control the report by setting the AUTOTRACE

system variable.

SET AUTOTRACE OFF - No AUTOTRACE report is generated. This is the default. SET AUTOTRACE ON EXPLAIN - The AUTOTRACE report shows only the optimizer execution path. SET AUTOTRACE ON STATISTICS - The AUTOTRACE report shows only the SQL statement execution

statistics.

SET AUTOTRACE ON - The AUTOTRACE report includes both the optimizer execution path and the SQLstatement execution statistics.

SET AUTOTRACE TRACEONLY - Like SET AUTOTRACE ON, but suppresses the printing of the user's queryoutput, if any.

41. A table has been created with below settings. What will be size of 4th extent?

storage (initial 200k

next 200k


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minextents 2

maxextents 100

pctincrease 40)

What will be size of 4th extent?

"NEXT" Specify in bytes the size of the next extent to be allocated to the object.Percent Increase allows your segment to grow at an increasing rate.

The first two extents will be of a size determined by the Initial and Next parameter (200k)

The third extent will be 1 + PCTINCREASE/100 times the second extent (1,4*200=280k).

AND The fourth extent will be 1 + PCTINCREASE/100 times the third extent (1,4*280=392k!!!), and so on...

42. What is DB Buffer Cache Advisor?

The Buffer Cache Advisor provides advice on how to size the Database Buffer Cache to obtain optimal cache hit

ratios.

Member of Performance Advisors --> Memory Advisor pack.

43. What is STATSPACK tool?

STATSPACK is a performance diagnosis tool provided by Oracle starting from Oracle 8i and above.

STATSPACK is a diagnosis tool for instance-wide performance problems; it also supports application tuning

activities by providing data which identifies high-load SQL statements.

Although AWR and ADDM (introduced in Oracle 10g) provide better statistics than STATSPACK,

users that are not licensed to use the Enterprise Manager Diagnostic Pack should continue to use statspack.

More information about STATSPACK, can be found in file $ORACLE_HOME/rdbms/admin/spdoc.txt.

44. Can you change SHARED_POOL_SIZE online?

YES. That's possible.SQL>alter system set shared_pool_size=500M scope=both;

System altered.

It's a lot quicker to bounce the instance when changing this.

45. Can you Redefine a table Online?

Yes you can. In any database system, it is occasionally necessary to modify the logical or physical structure of a

table to:

Improve the performance of queries or DML Accommodate application changes Manage storage

Oracle Database provides a mechanism to make table structure modifications without significantly affecting the

availability of the table.

The mechanism is called online table redefinition.

When a table is redefined online, it is accessible to both queries and DML during much of the redefinition process.

The table is locked in the exclusive mode only during a very small window that is independent of the size of the

table and complexity of the redefinition and that is completely transparent to users.

Online table redefinition requires an amount of free space that is approximately equivalent to the space used by

the table being redefined. More space may be required if new columns are added.


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Here is an example:

CREATE TABLESPACE example

DATAFILE '/oradata/ORA_SID/example01.dbf' SIZE 50M

EXTENT MANAGEMENT LOCAL UNIFORM SIZE 2M

SEGMENT SPACE MANAGEMENT AUTO;

The storage parameters PCTUSED, FREELISTS and FREELIST GROUPS specified while creating a table are ignored byOracle on a LMT ASSM tablespace. Oracle does not produce an error.

One huge benefit of having ASSM is to reduce the Buffer Busy Waits you see on segments.

Beware:

Using ASSM can hinder database DML performance, and most Oracle experts will use manual freelists and freelist

groups.

50. What is the difference between DELETE and TRUNCATE statements?

The DELETEcommand is used to remove rows from a table. A WHERE clause can be used to only remove some

rows.

If no WHERE condition is specified, all rows will be removed. After performing a DELETE operation you need to

COMMIT or ROLLBACK the transaction to make the change permanent or to undo it.DELETE will cause all DELETE triggers on the table to fire.

TRUNCATEremoves all rows from a table. A WHERE clause is not permited. The operation cannot be rolled back

and no triggers will be fired.

As such, TRUCATE is faster and doesn't use as much undo space as a DELETE.

51. What is COMPRESS and CONSISTENT setting in EXPORT utility?

COMPRESS

Simply: COMPRESS=n- Allocated space in database for imported table will be exactly as the space required to hold

the data.

COMPRESS=y- The INITIAL extent of the table would be as large as the sum of all the extents allocated to the table

in the original database.In other words:

The default, COMPRESS=y, causes Export to flag table data for consolidation into one initial extent upon import.

If extent sizes are large (for example, because of the PCTINCREASE parameter), the allocated space will be larger

than the space required to hold the data.

If you specify COMPRESS=n, Export uses the current storage parameters, including the values of initial extent size

and next extent size.

If you are using locally managed tablespaces you should always export with COMPRESS=n

CONSISTENT

Default: n. Specifies whether or not Export uses the SET TRANSACTION READ ONLY statement to ensure that the

data seen by Export is consistent to a single point in time and does not change during the execution of the exp

command.You should specify CONSISTENT=y when you anticipate that other applications will be updating the target data

after an export has started.

If you use CONSISTENT=n, each table is usually exported in a single transaction. However, if a table contains nested

tables, the outer table and each inner table are exported as separate transactions.

If a table is partitioned, each partition is exported as a separate transaction.

Therefore, if nested tables and partitioned tables are being updated by other applications, the data that is

exported could be inconsistent. To minimize this possibility, export those tables at a time when updates are not

being done.


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52. What is the difference between Direct Path and Conventional Path loading?

A conventional path load executes SQL INSERT statements to populate tables in an Oracle database.

A direct path load eliminates much of the Oracle database overhead by formatting Oracle data blocks and writing

the data blocks directly to the database files.

53. Can you disable and enable Primary key?

You can use the ALTER TABLE statement to enable, disable, modify, or drop a constraint.

When the database is using a UNIQUE or PRIMARY KEY index to enforce a constraint, and constraints associated

with that index are dropped or disabled, the index is dropped, unless you specify otherwise.

While enabled foreign keys reference a PRIMARY or UNIQUE key, you cannot disable or drop the PRIMARY or

UNIQUE key constraint or the index.

Disabling Enabled Constraints

The following statements disable integrity constraints. The second statement specifies that the associated indexes

are to be kept.ALTER TABLE dept DISABLE CONSTRAINT dname_ukey;

ALTER TABLE dept DISABLE PRIMARY KEY KEEP INDEX, DISABLE UNIQUE (dname, loc) KEEP INDEX;

The following statements enable novalidate disabled integrity constraints:ALTER TABLE dept ENABLE NOVALIDATE CONSTRAINT dname_ukey;

ALTER TABLE dept ENABLE NOVALIDATE PRIMARY KEY, ENABLE NOVALIDATE UNIQUE (dname, loc);

The following statements enable or validate disabled integrity constraints:ALTER TABLE dept MODIFY CONSTRAINT dname_key VALIDATE;

ALTER TABLE dept MODIFY PRIMARY KEY ENABLE NOVALIDATE;

The following statements enable disabled integrity constraints:ALTER TABLE dept ENABLE CONSTRAINT dname_ukey;

ALTER TABLE dept ENABLE PRIMARY KEY, ENABLE UNIQUE (dname, loc);

To disable or drop a UNIQUE key or PRIMARY KEY constraint and all dependent FOREIGN KEY constraints in a single

step, use the CASCADE option of the DISABLE or DROP clauses.

For example, the following statement disables a PRIMARY KEY constraint and any FOREIGN KEY constraints that

depend on it:ALTER TABLE dept DISABLE PRIMARY KEY CASCADE;

54. What is an Index Organized Table?

An index-organized table (IOT) is a type of table that stores data in a B*Tree index structure. Normal relational

tables, called heap-organized tables, store rows in any order (unsorted). In contrast to this, index-organized tables

store rows in a B-tree index structure that is logically sorted in primary key order. Unlike normal primary key

indexes, which store only the columns included in it definition, IOT indexes store all the columns of the table (anexception to this rule - is being called the overflow area).

Properties and restrictions:

An IOT must contain a primary key Rows are accessed via a logical rowid and not a physical rowid like in heap-organized tables An IOT cannot be in a cluster An IOT cannot contain a column of LONG data type


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You cannot modify an IOT index property using ALTER INDEX (error ORA-25176), you must use an ALTERTABLE instead.

Advantages of an IOT

As an IOT has the structure of an index and stores all the columns of the row, accesses via primary keyconditions are faster as they don't need to access the table to get additional column values.

As an IOT has the structure of an index and is thus sorted in the order of the primary key, accesses of arange of primary key values are also faster. As the index and the table are in the same segment, less storage space is needed. In addition, as rows are stored in the primary key order, you can further reduce space with key

compression.

As all indexes on an IOT uses logical rowids, they will not become unusable if the table is reorganized.Row overflow area

If some columns of the table are infrequently accessed, it is possible to offload them into another segment named

the overflow area. An overflow segment will decrease the size of the main (or top) segment and will increase the

performance of statements that do not need access the columns in the overflow area.

Notes:

The overflow area can contains only columns that are not part of the primary key.

If a row cannot fit in a block, you must define an overflow area.

Consequently, the primary key values of an IOT must fit in a single block.

The columns of the table that are recorded in the overflow segment are defined using the PCTHRESHOLD and/or

INCLUDING options of the OVERFLOW clause (examples on source website).

55. What is a Global Index and Local Index?

Local Index- each partition of a local index is associated with exactly one partition of the table.

Global Index- global index is associated with multiple partitions of the table.

Oracle offers two types of global partitioned index:

- Global RangePartitioned Indexes- Global HashPartitioned Indexes

Global Nonpartitioned Indexes- behave just like a nonpartitioned index.

56. What is the difference between Range Partitioning and Hash Partitioning?

Range Partitioning maps data to partitions based on a range of column values (e.g. a date column)

Hash Partitioning maps data to partitions based on a hashing algorithm, evenly distributing data between the

partitions.

This is typically used where ranges aren't appropriate, i.e. customer number, product ID

57. What is difference between Multithreaded/Shared Server and Dedicated Server?

Oracle Database creates server processes to handle the requests of user processes connected to an instance.

A server process can be either of the following:

- A dedicated server process, which services only one user process

- A shared server process, which can service multiple user processes

Your database is always enabled to allow dedicated server processes, but you must specifically configure and

enable shared server by setting one or more initialization parameters.


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58. Can you import objects from Oracle ver. 7.3 to 9i?

Different versions of the import utility are upwards compatible. This means that one can take an export file

created from an old export version, and import it using a later version of the import utility.

Oracle also ships some previous catexpX.sql scripts that can be executed as user SYS enabling older imp/exp

versions to work (for backwards compatibility).

For example, one can run $ORACLE_HOME/rdbms/admin/catexp7.sql on an Oracle 8 database to allow the Oracle

7.3 exp/imp utilities to run against an Oracle 8 database.

59. How do you move tables from one tablespace to another tablespace?

There are several methods to do this;

1) export the table, drop the table, create the table definition in the new

tablespace, and then import the data (imp ignore=y).

2) Create a new table in the new tablespace with the CREATE TABLE statement AS SELECT all from source table

command:CREATE TABLE temp_name TABLESPACE new_tablespace AS SELECT * FROM source_table;

Then drop the original table and rename the temporary table as the original:DROP TABLE real_table;

RENAME temp_name TO real_table;

Note: don't forget to rebuild any indexes.

60. How to display how much space is used and free in a tablespace?

Example query to check free and used space per tablespace:

SELECT

/* + RULE */df.tablespace_name "Tablespace", df.bytes / (1024 * 1024) "Size (MB)",

SUM(fs.bytes) / (1024 * 1024) "Free (MB)", NVL(

ROUND(SUM(fs.bytes) * 100 / df.bytes),1) "% Free",

ROUND((df.bytes - SUM(fs.bytes)) * 100 / df.bytes)

"% Used"

FROM dba_free_spacefs,

(

SELECT tablespace_name,SUM(bytes) bytes

FROM dba_data_files

GROUP BY tablespace_name

)

df

WHERE fs.tablespace_name (+) = df.tablespace_name

GROUP BY df.tablespace_name,df.bytes

UNION ALL

SELECT

/* + RULE */

df.tablespace_nametspace, fs.bytes / (1024 * 1024),

SUM(df.bytes_free) / (1024 * 1024), NVL(ROUND((SUM(fs.bytes)

- df.bytes_used) * 100 / fs.bytes), 1),

ROUND((SUM(fs.bytes) - df.bytes_free) * 100 / fs.bytes)

FROM dba_temp_filesfs,

(

SELECT tablespace_name,bytes_free,


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bytes_used

FROM v$temp_space_header

GROUP BY tablespace_name,bytes_free,

bytes_used

)

df

WHERE fs.tablespace_name (+) = df.tablespace_name

GROUP BY df.tablespace_name,fs.bytes,df.bytes_free,df.bytes_used;

Sample output:

Tablespace Size (MB) Free (MB) % Free % Used

------------------------------ ---------- ---------- ---------- ----------

UNDOTBS1 65 17.8125 27 73

EXAMPLE 100 22.625 23 77

USERS 5 1.0625 21 79

TEMP 20 2 10 90

SYSAUX 625.125 54.5 9 91

SYSTEM 700 9.0625 1 99


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Part 3:

What is a Database?

a) A database is an organized collection ofdata for one or more purposes, usually in digital form.b) A database is a collection of information that is organized so that it can easily be accessed,

managed, and updated.c) A database is the collection of files that contains the data. In Oracle its ending with .dbf

extension and these are binary files

Types of Databases:

There are several common types of databases; each type of database has its own data model (how the

data is structured).

They include: Flat Model, Hierarchical Model, Network Model, andRelational Model.

1. The Flat Model Database:In a flat model database, there is a two dimensional (flat structure) array of data. For instance, there isone column of information and within this column it is assumed that each data item will be related to

the other. For instance, a flat model database includes only zip codes. Within the database, there will

only be one column and each new row within that one column will be a new zip code.

2. The Hierarchical Model Database:The hierarchical model database resembles a tree like structure, such as howMicrosoft

Windows organizes folders and files. In a hierarchical model database, each upward link is nested in

order to keep data organized in a particular order on a same level list. For instance, a hierarchal

database of sales, may list each days sales as a separate file. Within this nested file are all of the sales

(same types of data) for the day.

3. The Network Model Database:In a network model, the defining feature is that a record is stored with a link to other recordsin effect

networked. These networks (or sometimes referred to as pointers) can be a variety of different types of

information such as node numbers or even a disk address.

4. The Relational Model Database:The relational model is the most popular type of database and an extremely powerful tool, not only to

store information, but to access it as well. Relational databases are organized as tables. The beauty of a

table is that the information can be accessed or added without reorganizing the tables. A table can havemany records and each record can have many fields.

What is an Instance?

a) The term instanceis typically used to describe a complete database environment, including theRDBMS software, table structure, stored procedures and other functionality. It is most

commonly used when administrators describe multiple instances of the same database.

b) An instance is just collection of background processes and memory structures, used to tap in tothe database.
http://en.wikipedia.org/wiki/Datahttp://whatis.techtarget.com/definition/0,289893,sid9_gci212343,00.htmlhttp://www.tech-faq.com/microsoft-windows.htmlhttp://www.tech-faq.com/microsoft-windows.htmlhttp://www.tech-faq.com/microsoft-windows.htmlhttp://www.tech-faq.com/microsoft-windows.htmlhttp://whatis.techtarget.com/definition/0,289893,sid9_gci212343,00.htmlhttp://en.wikipedia.org/wiki/Data


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Also Known As: environment

Example:

An organization with an employees database might have three different instances: Production(used to

contain live data), Pre-production(used to test new functionality prior to release into production) and

Development(used by database developers to create new functionality).

Data Management

Data Management is a broad field of study, but essentially is the process of managing data as a resource

that is valuable to an organization or business. One of the largest organizations that deal with data

management, DAMA (Data Management Association), states that data management is the process of

developing data architectures, practices and procedures dealing with data and then executing these

aspects on a regular basis.

There are many topics within data management, some of the more popular topics include data

modeling, data warehousing, data movement,database administration and data mining.

Data Modeling

Data modeling is first creating a structure for the data that you collect and use and then organizing this

data in a way that is easily accessible and efficient to store and pull the data for reports and analysis. In

order to create a structure for data, it must be named appropriately and show a relationship with other

data. It also must fit appropriately in a class. For instance, if you have a database of media, you might

have a hierarchal structure of objects that include photos, videos, and audio files. Within each category,

you can classify objects accordingly.

Data Warehousing

Data warehousing is storing data effectively so that it can be accessed and used efficiently. Different

organizations collect different types of data, but many organizations use their data the same way, inorder to create reports and analyze their data to make quality business decisions. Data warehousing is

usually an organizational wide repository of data, however for very large corporations in can encompass

just one office or one department.

Data Movement

Data movement is the ability to move data from one place to another. For instance, data needs to be

moved from where it is collected to a database and then to an end user, but this process takes quite a

bit of logistic insight. Not only do all hardware, applications and data collected need to be compatible

with one another, they must also be able to be classified, stored and accessed with ease within an

organization. Moving data can be very expensive and can require lots of resources to make sure that

data is moved efficiently, that data is secure in transit and that once it reaches the end user it can be

used effectively either to be printed out as a report, saved on a computer or sent as an email

attachment.

Data Mining

Another important topic regarding data management is data mining. Data mining is a process in which

large amounts of data are sifted through to show trends, relationships, and patterns. Data mining is a
http://www.tech-faq.com/data-management.htmlhttp://www.tech-faq.com/data-management.htmlhttp://www.tech-faq.com/data-management.html


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crucial component to data management because it exposes interesting information about the data

being collected. It is important to note that data is primarily collected so it can be used to find these

patterns, relationships and trends that can help a business grow or create profit.

Database Administration

Database administration is extremely important in managing data. Every organization or enterpriseneeds database administrators that are responsible for the database environment. Database

administrators are usually given the authority to do the following tasks that include recoverability,

integrity, security, availability, performance and development & testing support.

Some Basics command of ORACLE 10g on Windows Operating System:

SQL> show parameter db_name

SQL> show parameter instance_name

SQL> show parameterdb_block_size (it will show the default block size which is 8KB)

Abbreviations:

SGA: System Global Area

PGA: Program Global Area

pfile: Parameter File

spfile: Server Parameter File

CKPT: Check Point

DBW: Database Writer

LGWR: Log Writer

ARC0: Archiver

PMON: Process Monitor

SMON: System Monitor

What is a Table?

A Tableis the primary unit of physical storage in a database. Usually a database contains more than one

table.

What is an Entity?

An entity is a business object represents a group or category of data.

Data Types:

Alphanumeric (Text, Memo) Numeric (Number, Currency, etc.) Date/Time Boolean (Yes/No)


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Oracle Memory Structure

Figure 1:

CKPT (Check Point): ---------------------------------

DBW (Database Writer): ----------------------------

LGWR (Log Writer): -----------------------------------

ARC0 (Archiver): -------------------------------------

PMON (Process Monitor): -------------------------

SMON (System Monitor): ---------------------------

How to check Oracle services on Windows and UNIX systems:

a) On Windows system click on Task Manager and then processes tab, you will Oracle processb) On UNIX system use the below command:

# ps -ef | grep oracle


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Physical Structure of Database

Usually a database has the below files:

a) Data Files (*.dbf) -------------------b)

Redo Logs (*.log) -------------------

c) Undo (*.dbf) -------------------------d) Spfile or init file ---------------------e) Alert Log (*.log) ---------------------f) Trace Files (*.trc) --------------------g) Control Files (*.ctl) -------------------h) Backup Files ----------------------------

Figure 2:


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Figure 3: Log files location and size query

Logical Storage Structure

Figure 4:


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a) SYSTEM TABLESPACE:b) SYSAUX TABLESPACE:c) UNDO TABLESPACE:d) TEMP TABLESPACE:e) DATA TABLESPCAE:

Startup, Shutdown and Oracle Initialization File

Figure 5:


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Figure 6:


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Figure 7: startup database:

Data Dictionary Views

Figure 7: DataDictionary Views


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Figure 8: Data Dictionary View:


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Managing Tables, Views, Constraints and Sequences

What is Table?

What is View?

What is Constraint?

What is Sequence?

Data Moving Utilities:

Figure 9:

Four Types of Exports:

1. Full Database Export2. Schema Export3. Table Export4. Tablespace Export


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Transportable Tablespaces


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PL/SQL (Procedural Language / Structural Query Language


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1. What is database?

A database is a logically coherent collection of data with some inherent meaning, representing

some aspect of real world and which is designed, built and populated with data for a specific

purpose.

2. What is DBMS?

It is a collection of programs that enables user to create and maintain a database. In other

words it is general-purpose software that provides the users with the processes of defining,

constructing and manipulating the database for various applications.

3. What is a Database system?

The database and DBMS software together is called as Database system.

4. Advantages of DBMS?

Redundancy is controlled. Unauthorized access is restricted. Providing multiple user interfaces. Enforcing integrity constraints. Providing backup and recovery.

5. Disadvantage in File Processing System?

Data redundancy & inconsistency. Difficult in accessing data. Data isolation. Data integrity. Concurrent access is not possible. Security Problems.

6. Describe the three levels of data abstraction?

There are three levels of abstraction:

Physical level: The lowest level of abstraction describes how data are stored. Logical level: The next higher level of abstraction, describes what data are stored in

database and what relationship among those data.

View level: The highest level of abstraction describes only part of entire database.


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7. Define the integrity rules

There are two Integrity rules.

Entity Integrity: States that Primary key cannot have NULL value Referential Integrity: States that Foreign Key can be either a NULL value or should be

Primary Key value of other relation.

8. What is extension and intension?

Extension -

It is the number of tuples present in a table at any instance. This is time dependent.

Intension -

It is a constant value that gives the name, structure of table and the constraints laid on it.

9. What is System R? What are its two major subsystems?

System R was designed and developed over a period of 1974-79 at IBM San Jose Research

Center. It is a prototype and its purpose was to demonstrate that it is possible to build a

Relational System that can be used in a real life environment to solve real life problems, with

performance at least comparable to that of existing system.

Its two subsystems are

Research Storage System Relational Data System.

10. How is the data structure of System R different from the relational structure?

Unlike Relational systems in System R

Domains are not supported Enforcement of candidate key uniqueness is optional Enforcement of entity integrity is optional Referential integrity is not enforced

11. What is Data Independence?

Data independence means that the application is independent of the storage structure and

access strategy of data. In other words, The ability to modify the schema definition in one level

should not affect the schema definition in the next higher level.

Two types of Data Independence:

Physical Data Independence: Modification in physical level should not affect the logicallevel.

Logical Data Independence: Modification in logical level should affect the view level.


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NOTE: Logical Data Independence is more difficult to achieve

12. What is a view? How it is related to data independence?

A view may be thought of as a virtual table, that is, a table that does not really exist in its own

right but is instead derived from one or more underlying base table. In other words, there is nostored file that direct represents the view instead a definition of view is stored in data

dictionary.

Growth and restructuring of base tables is not reflected in views. Thus the view can insulate

users from the effects of restructuring and growth in the database. Hence accounts for logical

data independence.

13. What is Data Model?

A collection of conceptual tools for describing data, data relationships data semantics and

constraints.

14. What is E-R model?

This data model is based on real world that consists of basic objects called entities and of

relationship among these objects. Entities are described in a database by a set of attributes.

15. What is Object Oriented model?

This model is based on collection of objects. An object contains values stored in instance

variables with in the object. An object also contains bodies of code that operate on the object.

These bodies of code are called methods. Objects that contain same types of values and thesame methods are grouped together into classes.

16. What is an Entity?

It is a thing in the real world with an independent existence.

17. What is an Entity type?

It is a collection (set) of entities that have same attributes.

18. What is an Entity set?

It is a collection of all entities of particular entity type in the database.

19. What is an Extension of entity type?

The collections of entities of a particular entity type are grouped together into an entity set.


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20. What is Weak Entity set?

An entity set may not have sufficient attributes to form a primary key, and its primary key

compromises of its partial key and primary key of its parent entity, then it is said to be Weak

Entity set.

21. What is an attribute?

It is a particular property, which describes the entity.

22. What is a Relation Schema and a Relation?

A relation Schema denoted by R(A1, A2, , An) is made up of the relation name R and the list of

attributes Ai that it contains. A relation is defined as a set of tuples. Let r be the relation which

contains set tuples (t1, t2, t3, ,tn). Each tuple is an ordered list of n-values t=(v1,v2, , vn).

23. What is degree of a Relation?

It is the number of attribute of its relation schema.

24. What is Relationship?

It is an association among two or more entities.

25. What is Relationship set?

The collection (or set) of similar relationships.

26. What is Relationship type?

Relationship type defines a set of associations or a relationship set among a given set of entity

types.

27. What is degree of Relationship type?

It is the number of entity type participating.

25. What is DDL (Data Definition Language)?

A data base schema is specifies by a set of definitions expressed by a special language called

DDL.

26. What is VDL (View Definition Language)?


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It specifies user views and their mappings to the conceptual schema.

27. What is SDL (Storage Definition Language)?

This language is to specify the internal schema. This language may specify the mappingbetween two schemas.

28. What is Data Storage - Definition Language?

The storage structures and access methods used by database system are specified by a set of

definition in a special type of DDL called data storage-definition language.

29. What is DML (Data Manipulation Language)?

This language that enable user to access or manipulate data as organised by appropriate data

model.

Procedural DML or Low level: DML requires a user to specify what data are needed andhow to get those data.

Non-Procedural DML or High level: DML requires a user to specify what data are neededwithout specifying how to get those data.

31. What is DML Compiler?

It translates DML statements in a query language into low-level instruction that the queryevaluation engine can understand.

32. What is Query evaluation engine?

It executes low-level instruction generated by compiler.

33. What is DDL Interpreter?

It interprets DDL statements and record them in tables containing metadata.

34. What is Record-at-a-time?

The Low level or Procedural DML can specify and retrieve each record from a set of records.

This retrieve of a record is said to be Record-at-a-time.

35. What is Set-at-a-time or Set-oriented?


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The High level or Non-procedural DML can specify and retrieve many records in a single DML

statement. This retrieve of a record is said to be Set-at-a-time or Set-oriented.

36. What is Relational Algebra?

It is procedural query language. It consists of a set of operations that take one or two relationsas input and produce a new relation.

37. What is Relational Calculus?

It is an applied predicate calculus specifically tailored for relational databases proposed by E.F.

Codd. E.g. of languages based on it are DSL ALPHA, QUEL.

38. How does Tuple-oriented relational calculus differ from domain-oriented relational

calculus?

The tuple-oriented calculus uses a tuple variables i.e., variable whose only permitted values are

tuples of that relation. E.g. QUEL

The domain-oriented calculus has domain variables i.e., variables that range over the

underlying domains instead of over relation. E.g. ILL, DEDUCE.

39. What is normalization?

It is a process of analysing the given relation schemas based on their Functional Dependencies

(FDs) and primary key to achieve the properties

Minimizing redundancy Minimizing insertion, deletion and update anomalies.

40. What is Functional Dependency?

A Functional dependency is denoted by X Y between two sets of attributes X and Y that are

subsets of R specifies a constraint on the possible tuple that can form a relation state r of R. The

constraint is for any two tuples t1 and t2 in r if t1[X] = t2[X] then they have t1[Y] = t2[Y]. This

means the value of X component of a tuple uniquely determines the value of component Y.

41. When is a functional dependency F said to be minimal?

Every dependency in F has a single attribute for its right hand side. We cannot replace any dependency X A in F with a dependency Y A where Y is a proper

subset of X and still have a set of dependency that is equivalent to F.

We cannot remove any dependency from F and still have set of dependency that isequivalent to F.


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42. What is Multivalued dependency?

Multivalued dependency denoted by X Y specified on relation schema R, where X and Y are

both subsets of R, specifies the following constraint on any relation r of R: if two tuples t1 and

t2 exist in r such that t1[X] = t2[X] then t3 and t4 should also exist in r with the followingproperties

t3[x] = t4[X] = t1[X] = t2[X] t3[Y] = t1[Y] and t4[Y] = t2[Y] t3[Z] = t2[Z] and t4[Z] = t1[Z]

where [Z = (R-(X U Y)) ]

43. What is Lossless join property?

It guarantees that the spurious tuple generation does not occur with respect to relation

schemas after decomposition.

44. What is 1 NF (Normal Form)?

The domain of attribute must include only atomic (simple, indivisible) values.

45. What is Fully Functional dependency?

It is based on concept of full functional dependency. A functional dependency X Y is full

functional dependency if removal of any attribute A from X means that the dependency doesnot hold any more.

46. What is 2NF?

A relation schema R is in 2NF if it is in 1NF and every non-prime attribute A in R is fully

functionally dependent on primary key.

47. What is 3NF?

A relation schema R is in 3NF if it is in 2NF and for every FD X A either of the following is true

X is a Super-key of R. A is a prime attribute of R.

In other words, if every non prime attribute is non-transitively dependent on primary key.

48. What is BCNF (Boyce-Codd Normal Form)?


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A relation schema R is in BCNF if it is in 3NF and satisfies an additional constraint that for every

FD X A, X must be a candidate key.

49. What is 4NF?

A relation schema R is said to be in 4NF if for every Multivalued dependency X Y that holds over

R, one of following is true

X is subset or equal to (or) XY = R. X is a super key.

50. What is 5NF?

A Relation schema R is said to be 5NF if for every join dependency {R1, R2, ,Rn} that holds R,

one the following is true

Ri = R for some i. The join dependency is implied by the set of FD, over R in which the left side is key of R.

51. What is Domain-Key Normal Form?

A relation is said to be in DKNF if all constraints and dependencies that should hold on the the

constraint can be enforced by simply enforcing the domain constraint and key constraint on the

relation.

52. What are partial, alternate,, artificial, compound and natural key?

Partial Key:

It is a set of attributes that can uniquely identify weak entities and that are related to same

owner entity. It is sometime called as Discriminator.

Alternate Key:

All Candidate Keys excluding the Primary Key are known as Alternate Keys.

Artificial Key:

If no obvious key, either stand alone or compound is available, then the last resort is to simply

create a key, by assigning a unique number to each record or occurrence. Then this is known as

developing an artificial key.

Compound Key:

If no single data element uniquely identifies occurrences within a construct, then combining

multiple elements to create a unique identifier for the construct is known as creating a

compound key.

Natural Key:


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When one of the data elements stored within a construct is utilized as the primary key, then it

is called the natural key.

53. What is indexing and what are the different kinds of indexing?

Indexing is a technique for determining how quickly specific data can be found.

Types:

Binary search style indexing B-Tree indexing Inverted list indexing Memory resident table Table indexing

54. What is system catalog or catalog relation? How is better known as?

A RDBMS maintains a description of all the data that it contains, information about every

relation and index that it contains. This information is stored in a collection of relations

maintained by the system called metadata. It is also called data dictionary.

55. What is meant by query optimization?

The phase that identifies an efficient execution plan for evaluating a query that has the least

estimated cost is referred to as query optimization.

56. What is join dependency and inclusion dependency?

Join Dependency:

A Join dependency is generalization of Multivalued dependency.A JD {R1, R2, ,Rn} is said to

hold over a relation R if R1, R2, R3, , Rn is a lossless-join decomposition of R . There is no set of

sound and complete inference rules for JD.

Inclusion Dependency:

An Inclusion Dependency is a statement of the form that some columns of a relation are

contained in other columns. A foreign key constraint is an example of inclusion dependency.

57. What is durability in DBMS?

Once the DBMS informs the user that a transaction has successfully completed, its effectsshould persist even if the system crashes before all its changes are reflected on disk. This

property is called durability.

58. What do you mean by atomicity and aggregation?

Atomicity:


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Either all actions are carried out or none are. Users should not have to worry about the effect of

incomplete transactions. DBMS ensures this by undoing the actions of incomplete transactions.

Aggregation:

A concept which is used to model a relationship between a collection of entities and

relationships. It is used when we need to express a relationship among relationships.

59. What is a Phantom Deadlock?

In distributed deadlock detection, the delay in propagating local information might cause the

deadlock detection algorithms to identify deadlocks that do not really exist. Such situations are

called phantom deadlocks and they lead to unnecessary aborts.

60. What is a checkpoint and When does it occur?

A Checkpoint is like a snapshot of the DBMS state. By taking checkpoints, the DBMS can reduce

the amount of work to be done during restart in the event of subsequent crashes.

61. What are the different phases of transaction?

Different phases are

Analysis phase Redo Phase Undo phase

62. What do you mean by flat file database?

It is a database in which there are no programs or user access languages. It has no cross-file

capabilities but is user-friendly and provides user-interface management.

63. What is transparent DBMS?

It is one, which keeps its Physical Structure hidden from user.

64. Brief theory of Network, Hierarchical schemas and their properties

Network schema uses a graph data structure to organize records example for such a databasemanagement system is CTCG while a hierarchical schema uses a tree data structure example for

such a system is IMS.

65. What is a query?

A query with respect to DBMS relates to user commands that are used to interact with a data

base. The query language can be classified into data definition language and data manipulation


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language.

66. What do you mean by Correlated subquery?

Subqueries, or nested queries, are used to bring back a set of rows to be used by the parent

query. Depending on how the subquery is written, it can be executed once for the parent queryor it can be executed once for each row returned by the parent query. If the subquery is

executed for each row of the parent, this is called a correlated subquery.

A correlated subquery can be easily identified if it contains any references to the parent

subquery columns in its WHERE clause. Columns from the subquery cannot be referenced

anywhere else in the parent query. The following example demonstrates a non-correlated

subquery.

E.g. Select * From CUST Where 10/03/1990? IN (Select ODATE From ORDER Where

CUST.CNUM = ORDER.CNUM)

67. What are the primitive operations common to all record management systems?

Addition, deletion and modification.

68. Name the buffer in which all the commands that are typed in are stored

Edit Buffer

69. What are the unary operations in Relational Algebra?

PROJECTION and SELECTION.

70. Are the resulting relations of PRODUCT and JOIN operation the same?

No.

PRODUCT: Concatenation of every row in one relation with every row in another.

JOIN: Concatenation of rows from one relation and related rows from another.

71. What is RDBMS KERNEL?

Two important pieces of RDBMS architecture are the kernel, which is the software, and

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