Sunday, August 21, 2016

Oracle Data Base Administration (DBA) Interview Questions and Answers (FAQS)

1. What is a Database instance ? Explain
A  database  instance  (Server) is a set of memory structure and background processes that access a set of database files.
The process can be shared by all users.
The  memory  structure  that  are  used  to  store  most  queried data from database.  This  helps up to improve database performance by decreasing the amount of I/O performed against data file.
2. What is Parallel Server ?
Multiple   instances   accessing  the  same  database  (Only  In  Multi-CPU environments)
3. What is a Schema ?
The set of objects owned by user account is called the schema.
4.  What is an Index ? How it is implemented in Oracle Database ?
An  index  is a database structure used by the server to have direct access of a row in a table.
An  index  is automatically created when a unique of primary key constraint clause is specified in create table comman (Ver 7.0)
5. What is clusters ?
Group  of  tables  physically  stored  together  because  they share common columns and are often used together is called Cluster.
6. What is a cluster Key ?
The  related columns of the tables are called the cluster key.  The cluster key  is indexed using a cluster index and its value is stored only once for multiple tables in the cluster.
7. What are the basic element of Base configuration of an oracle Database ?
It consists of
     one or more data files.
     one or more control files.
     two or more redo log files.
The Database contains
     multiple users/schemas
     one or more rollback segments
     one or more tablespaces
     Data dictionary tables
     User objects (table,indexes,views etc.,)
The server that access the database consists of
     SGA  (Database  buffer,  Dictionary  Cache  Buffers, Redo log buffers, Shared SQL pool)
     SMON (System MONito)
     PMON (Process MONitor)
     LGWR (LoG  Write)
     DBWR (Data Base Write)
     ARCH (ARCHiver)
     CKPT  (Check Point)
     User Process with associated PGS
8. What is a deadlock ? Explain .
Two  processes wating to update the rows of a table which are locked by the other process then deadlock arises.
In  a  database  environment  this will often happen because of not issuing proper  row  lock  commands. Poor design of front-end application may cause this situation and the performance of server will reduce drastically.
These locks will be released automatically when a commit/rollback operation performed or any one of this processes being killed externally.
9. What is SGA ? How it is different from  Ver 6.0 and Ver 7.0 ?
The  System   Global  Area  in  a  Oracle database is the area in memory to facilitates  the  transfer  of information between users. It holds the most recently  requested structural information between users. It holds the most recently requested structural information about the database.
The  structure  is  Database buffers, Dictionary cache, Redo Log Buffer and Shared SQL pool (ver 7.0 only) area.
10. What is a Shared SQL pool ?
The data dictionary cache is stored in an area in SGA called the Shared SQL Pool.  This  will  allow  sharing of parsed SQL statements among concurrent users.
11. What is mean by Program Global Area (PGA) ?
It is area in memory that is used by a Single Oracle User Process.
12. What is a data segment ?
Data  segment  are  the physical areas within a database block in which the data associated with tables and clusters are stored.
13. What are the factors causing the reparsing of SQL statements in SGA? 
Due to insufficient Shared SQL pool size.
Monitor   the  ratio  of  the  reloads  takes  place  while  executing  SQL statements.   If   the   ratio   is   greater  than  1  then  increase  the SHARED_POOL_SIZE.
14. What is  Database Buffers ?
Database buffers are cache in the SGA used to hold the data blocks that are read  from  the  data  segments in the database such as tables, indexes and clusters DB_BLOCK_BUFFERS parameter in INIT.ORA decides the size.
15. What is dictionary cache ?
Dictionary  cache is information about the databse objects stored in a data dictionary table.
16. What is meant by recursive hints ?
Number  of  times processes repeatedly query the dictionary table is called recursive  hints.  It  is due to the data dictionary cache is too small. By increasing  the SHARED_POOL_SIZE parameter we can optimize the size of Data Dictionary Cache.
17. What is meant by redo log buffer ?
Change  made  to entries are written to the on-line redo log files. So that they  can  be  used  in roll forward operations during database recoveries. Before  writing  them  into  the redo log files, they will first brought to redo  log  buffers  in  SGA  and  LGWR  will  write  into files frequently.
LOG_BUFFER parameter will decide the size.
18.  How will you swap objects into a different table space for an existing database ?
Export the user
Perform import  using  the  command  imp  system/manager   file=export.dmp indexfile=newrite.sql. This will create all definitions into newfile.sql.
Drop necessary objects.
Run the script newfile.sql after altering the tablespaces.
Import from the backup for the necessary objects.
19.  List  the Optional Flexible Architecture (OFA) of Oracle database ? or How  can  we  organise  the  tablespaces in Oracle database to have maximum performance ?
SYSTEM - Data dictionary tables.
DATA  - Standard operational tables.
DATA2- Static tables used for standard operations
INDEXES - Indexes for Standard operational tables.
INDEXES1 - Indexes of static tables used for standard operations.
TOOLS - Tools table.
TOOLS1 - Indexes for tools table.
RBS - Standard Operations Rollback Segments,
RBS1,RBS2 - Additional/Special Rollback segments.
TEMP - Temporary purpose tablespace
TEMP_USER - Temporary tablespace for users.
USERS - User tablespace.
20. How will you force database to use particular rollback segment ?
21. What is meant by free extent ?
A free extent is a collection of continuous free blocks in tablespace. When a segment is dropped its extents are reallocated and are marked as free.
22. How free extents are managed in Ver 6.0 and Ver 7.0 ?
Free extents cannot be merged together in Ver 6.0.
Free extents are periodically coalesces with the neighboring free extent in Ver 7.0
23.Which parameter in Storage clause will reduce no. of rows per block?
PCTFREE parameter
Row size also reduces no of rows per block.
24. What is the significance of having storage clause ?
We  can  plan  the  storage  for  a  table  as how much initial extents are required,  how  much can be extended next, how much % should leave free for managing row updations etc.,
25. How does Space allocation table place within a block ?
Each block contains entries as follows
Fixied block header
Variable block header
Row Header,row date (multiple rows may exists)
PCTEREE (% of  free space for row updation in future)
26. What is the role of PCTFREE parameter is Storage clause ?
This is used to reserve certain amount of space in a block for expansion of rows.
27. What is the OPTIMAL parameter ?
It is used to set the optimal length of a rollback segment.
28. What is the functionality of SYSTEM table space ?
To manage the database level transactions such as modifications of the data dictionary table that record information about the free space usage.
29. How will you create multiple rollback segments in a database ?
Create  a  database which implicitly creates a SYSTEM Rollback Segment in a SYSTEM tablespace.
Create a Second Rollback Segment name R0  in the SYSTEM tablespace.
Make  new  rollback segment available (After shutdown, modify init.ora file and Start database)
Create other tablespaces (RBS)  for rollback segments.
Deactivate  Rollback  Segment  R0  and  activate the newly created rollback segments.
30. How the space utilisation takes place within rollback segments ?
It  will  try  to  fit  the transaction in a cyclic fashion to all existing extents.  Once it found an extent is in use then it forced to acquire a new extent (No. of extents is based on the optimal size)
31. Why query fails sometimes ?
Rollback  segment  dynamically  extent  to handle larger transactions entry loads.
A  single  transaction may wipeout all avaliable free space in the Rollback Segment Tablespace. This prevents other user using Rollback segments.
32. How will you monitor the space allocation ?
By quering DBA_SEGMENT table/view.
33. How  will you monitor rollback segment status ?
Querying the DBA_ROLLBACK_SEGS view
IN USE         - Rollback Segment is on-line.
AVAILABLE - Rollback Segment available but not on-line.
OFF-LINE  - Rollback Segment off-line
INVALID        - Rollback Segment Dropped.
NEEDS RECOVERY  - Contains data but need recovery or corupted.
PARTLY  AVAILABLE  - Contains data from an unresolved transaction involving a distributed database.
34.  List  the  sequence  of  events  when a large transaction that exceeds beyond  its  optimal  value  when  an  entry  wraps and causes the rollback segment to expand into another extend.
Transaction Begins.
An entry is made in the RES header for new transactions entry
Transaction acquires blocks in an extent of RBS
The entry attempts to wrap into second extent. None is available, so that the RBS must extent.
The RBS checks to see if it is part of its OPTIMAL size.
RBS chooses its oldest inactive segment.
Oldest inactive segment is eliminated.
RBS extents
The Data dictionary table for space management are updated.
Transaction Completes.
35. How can we plan storage for very large tables ?
Limit the number of extents in the table
Separate Table from its indexes.
Allocate Sufficient temporary storage.
36. How will you estimate the space required by a non-clustered tables?
Calculate the total header size
Calculate the available dataspace per data block
Calculate the combined column lengths of the average row
Calculate the total average row size.
Calculate the average number rows that can fit in a block
Calculate the number of blocks and bytes required for the table.
After  arriving the calculation, add 10 % additional space to calculate the initial extent size for a working table.
37.  It is possible to use raw devices as data files and what is the advantages over file. system files ?
The advantages over file system files.
I/O  will  be  improved  because  Oracle  is  bye-passing the kernnel which writing into disk.
Disk Corruption will be very less.
38. What is a Control file ?
Database's  overall  physical  architecture  is maintained in a file called control  file.  It  will be used to maintain internal consistency and guide recovery operations. Multiple copies of control files are advisable.
39. How to implement the multiple control files for an existing database ?
Shutdown the databse
Copy one of the existing control file to new location
Edit Config ora file by adding new control
Restart the database.
40. What is meant by Redo Log file mirrorring ? How it can be achieved?
Process of having a copy of redo log files is called mirroring.
This  can be achieved by creating group of log files together, so that LGWR will  automatically  writes  them to all the members of the current on-line redo  log  group. If any one group fails then database automatically switch over to next group. It degrades performance.
41. What is advantage of having disk shadowing/ Mirroring ?
Shadow  set of disks save as a backup in the event of disk failure. In most Operating  System if any disk failure occurs it automatically switchover to place of failed disk.
Improved performance  because  most OS support volume shadowing can direct file  I/O request to use the shadow set of files instead of the main set of files. This reduces I/O load on the main set of disks.
42. What is use of Rollback Segments In Database ?
They  allow  the  database  to  maintain  read consistency between multiple transactions.
43. What is a Rollback segment entry ?
It  is  the  set  of  before  image  data blocks that contain rows that are modified by a transaction.
Each   Rollback  Segment  entry  must  be  completed  within   one rollback segment.
A single rollback segment can have multiple rollback segment entries.
44. What is hit  ratio ?
It  is  a  measure  of  well the data cache buffer is handling requests for data.
Hit Ratio = (Logical Reads - Physical Reads - Hits Misses)/ Logical Reads.
45. When will be a segment released ?
When Segment is dropped.
When Shrink (RBS only)
When truncated (TRUNCATE used with drop storage option)
46. What are disadvanteges of having raw devices ?
We  should  depend  on  export/import  utility  for  backup/recovery (fully reliable)
The tar command cannot be used for physical file backup, instead we can use dd command which is less flexible and has limited recoveries.
47. List the factors that can affect the accuracy of the estimations ?
The space used transaction entries and deleted records does not become free immediately after completion due to delayed cleanout.
Trailling nulls and length bytes are not stored.
Inserts of, updates to and deletes of rows as well as columns larger than a single datablock, can cause fragmentation an chained row pieces.
48. What is user Account in Oracle database ?
An  user  account  is not a physical structure in Database but it is having important  relationship  to  the objects in the database and will be having certain privileges.
49. How will you enforce security using stored procedures ?
Don't grant user access directly to tables within the application.
Instead grant the ability to access the procedures that access the tables.
When  procedure executed it will execute the privilege of procedures owner. Users cannot access tables except via the procedure.
50. What are the dictionary tables used to monitor a database spaces ?
51. What are the responsibilities of a Database Administrator ?
Installing and upgrading the Oracle Server and application tools.
Allocating  system storage and planning future storage requirements for the database  system.
Managing primary database structures (tablespaces)
Managing primary objects (table,views,indexes)
Enrolling users and maintaining system security.
Ensuring compliance with Oralce license agreement
Controlling and monitoring user access to the database.
Monitoring and optimising the performance of the database.
Planning for backup and recovery of database information.
Maintain archived data on tape
Backing up and restoring the database.
Contacting Oracle Corporation for technical support.
52.  What  are  the roles  and user accounts created automatically with the database ?
DBA - role Contains all database system privileges.
SYS  user  account  - The DBA role will be assigned to this account. All of the  basetables  and  views for the database's dictionary are store in this schema and are manipulated only by ORACLE.
SYSTEM user account - It has all the system privileges for the database and additional  tables  and  views  that display administrative information and internal  tables  and  views  used  by  oracle tools are created using this username.
54. What are the database administrators utilities avaliable ?
SQL * DBA - This allows DBA to monitor and control an ORACLE database.
SQL  *  Loader  -  It loads data from standard operating system files (Flat files) into ORACLE database tables.
Export  (EXP) and Import (imp) utilities allow you to move existing data in ORACLE format to and from ORACLE database.
55.  What  are  the  minimum  parameters should exist in the parameter file (init.ora) ?
DB  NAME  -  Must  set to a text string of no more than 8 characters and it will  be  stored inside the datafiles, redo log files and control files and control file while database creation.
DB_DOMAIN  -  It  is  string  that  specifies  the network domain where the database  is  created.  The  global  database name is identified by setting these parameters (DB_NAME & DB_DOMAIN)
CONTORL  FILES  - List of control filenames of the database. If name is not mentioned then default name will be used.
DB_BLOCK_BUFFERS  -  To  determine the no of buffers in the buffer cache in SGA.
PROCESSES  -  To determine number of operating system processes that can be connected  to  ORACLE  concurrently.  The  value  should  be  5 (background process) and additional 1 for each user.
ROLLBACK_SEGMENTS  -  List of rollback segments an ORACLE instance acquires at database startup.
56. What is a trace file and how is it created ?
Each server and background process can write an associated trace file. When an   internal  error  is  detected  by  a  process or user process, it dumps information  about  the error to its trace. This can be used for tuning the database.
57. What are roles ? How can we implement roles ?
Roles  are  the easiest way to grant and manage common privileges needed by different groups of database users.
Creating roles and assigning provies to roles.
Assign each role to group of users. This will simplify the job of assigning privileges to individual users.
58.  What  are the steps to switch a database's archiving mode between   NO ARCHIVELOG and ARCHIVELOG mode ?
     1. Shutdown the database instance.
     2. Backup the databse
     3. Perform any operating system specific steps (optional)
     4. Start up a new instance and mount but do not open the databse.
     5. Switch the databse's archiving mode.
59. How can you enable automatic archiving ?
     Shut the database
     Backup the database
     Modify/Include LOG_ARCHIVE_START_TRUE in init.ora file.
     Start up the databse.
60. How can we specify the Archived log file name format and destination ?
By setting the following values in init.ora file.
LOG_ARCHIVE_FORMAT = arch %S/s/T/tarc (%S - Log sequence number and is zero left  paded,  %s  -  Log  sequence  number  not  padded. %T - Thread number lef-zero-paded  and  %t -  Thread number not padded). The file name created is arch 0001 are if %S is used.
61. What is the use of  ANALYZE command  ?
To perform one of these function on an index,table, or cluster:
     -  to  collect statisties about object used by the optimizer and store them in the data           dictionary.
     -  to  delete statistics about the object used by object from the data dictionary.
     - to validate the structure of the object.
     - to identify migrated and chained rows of the table or cluster.
62. How can we reduce the network traffic ?
     - Replictaion of data in distributed environment.
     - Using snapshots to replicate data.
     - Using remote procedure calls.
63. What is snapshots ?
Snapshot is an object used to dynamically replicate data between distribute database at specified  time intervals. In ver 7.0 they are read only.
64. What are the various type of snapshots ?
Simple and Complex.
65. Differentiate simple and complex, snapshots ?
     -  A  simple snapshot is based on a query that does not contains GROUP BY clauses, CONNECT BY clauses, JOINs, sub-query or snashot of operations.
     - A complex snapshots contain atleast any one of the above.
66. What dynamic data replication ?
Updating or Inserting records in remote database through database triggers. It may fail if remote database is having any problem.
67. How can you Enforce Refrencial Integrity  in snapshots ?
Time the references to occur when master tables are not in use.
 Peform the reference the manually immdiately locking the master tables. We  can join  tables in snopshots by creating a complex snapshots that will based on the master tables.
68. What are the options available to  refresh snapshots ?
COMPLETE   -   Tables   are  completly  regenerated  using  the  snapshot's query and the master tables every time the snapshot referenced.
FAST  - If simple snapshot used then a snapshot log can be used to send the changes to  the snapshot tables.
FORCE  -  Default value. If possible it performs a  FAST refresh; Otherwise it will perform a complete refresh.
69. what is snapshot log ?
It  is a table that maintains a record of modifications to the master table in  a  snapshot.  It is stored  in the same database as master table and is only  available for simple snapshots.  It should be created before creating snapshots.
70. When will the data in the snapshot log be used ?
We  must be able to create a after row trigger on table (i.e., it should be not be already available )
After giving table privileges.
We  cannot  specify  snapshot  log name because oracle uses the name of the master  table in the name of the database objects that support its snapshot log.
The master table name should be less than or equal to 23 characters.
(The  table  name created will be MLOGS_tablename, and trigger name will be TLOGS name).
72. What are the benefits of distributed options in databases ?
Database  on  other  servers  can  be updated and those transactions can be grouped together with others in a logical unit.
Database uses a two phase commit.
73. What are the different methods of backing up oracle database ?
     - Logical Backups
     - Cold Backups
     - Hot Backups (Archive log)
74. What is a logical backup ?
Logical  backup  involves reading a set of databse records and writing them into a file. Export utility is used for taking backup and Import utility is used to recover from backup.
75. What is cold backup ? What are the elements of it ?
Cold backup is taking backup of all physical files after normal shutdown of database.  We need to take.
     - All Data files.
     - All Control files.
     - All on-line redo log files.
     - The init.ora file (Optional)
76. What are the different kind of export backups ?
Full back      - Complete database
Incremental          - Only affected tables from last incremental date/full backup date.
Cumulative  backup - Only affected table from the last cumulative date/full backup date.
77. What is hot backup and how it can be taken ?
Taking  backup  of  archive  log  files when database is open. For this the ARCHIVELOG  mode  should  be enabled. The following files need to be backed up. 
All data files. All  Archive log, redo log files. All control files.
78. What is the use of FILE option in EXP command ?
To give the export file name.
79. What is the use of COMPRESS option in EXP command ?
Flag  to  indicate  whether export should compress fragmented segments into single extents.
80. What is the use of GRANT option in EXP command ?
A  flag  to  indicate whether grants on databse objects will be exported or not. Value is 'Y'  or 'N'.
81. What is the use of INDEXES option in EXP command ?
A flag to indicate whether indexes on tables will be exported.
82. What is the use of ROWS option in EXP command ?
Flag  to  indicate  whether  table rows should be exported. If 'N' only DDL statements for the databse objects will be created.
83. What is the use of CONSTRAINTS option in EXP command ?
A flag to indicate whether constraints on table need to be exported.
84. What is the use of FULL option in EXP command ?
A flag to indicate whether full databse export should be performed.
85. What is the use of OWNER option in EXP command ?
List of table accounts should be exported.
86. What is the use of TABLES option in EXP command ?
List of tables should be exported.
87. What is the use of RECORD LENGTH option in EXP command ?
Record length in bytes.
88. What is the use of INCTYPE option in EXP command ?
Type export should be performed COMPLETE,CUMULATIVE,INCREMENTAL.
89. What is the use of RECORD option in EXP command ?
For Incremental exports, the flag indirects whether a record will be stores data dictionary tables recording the export.
90. What is the use of PARFILE option in EXP command ?
Name of the parameter file to be passed for export.
91. What is the use of PARFILE option in EXP command ?
Name of the parameter file to be passed for export.
92.  What is the use of ANALYSE ( Ver 7) option in  EXP command ?
A  flag  to  indicate  whether  statistical  information about the exported objects should be written to export dump file.
93.  What is the use of CONSISTENT (Ver 7) option in  EXP command ?
A  flag  to indicate whether  a read consistent version of all the exported objects should be maintained.
94.   What is use of LOG (Ver 7) option in  EXP command ?
The name of the file which log of the export will be written.
95.What is the use of FILE option in  IMP command ?
The name of the file from which import should be performed.
96.  What is the use of SHOW option in IMP command ?
A flag to indicate whether file content should be displayed or not.
97.  What is the use of IGNORE option in IMP command ?
A  flag  to indicate whether the import should ignore errors encounter when issuing CREATE commands.
98. What is the use of GRANT option in IMP command ?
A flag to indicate whether grants on database objects will be imported.
99. What is the use of INDEXES option in IMP command ?
A flag to indicate whether import should import index on tables or not.
100. What is the use of ROWS option in IMP command ?
A  flag  to indicate whether rows should be imported. If this is set to 'N' then only DDL for database  objects  will be exectued.

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