Saturday, August 20, 2016

Oracle Architecture and Database Interview Questions and Answers (FAQS)

Oracle Concepts and Architecture Database Structures.
1.  What  are  the  components of Physical database structure of Oracle Database?.
ORACLE  database  is  comprised  of  three types of files. One or more Data files, two are more Redo Log files, and one or more Control files.
2.  What  are  the  components  of  Logical  database  structure  of ORACLE database?
Tablespaces and the Database's Schema Objects.
3. What is a Tablespace?
A  database  is  divided  into  Logical  Storage Unit called tablespaces. A tablespace is used to grouped related logical structures together.
4. What is SYSTEM tablespace and When is it Created?
Every  ORACLE  database  contains  a  tablespace  named  SYSTEM,  which  is automatically created when the database is created. The SYSTEM tablespace always contains the data dictionary tables for the entire database.
5. Explain the relationship among Database, Tablespace and Data file.
Each  databases  logically divided into one or more tablespaces One or more data files are explicitly created for each tablespace.
6. What is schema?
A schema is collection of database objects of a User.
7. What are Schema Objects ?
Schema objects are the logical structures that directly refer to the database's data. Schema objects include tables, views, sequences, synonyms, indexes, clusters, database triggers, procedures, functions packages anddatabase links.
8. Can objects of the same Schema reside in different tablespaces.?
9. Can a Tablespace hold objects from different Schemes ?
10. what is Table ?
A table is the basic unit of data storage in an ORACLE database. The tables of a database hold all of the user accessible data. Table data is stored in rows and columns.
11. What is a View ?
A view is a virtual table. Every view has a Query attached to it. (The Query  is a SELECT statement that identifies the columns and rows of the table(s) the view uses.)
12. Do View contain Data ?
Views do not contain or store data.
13. Can a View based on another View ?
14. What are the advantages of Views ?
Provide an additional level of table security, by restricting access to a predetermined set of rows and columns of a table.
Hide data complexity.
Simplify commands for the user.
Present the data in a different perpecetive from that of the base table.
Store complex queries.
15. What is a Sequence ?
A  sequence generates a serial list of unique numbers for numerical columns of a database's tables.
16.  What is a Synonym ?
A synonym is an alias for a table, view, sequence or program unit.
17. What are the type of Synonyms?
There are two types of Synonyms Private and Public.
18. What is a Private Synonyms ?
A Private Synonyms can be accessed only by the owner.
19. What is a Public Synonyms ?
A Public synonyms can be accessed by any user on the database.
20. What are synonyms used for ?
Synonyms are used to : Mask the real name and owner of an object.
Provide public access to an object
Provide location transparency for tables, views or program units of a remote database.
Simplify the SQL statements for database users.
21. What is an Index ?
An  Index is an optional structure associated with a table to have direct access  to  rows, which  can  be created to increase the performance of data retrieval.  Index can be created on one or more columns of a table.
22.  How are Indexes Update ?
Indexes  are  automatically maintained and used by ORACLE. Changes to table data are automatically incorporated into all relevant indexes.
23. What are Clusters ?
Clusters  are  groups  of  one or more tables physically stores together to share common columns and are often used together.
24. What is cluster Key ?
The related columns of the tables in a cluster is called the Cluster Key.
25. What is Index Cluster ?
A Cluster with an index on the Cluster Key.
26. What is Hash Cluster ?
A  row  is  stored in a hash cluster based on the result of applying a hash function  to  the  row's cluster key value. All rows with the same hash key value are stores together on disk.
27. When can Hash Cluster used ?
Hash clusters are better choice when a table is often queried with equality queries.  For  such  queries the specified cluster key value is hashed. The resulting  hash  key  value points directly to the area on disk that stores the specified rows.
28. What is  Database Link ?
A database link is a named object that describes a "path" from one database to another.
29. What are the types of Database Links ?
Private Database Link, Public Database Link & Network Database Link.
30. What is Private Database Link ?
Private database link is created on behalf of a specific user. A private database link can be used only when the owner of the link specifies a global  object  name in a SQL statement or in the definition of the owner's views or procedures.
31. What is Public Database Link ?
Public database link is created for the special user group PUBLIC. A public database  link  can  be  used  when  any  user  in  the associated database specifies a global object name in a SQL  statement or object definition.
32.  What is Network Database link ?
Network database link is created and managed by a network domain service. A network database link can be used when any user of any database in the network  specifies  a  global  object  name  in  a  SQL statement or object definition.
33. What is Data Block ?
ORACLE database's data is stored in data blocks. One data block corresponds to a specific number of bytes of physical database space on disk.
34. How to define Data Block size ?
A data block size is specified for each ORACLE database when the database is  created.  A database users and allocated free database space in ORACLE datablocks.  Block size is specified in INIT.ORA file and cann't be changed latter.
35. What is Row Chaining ?
In Circumstances,  all of the data for a row in a table may not be able to fit  in  the  same  data  block. When this occurs , the data for the row is stored in a chain of data block (one or more) reserved for that segment.
36. What is an Extent ?
An   Extent  is  a specific number of contiguous data blocks, obtained in a single allocation, used to store a specific type of information.
37.  What is a Segment ?
A segment is a set of extents allocated for a certain logical structure.
38. What are the different type of Segments ?
Data Segment, Index Segment, Rollback Segment and Temporary Segment.
39. What is a Data Segment ?
Each  Non-clustered  table  has  a data segment. All of the table's data is stored in the extents of its data segment. Each cluster has a data segment. The  data  of  every  table  in the cluster is stored in the cluster's data segment.
40. What is an Index Segment ?
Each Index has an Index segment that stores all of its data.
41. What is  Rollback Segment ?
A  Database  contains  one  or  more Rollback Segments to temporarily store "undo" information.
42. What are the uses of Rollback Segment ?
Rollback Segments are used :
To generate read-consistent database information during database recovery to rollback uncommitted transactions for users.
43. What is a Temporary Segment ?
Temporary segments are created by ORACLE when a SQL statement needs a temporary work  area  to  complete  execution. When the statement finishes execution, the temporary segment extents are released to the system for future use.
44. What is a Data File ?
Every  ORACLE  database  has  one or more physical data files. A database's data  files  contain  all  the  database data. The data of logical database structures  such  as  tables  and  indexes is physically stored in the data files allocated for a database.
45. What are the Characteristics of Data Files ?
A  data  file  can be associated with only one database.Once created a data file can't change size.
One  or  more  data  files form a logical unit of database storage called a tablespace.
46. What is a Redo Log ?
The  set  of  Redo  Log  files  for a database is collectively known as the database's redo log.
47. What is the function of Redo Log ?
The Primary function of the redo log is to record all changes made to data.
48. What is the use of Redo Log Information ?
The Information in a redo log file is used only to recover the database from a system or media failure prevents database data from being written to a database's data files.
49. What does a Control file Contain ?
A  Control file records the physical structure of the database. It contains the following information.
Database Name
Names and locations of a database's files and redolog files.
Time stamp of database creation.
50. What is the use of Control File ?
When an instance of an ORACLE database is started, its control file is used to  identify  the  database  and  redo  log  files  that must be opened for database operation to proceed. It is also used in database recovery.
51. What is a Data Dictionary ?
The data dictionary of an ORACLE database is a set of tables and views that are used  as a read-only reference about the database.
It  stores information about both the logical and physical structure of the database,  the  valid  users  of  an ORACLE database, integrity constraints defined  for tables in the database and space allocated for a schema object and how much of it is being used.
52. What is an Integrity Constrains ?
An  integrity constraint is a declarative way to define a business rule for a column of a table.
53.  Can an Integrity Constraint be enforced on a table if some existing table data does not satisfy the constraint ?
54.  Describe the different type  of  Integrity Constraints supported by ORACLE ? 
NOT NULL Constraint    - Disallows NULLs in a table's column.
UNIQUE   Constraint      - Disallows duplicate values in a column or set of columns.
PRIMARY  KEY  Constraint - Disallows duplicate values and NULLs in a column or set of columns.
FOREIGN  KEY   Constrain - Require each value in a column or set of columns match a value in a related table's UNIQUE or PRIMARY KEY.
CHECK  Constraint        - Disallows values that do not satisfy the logical expression of the constraint.
55. What is difference between UNIQUE constraint and PRIMARY KEY constraint ?
A  column  defined  as   UNIQUE can contain NULLs while a column defined as PRIMARY KEY can't contain Nulls.
56. Describe Referential Integrity ?
A rule defined on a column (or set of columns) in one table that allows the insert or update of a  row  only  if the value for the column or set of columns  (the  dependent  value)  matches  a value in a column of a related table   (the  referenced  value).  It also specifies the type of data manipulation allowed  on referenced data and the action to be performed on dependent data as a result of any action on referenced data.
57.  What are the  Referential actions supported by FOREIGN KEY integrity constraint ?
UPDATE  and  DELETE  Restrict - A referential integrity rule that disallows the update or deletion of referenced data.
DELETE  Cascade - When a referenced row is deleted all associated dependent rows are deleted.
58. What is self-referential integrity constraint ?
If  a  foreign  key  reference  a  parent  key  of the same table is called self-referential integrity constraint.
59. What are the Limitations of a CHECK Constraint ?
The  condition  must  be a Boolean expression evaluated using the values in the row being inserted or updated and can't contain subqueries, sequence, the SYSDATE,UID,USER  or  USERENV  SQL functions, or the pseudo columns LEVEL or ROWNUM.
60.  What is the maximum number of CHECK constraints that can be defined on a column ?
No Limit.
61. What constitute an ORACLE Instance ?
SGA  and  ORACLE  background  processes constitute an ORACLE instance. (or) Combination of memory structure and background process.
62. What is SGA ?
The  System Global Area (SGA) is a shared memory region allocated by ORACLE that contains data and control information for one ORACLE instance.
63. What are the components of SGA ?
Database buffers, Redo Log Buffer the Shared Pool and   Cursors.
 64. What do Database Buffers contain ?
Database  buffers store the most recently  used blocks of database data. It can also contain modified data that has not yet been permanently written to disk.
65. What do Redo Log Buffers contain ?
Redo Log Buffer stores redo entries a log of changes made to the database.
66. What is Shared Pool ?
Shared  Pool is a portion of the SGA that contains shared memory constructs such as shared SQL areas.
67. What is Shared SQL Area ?
A  Shared  SQL  area  is  required  to  process  every unique SQL statement submitted to a database and contains information such as the parse tree and execution plan for the corresponding statement.
68. What is Cursor ?
A Cursor is a handle ( a name or pointer) for the memory associated with a specific statement.
69. What is PGA ?
Program Global Area (PGA) is a memory buffer that contains data and control information for a server process.
70. What is User Process ?
A user process is created and maintained to execute the software code of an application  program.  It  is  a  shadow  process  created automatically to facilitate communication between the user and the server process.
71. What is Server Process ?
Server  Process  handle  requests  from  connected  user  process. A server process is in charge of communicating with the user process and interacting with ORACLE carry out requests of the associated user process.
72. What are the two types of Server Configurations ?
Dedicated Server Configuration and Multi-threaded Server Configuration.
73. What is Dedicated Server Configuration ?
In a Dedicated Server Configuration a Server Process handles requests for a Single User Process.
74. What is a Multi-threaded Server Configuration ?
In  a Multi-threaded Server Configuration many user processes share a group of server process.
75. What is a Parallel Server option in ORACLE ?
A configuration for loosely coupled systems where multiple instance share a single physical database is called Parallel Server.
76. Name the ORACLE Background Process ?
DBWR - Database Writer.
LGWR - Log Writer
CKPT - Check Point
SMON - System Monitor
PMON - Process Monitor
ARCH - Archiver
RECO - Recover
Dnnn - Dispatcher, and
LCKn - Lock
Snnn - Server.
77.  What Does DBWR do ?
Database  writer  writes  modified blocks from the database buffer cache to the data files.
78.When Does DBWR write to the database ?
DBWR  writes  when  more  data  needs  to  be read into the SGA and too few database  buffers  are free. The least recently used data is written to the data files first. DBWR also writes when CheckPoint occurs.
79. What does LGWR do ?
Log  Writer (LGWR) writes redo log entries generated in the redo log buffer of the SGA to on-line Redo Log File.
80. When does LGWR write to the database ?
LGWR   writes  redo  log  entries  into  an  on-line  redo  log  file  when transactions commit and the log buffer files are full.
81. What is the function of checkpoint(CKPT)?
The  Checkpoint  (CKPT)  process  is  responsible  for  signaling  DBWR  at checkpoints  and  updating  all  the  data  files  and control files of the database.
82. What are the functions of SMON ?
System  Monitor (SMON)  performs instance recovery at instance start-up. In a multiple instance system (one that uses the Parallel Server), SMON of one instance  can  also  perform instance recovery for other instance that have failed SMON also cleans up temporary segments that are no longer in use and recovers  dead  transactions  skipped  during  crash  and instance recovery because of  file-read or off-line errors. These transactions are eventually recovered  by SMON when the tablespace or file is brought back on-line SMON also  coalesces  free  extents  within  the  database  to  make  free space contiguous and easier to allocate.
83.  What are functions of PMON ?
Process Monitor (PMON)  performs process recovery when a user process fails PMON  is  responsible  for cleaning up the cache and Freeing resources that the  process  was using PMON also checks on dispatcher and server processes and restarts them if they have failed.
84.  What is the function of ARCH ?
Archiver  (ARCH) copies the on-line redo log files to archival storage when they  are  full.  ARCH is active only when a database's redo log is used in ARCHIVELOG mode.
85. What is function of RECO ?
RECOver (RECO) is used to resolve distributed transactions that are pending due  to  a  network  or  system failure in a distributed database. At timed intervals,the  local  RECO  attempts  to  connect  to  remote databases and automatically  complete  the commit or rollback of the local portion of any pending distributed transactions.
86. What is the function of Dispatcher (Dnnn) ?
Dispatcher   (Dnnn)  process  is  responsible  for  routing  requests  from connected user processes to available shared server processes and returning the responses back to the appropriate user processes.
87. How many Dispatcher Processes are created ?
Atleast one Dispatcher process is created for every communication protocol in use.
88. What is the function of Lock (LCKn) Process ?
Lock  (LCKn)  are  used for inter-instance locking when the ORACLE Parallel Server option is used.
89. What is the maximum number of Lock Processes used ?
Though  a single LCK process is sufficient for most Parallel Server systems
upto Ten Locks (LCK0,....LCK9) are used for inter-instance locking.
90. Define Transaction ?
A  Transaction  is  a  logical  unit of work that comprises one or more SQL statements executed by a single user.
91. When does a Transaction end ?
When it is committed or Rollbacked.
92. What does COMMIT do ?
COMMIT makes permanent the changes resulting from all SQL statements in the transaction. The changes made by the SQL statements of a transaction become visible  to other  user  sessions  transactions  that  start  only  after transaction is committed.
93. What does ROLLBACK do ?
ROLLBACK retracts any of the changes resulting from the SQL statements in the transaction.
94. What is SAVE POINT ?
For  long  transactions  that  contain  many  SQL  statements, intermediate markers  or  savepoints  can  be  declared  which  can  be used to divide a transaction  into  smaller  parts.  This allows the option of later rolling back  all  work  performed  from  the current point in the transaction to a declared savepoint within the transaction.
95. What is Read-Only Transaction ?
A  Read-Only transaction ensures that the results of each query executed in the transaction are consistant with respect to the same point in time.
96. What is the function of Optimizer ?
The  goal of the optimizer is to choose the most efficient way to execute a SQL statement.
97. What is Execution Plan ?
The combinations of the steps the optimizer chooses to execute a statement is called an execution plan.
98.  What are the different approaches used by Optimizer in choosing an execution plan ?
Rule-based and Cost-based.
99. What are the factors that affect OPTIMIZER in choosing an Optimization approach ?
The   OPTIMIZER_MODE   initialization  parameter  Statistics  in  the  Data Dictionary  the OPTIMIZER_GOAL parameter of the ALTER SESSION command hints in the statement.
100. What are the values that can be specified for OPTIMIZER MODE Parameter ?
101. Will the Optimizer always use COST-based approach if OPTIMIZER_MODE is set to "Cost'?
Presence of statistics in the data dictionary for atleast one of the tables accessed  by  the  SQL  statements  is  necessary  for the OPTIMIZER to use COST-based approach. Otherwise OPTIMIZER chooses RULE-based approach.
102. What is the effect of setting the value of OPTIMIZER_MODE to 'RULE' ?
This  value  causes the optimizer to choose the rule_based approach for all SQL  statements  issued  to  the  instance  regardless  of  the presence of statistics.
103. What are the values that can be specified for OPTIMIZER_GOAL parameter of the ALTER SESSION Command ?
104.  What  is the effect of setting the value "CHOOSE" for OPTIMIZER_GOAL, parameter of the ALTER SESSION Command ?
The  Optimizer  chooses  Cost_based approach and optimizes with the goal of best throughput if statistics for atleast one of the tables accessed by the SQL statement exist in the data dictionary. Otherwise the OPTIMIZER chooses RULE_based approach.
105. What is the effect of setting the value "ALL_ROWS"  for OPTIMIZER_GOAL parameter of the ALTER SESSION  command ?
This  value  causes  the  optimizer to the cost-based approach for  all SQL statements  in  the session regardless of the presence of statistics and to optimize with a goal of best throughput.
106.    What   is   the  effect  of  setting  the  value  'FIRST_ROWS'  for OPTIMIZER_GOAL parameter of the ALTER SESSION command ?
This  value causes the optimizer to use the cost-based approach for all SQL statements  in  the session regardless of the presence of statistics and to optimize with a goal of best response time.
107.  What  is the effect of setting the 'RULE' for OPTIMIER_GOAL parameter of the ALTER SESSION Command ?
This  value  causes the optimizer to choose the rule-based approach for all SQL statements in a session regardless of the presence of statistics.
108. What is RULE-based approach to optimization ?
Choosing an executing planbased on the access paths available and the ranks of these access paths.
109. What is COST-based approach to optimization ?
Considering  available  access  paths  and  determining  the most efficient execution  plan  based  on statistics in the data dictionary for the tables accessed by the statement and their associated  clusters and indexes.
110.  What  are  the  different  types  of PL/SQL program units that can be defined and stored in ORACLE database ?
Procedures and Functions,Packages and Database Triggers.
111. What is a Procedure ?
A  Procedure consist of a set of SQL and PL/SQL statements that are grouped together  as a unit to solve a specific problem or perform a set of related tasks.
112. What is difference between Procedures and Functions ?
A Function returns a value to the caller where as a Procedure does not.
113. What is a Package ?
A  Package  is a collection of related procedures, functions, variables and other package constructs together as a unit in the database.
114. What are the advantages of having a Package ?
Increased  functionality  (for  example,global  package  variables  can  be declared  and  used  by  any proecdure in the package) and performance (for example  all  objects  of  the package are parsed compiled, and loaded into memory once)
115. What is Database Trigger ?
A Database Trigger is procedure (set of SQL  and PL/SQL statements) that is automatically  executed  as  a  result of an insert in, update to, or delete from a table.
116. What are the uses of Database Trigger ?
Database  triggers  can  be  used  to automatic data generation, audit data modifications, enforce complex Integrity constraints, and customize complex security authorizations.
117.  What  are  the  differences  between  Database  Trigger and Integrity constraints ?
A  declarative  integrity constraint is a statement about the database that is  always true. A constraint applies to existing data in the table and any statement that manipulates the table.
A  trigger  does  not  apply  to  data  loaded before the definition of the trigger,  therefore,  it does not guarantee all data in a table conforms to the rules established by an associated trigger.
A  trigger  can  be  used  to  enforce  transitional constraints where as a declarative integrity constraint cannot be used.
118.  What are Roles ?
Roles  are  named groups of related privileges that are granted to users or other roles.
119. What are the use of Roles ?
REDUCED  GRANTING  OF PRIVILEGES - Rather than explicitly granting the same set  of  privileges  to  many users a database administrator  can grant the privileges  for  a  group of related users granted to a role and then grant only the role to each member of the group.
DYNAMIC PRIVILEGE MANAGEMENT  - When the privileges of a group must change, only  the  privileges of the role need to be modified. The security domains of  all  users  granted  the group's role automatically reflect the changes made to the role.
SELECTIVE  AVAILABILITY  OF PRIVILEGES - The roles granted to a user can be selectively  enable  (available  for  use)  or disabled  (not available for use).  This  allows  specific  control  of a user's privileges in any given situation.
APPLICATION   AWARENESS  -  A  database  application  can  be  designed  to automatically  enable  and  disable selective roles when a user attempts to use the application.
120. How to prevent unauthorized use of privileges granted to a Role ?
By creating a Role with a password.
121. What is default tablespace ?
The  Tablespace  to  contain  schema  objects  created without specifying a tablespace name.
122. What is Tablespace Quota ?
The collective amount of disk space available to the objects in a schema on a particular tablespace.
123. What is a profile ?
Each  database  user  is  assigned  a Profile that specifies limitations on various system resources available to the user.
124. What are the system resources that can be controlled through Profile ?
The number of concurrent sessions the user can establish the CPU processing time available to the user's session the CPU processing time available to a single  call  to  ORACLE  made by a SQL statement the amount of logical I/O available  to  the  user's  session the amout of logical I/O available to a single  call  to  ORACLE made by a SQL statement the allowed amount of idle time  for  the  user's  session  the allowed amount of connect time for the user's session.
125. What is Auditing ?
Monitoring of user access to aid in the investigation of database use.
126. What are the different Levels of Auditing ?
Statement Auditing, Privilege Auditing and Object Auditing.
127. What is Statement Auditing ?
Statement  auditing  is  the  auditing  of  the  powerful system privileges without regard to specifically named objects.
128. What is Privilege Auditing ?
Privilege auditing is the auditing of the use of powerful system privileges without regard to specifically named objects.
129. What is Object  Auditing ?
Object  auditing  is  the  auditing  of accesses to specific schema objects without regard to user.
130. What is Distributed database ?
A  distributed  database  is  a  network  of  databases managed by multiple database  servers  that  appears  to a user as single logical database. The data  of  all  databases  in the distributed database can be simultaneously accessed and modified.
131.  What is Two-Phase Commit ?
Two-phase  commit  is  mechanism  that guarantees a distributed transaction either commits on all involved nodes or rolls back on all involved nodes to maintain  data  consistency  across the global distributed database. It has two phase, a Prepare Phase and a Commit Phase.
132. Describe two phases of Two-phase commit ?
Prepare phase - The global coordinator (initiating node) ask a participants to  prepare   (to  promise  to  commit or rollback the transaction, even if there is a  failure)
Commit  -  Phase - If all participants respond to the coordinator that they are  prepared, the coordinator asks all nodes to commit the transaction, if all  participants  cannot  prepare,  the coordinator asks all nodes to roll back the transaction.
133. What is the mechanism provided by ORACLE for table replication ?
Snapshots and SNAPSHOT LOGs
134. What is a SNAPSHOT ?
Snapshots  are  read-only copies of a master table located on a remote node which  is  periodically  refreshed  to  reflect  changes made to the master table.
135. What is a SNAPSHOT LOG ?
A  snapshot  log  is a table in the master database that is associated with the  master  table.  ORACLE uses a snapshot log to track the rows that have been  updated  in  the master table. Snapshot logs are used in updating the snapshots based on the master table.
136. What is a SQL * NET?
SQL  *NET  is  ORACLE's  mechanism  for  interfacing with the communication protocols  used  by the networks that facilitate distributed processing and distributed  databases.  It  is  used  in  Clint-Server  and  Server-Server communications.
137. What are the steps involved in Database Startup ?
Start an instance, Mount the Database and Open the Database.
138. What are the steps involved in Database Shutdown ?
Close the Database, Dismount the Database and Shutdown the Instance.
139. What is  Restricted Mode of Instance Startup ?
An  instance  can be started in (or later altered to be in) restricted mode so  that  when  the  database is open connections are limited only to those whose  user  accounts  have  been  granted  the  RESTRICTED  SESSION system privilege.
140.  What are the different modes of mounting a Database with the Parallel Server ?
Exclusive  Mode  If  the  first  instance that mounts a database does so in exclusive mode, only that Instance can mount the database.
Parallel  Mode   If the first instance that mounts a database is started in parallel  mode,  other instances that are started in parallel mode can also mount the database.
141. What is Full Backup ?
A full backup is an operating system backup of all data files, on-line redo log  files  and  control  file  that  constitute  ORACLE  database  and the parameter.
142. Can Full Backup be performed when the database is open ?
143. What is Partial Backup ?
A   Partial  Backup  is any operating system backup short of a full backup, taken while the database is open or shut down.
The On-line Redo Log is a set of tow or more on-line redo files that record all  committed  changes  made  to  the  database. Whenever a transaction is committed,  the  corresponding  redo entries temporarily stores in redo log buffers  of  the  SGA  are  written  to  an  on-line  redo  log file by the background  process  LGWR.  The on-line redo log files are used in cyclical fashion.
145.  What is Mirrored on-line Redo Log ?
A  mirrored  on-line  redo log consists of copies of on-line redo log files physically  located  on  separate  disks, changes made to one member of the group are made to all members.
146. What is Archived Redo Log ?
Archived  Redo  Log  consists  of Redo Log files that have         archived before being reused.
147.  What  are  the  advantages of operating a database in ARCHIVELOG mode over operating it in NO ARCHIVELOG mode ?
Complete database recovery from disk failure is possible only in ARCHIVELOG mode.
Online database backup is possible only in ARCHIVELOG     mode.
148. What is Log  Switch ?
The  point  at  which  ORACLE  ends writing to one online redo log file and begins writing to another is called a log switch.
149. What are the steps involved in Instance  Recovery ?
Rolling  forward to recover data that has not been recorded in data files, yet  has  been recorded  in the on-line redo log, including the contents of rollback segments.
Rolling back transactions that have been explicitly rolled back or have not been committed as indicated by the rollback segments regenerated in step a. 
Releasing any resources (locks) held by transactions in process at the time of the failure.
Resolving  any  pending  distributed  transactions  undergoing  a two-phase commit at the time of the instance failure.

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