oracle core dba questions for freshers

1. What are the components of physical database structure of Oracle database?

Oracle database is comprised of three types of files. One or more datafiles, two are more redo log files, and one

or more control files.

2. What are the components of logical database structure of Oracle database?

There are tablespaces and 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 and database

links.

8. Can objects of the same schema reside in different table spaces?

Yes.

9. Can a tablespace hold objects from different schemes?

Yes.

10. What is Oracle 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 an Oracle 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 a view contain data?

Views do not contain or store data.

13. Can a view based on another view?

Yes.

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 perspective from that of the base table.

- Store complex queries.

15. What is an Oracle 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 types of synonyms?

There are two types of synonyms private and public.

18. What is a private synonym?

Only its owner can access a private synonym.

19. What is a public synonym?

Any database user can access a public synonym.

20. What are synonyms used for?

- 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 Oracle 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 the index updates?

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 are 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 data blocks. Block size is specified in init.ora file and cannot 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 and 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 types 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?

To generate read-consistent database information during database recovery and to rollback uncommitted

transactions by the 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 datafile?

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 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 contains?

- 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.

Data Base Administration

51. 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 processes can be shared by all of the users.

The memory structure that is used to store the most queried data from database. This helps up to improve

database performance by decreasing the amount of I/O performed against data file.

52. What is Parallel Server?

Multiple instances accessing the same database (only in multi-CPU environments)

53. What is a schema?

The set of objects owned by user account is called the schema.

54. 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 command

55. What are clusters?

Group of tables physically stored together because they share common columns and are often used together is

called cluster.

56. 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.

57. What is 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)

RECO

Dispatcher

User Process with associated PGS

58. What is a deadlock? Explain.

Two processes waiting to update the rows of a table, which are locked by other processes then deadlock arises.

In a database environment this will often happen because of not issuing the 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.

Memory Management

59. What is SGA?

The System Global Area in an Oracle database is the area in memory to facilitate 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 pool area.

60. What is a shared pool?

The data dictionary cache is stored in an area in SGA called the shared pool. This will allow sharing of parsed

SQL statements among concurrent users.

61. What is mean by Program Global Area (PGA)?

It is area in memory that is used by a single Oracle user process.

62. 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.

63. What are the factors causing the reparsing of SQL statements in SGA?

Due to insufficient shared 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.

Database Logical & Physical Architecture

64. 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.

65. What is dictionary cache?

Dictionary cache is information about the database objects stored in a data dictionary table.

66. 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.

67. What is redo log buffer?

Changes made to the records 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.

68. 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.

69. List the Optional Flexible Architecture (OFA) of Oracle database? How can we organize 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.

70. How will you force database to use particular rollback segment?

SET TRANSACTION USE ROLLBACK SEGMENT rbs_name.

71. 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.

72.Which parameter in Storage clause will reduce number of rows per block?

PCTFREE parameter

Row size also reduces no of rows per block.

73. 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 updating, etc.,

74. How does Space allocation table place within a block?

Each block contains entries as follows

Fixed block header

Variable block header

Row Header, row date (multiple rows may exists)

PCTEREE (% of free space for row updating in future)

75. 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.

76. What is the OPTIMAL parameter?

It is used to set the optimal length of a rollback segment.

77. 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.