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 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 tablespaces?
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 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) 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.
78. 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.
79. How the space
utilization 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 (number of extents is based on the optimal size)
80. Why query
fails sometimes?
Rollback segment dynamically extent to handle
larger transactions entry loads. A single transaction may wipeout all available
free space in the rollback segment tablespace. This prevents other user using
rollback segments.
81. How will you
monitor the space allocation?
By querying DBA_SEGMENT table/view
82. 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 corrupted. PARTLY
AVAILABLE - Contains data from an unresolved transaction involving a distributed
database.
83. 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 tables for space management are updated.
Transaction Completes.
84. 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.
85. How will you
estimate the space required by a non-clustered tables?
Calculate the total header size Calculate the
available data space 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.
86. It is
possible to use raw devices as data files and what are the advantages over file
system files?
Yes.
The advantages over file system files are that
I/O will be improved because Oracle is bye-passing the kernel which writing into
disk. Disk corruption will be very less.
87. 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.
88. How to
implement the multiple control files for an existing database?
Shutdown the database Copy one of the existing
controlfile to new location
Edit Config ora file by adding new control
filename Restart the database.
89. What is redo
log file mirroring? How 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.
90. 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 systems 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.
91. What is use
of rollback segments in Oracle database?
They allow the database to maintain read consistency
between multiple transactions.
92. 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.
93. 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.
94. When will be
a segment released?
When Segment is dropped. When Shrink (RBS only) When
truncated (TRUNCATE used with drop storage option)
95. What are
disadvantages 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.
96. 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. - Trailing nulls and length bytes are not stored. - Inserts of,
updates to and deletes of rows as well as columns larger than a single data
block, can cause fragmentation a chained row pieces.
Database Security
& Administration
97. What is user
Account in Oracle database?
A 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.
98. 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.
99. What are the
dictionary tables used to monitor a database space?
DBA_FREE_SPACE
DBA_SEGMENTS
DBA_DATA_FILES.
SQL*Plus
Statements
100. What are the
types of SQL statement?
Data Definition Language: CREATE, ALTER, DROP,
TRUNCATE, REVOKE, NO AUDIT & COMMIT. Data Manipulation Language: INSERT, UPDATE,
DELETE, LOCK TABLE, EXPLAIN PLAN & SELECT. Transactional Control: COMMIT
&
ROLLBACK Session Control: ALTERSESSION & SET
ROLE System Control: ALTER SYSTEM.
101. What is a
transaction?
Transaction is logical unit between two commits
and commit and rollback.
102. What is
difference between TRUNCATE & DELETE?
TRUNCATE commits after deleting entire table
i.e., cannot be rolled back. Database triggers do not fire on TRUNCATE DELETE
allows the filtered deletion. Deleted records can be rolled back or committed. Database
triggers fire on DELETE.
103. What is a
join? Explain the different types of joins?
Join is a query, which retrieves related columns
or rows from multiple tables. Self Join - Joining the table with itself. Equi
Join - Joining two tables by equating two common columns. Non-Equi Join - Joining two tables by
equating two common columns.
Outer Join - Joining two tables in such a way
that query can also retrieve rows that do not have corresponding join value in
the other table.
104. What is the
sub-query?
Sub-query is a query whose return values are used
in filtering conditions of the main query.
105. What is
correlated sub-query?
Correlated sub-query is a sub-query, which has
reference to the main query.
106. Explain
CONNECT BY PRIOR?
Retrieves rows in hierarchical order eg. select
empno, ename from emp where.
107. Difference
between SUBSTR and INSTR?
INSTR (String1, String2 (n, (m)), INSTR returns
the position of the m-th occurrence of the string 2 in string1. The search
begins from nth position of string1.
SUBSTR (String1 n, m) SUBSTR returns a character
string of size m in string1, starting from n-th position of string1.
108. Explain
UNION, MINUS, UNION ALL and INTERSECT?
INTERSECT - returns all distinct rows selected by
both queries.
MINUS - returns all distinct rows selected by the
first query but not by the second.
UNION - returns all distinct rows selected by
either query
UNION ALL - returns all rows selected by either
query, including all duplicates.
109. What is
ROWID?
ROWID is a pseudo column attached to each row of
a table. It is 18 characters long, blockno, rownumber are the components of
ROWID.
110. What is the
fastest way of accessing a row in a table?
Using ROWID. CONSTRAINTS
111. What is an
integrity constraint?
Integrity constraint is a rule that restricts values
to a column in a table.
112. What is
referential integrity constraint?
Maintaining data integrity through a set of rules
that restrict the values of one or more columns of the tables based on the values
of primary key or unique key of the referenced table.
113. What is the
usage of SAVEPOINTS?
SAVEPOINTS are used to subdivide a transaction
into smaller parts. It enables rolling back part of a transaction. Maximum of
five save points are allowed.
114. What is ON
DELETE CASCADE?
When ON DELETE CASCADE is specified Oracle
maintains referential integrity by automatically removing dependent foreign key
values if a referenced primary or unique key value is removed.
115. What are the
data types allowed in a table?
CHAR, VARCHAR2, NUMBER, DATE, RAW, LONG and LONG
RAW.
116. What is
difference between CHAR and VARCHAR2? What is the maximum SIZE allowed for each
type?
CHAR pads blank spaces to the maximum length. VARCHAR2
does not pad blank spaces. For CHAR the
maximum length is 255 and 2000 for VARCHAR2.
117. How many
LONG columns are allowed in a table? Is it possible to use LONG columns in
WHERE clause or ORDER BY?
Only one LONG column is allowed. It is not
possible to use LONG column in WHERE or ORDER BY clause.
118. What are the
pre-requisites to modify datatype of a column and to add a column with NOT NULL
constraint?
- To modify the datatype of a column the column
must be empty.
- To add a column with NOT NULL constrain, the
table must be empty.
119. Where the
integrity constraints are stored in data dictionary?
The integrity constraints are stored In USER_CONSTRAINTS.
120. How will you
activate/deactivate integrity constraints?
The integrity constraints can be enabled or
disabled by ALTER TABLE ENABLE CONSTRAINT / DISABLE CONSTRAINT.
121. If unique
key constraint on DATE column is created, will it validate the rows that are
inserted with SYSDATE?
It won't, Because SYSDATE format contains time
attached with it.
122. What is a
database link?
Database link is a named path through which a
remote database can be accessed.
123. How to
access the current value and next value from a sequence? Is it possible to
access the current value in a session before accessing next value?
Sequence name CURRVAL, sequence name NEXTVAL. It
is not possible. Only if you access next value in the session, current value
can be accessed.
124. What is
CYCLE/NO CYCLE in a Sequence?
CYCLE specifies that the sequence continue to
generate values after reaching either maximum or minimum value. After
pan-ascending sequence reaches its maximum value, it generates its minimum
value. After a descending sequence reaches its minimum, it generates its maximum.
NO CYCLE specifies that the sequence cannot generate more values after reaching
its maximum or minimum value.
125. What are the
advantages of VIEW?
- To protect some of the columns of a table from
other users.
- To hide complexity of a query.
- To hide complexity of calculations.
126. Can a view
be updated/inserted/deleted? If Yes - under what conditions?
A View can be updated/deleted/inserted if it has
only one base table if the view is based on columns from one or more tables
then insert, update and delete is not possible.
127. If a view on
a single base table is manipulated will the changes be reflected on the base
table?
If changes are made to the tables and these
tables are the base tables of a view, then the changes will be reference on the
view.
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