Oracle Concepts and Architecture Database Structures
1. What are the components of physical database structure of Oracle database?
Physical Structure
The physical layer of the database consists of three types of files:
- One or more Datafiles
- Two or more redo log files
- One or more control files
Datafiles (.dbf files):
Datafiles store the information contained in the database. One can have as few as one data file or as many as hundreds of datafiles. The information for a single table can span many datafiles or many tables can share a set of datafiles. Spreading tablespaces over many datafiles can have a significant positive effect on performance. The number of datafiles that can be configured is limited by the Oracle parameter MAXDATAFILES.
Redo Log Files (.rdo & .arc):
Oracle maintains logs of all the transaction against the database. These transactions are recorded in files called Online Redo Log Files (Redo Logs). The main purpose of the Redo log files is to hold information as recovery in the event of a system failure. Redo log stores a log of all changes made to the database. The redo log files must perform well and be protected against hardware failures (through software or hardware fault tolerance). If redo log information is lost, one cannot recover the system.
When a transaction occurs in the database, it is entered in the redo log buffers, while the data blocks affected by the transactions are not immediately written to disk. In anOracle database there are at least three or more Redo Log files.
Oracle writes to redo log file in a cyclical order i.e. after the first log file is filled, it writes to the second log file, until that one is filled. When all the Redo Log files have been filled, it returns to the first log file and begin overwrite its content with new transaction data. Note, if the database is running in the ARCHIVELOG Mode, the database will make a copy of the online redo log files before overwriting them.
Control Files (.ctl):
Control files record control information about all of the files within the database. These files maintain internal consistency and guide recovery operation. Control files contain information used to start an instance, such as the location of datafiles and redo log files; Oracle needs this information to start the database instance. Control files must be protected. Oracle provides a mechanism for storing multiple copies of control files. These multiple copies are stored on separate disks to minimize the potential damage due to disk failure. The names of the database’s control files are specified via the CONTROL_FILES initialization parameter.
2. What are the components of logical database structure of Oracle database?
Oracle is an RDBMS (Relational Database Management System). The Oracle database architecture can be described in terms of logical and physical structures. The advantage of separating the logical and physical structure is that the physical storage structure can be changed without affecting the logical structure.
Logical Structure
The logical structure for Oracle RDBMS consists of the following elements:
- Tablespace
- Schema
Tablespace
The Oracle database consists of one or more logical portions called as ‘Tablespaces’. A tablespace is a logical grouping of related data.
A database administrator can use Tablespaces to do the following:
- Control disk space allocation for database data.
- Assign specific space quotas for database users.
- Perform partial database backup or recovery operations.
- Allocate data storage across devices to improve performance.
Each database has at least one Tablespace called SYSTEM Tablespace. As part of the process of creating the database, Oracle automatically creates the SYSTEM tablespace. Although a small database can fit within the SYSTEM tablespace, it’s recommended that to create a separate tablespace for user data.
Oracle uses the SYSTEM tablespace to store information like the data dictionary. Data dictionary stores the metadata (or the data about data). This includes information like table access permissions, information about keys etc.
Data is stored in the database in form of files called as datafiles. Each Tablespace is a collection of one or more Datafiles. Each data file consists of ‘Data blocks’, ‘extents’ and ‘segments’.
Data Blocks
At the finest level of granularity, an ORACLE database’s data is stored in data blocks (also called logical blocks, ORACLE blocks, or pages). An ORACLE database uses and allocates free database space in ORACLE data blocks.
Extents
The next level of logical database space is called an extent. An extent is a specific number of contiguous data blocks that are allocated for storing a specific type of information.
Segments
The level of logical database storage above an extent is called a segment. A segment is a set of extents that have been allocated for a specific type of data structure, and all are stored in the same tablespace. For example, each table’s data is stored in its own data segment, while each index’s data is stored in its own index segment. ORACLE allocates space for segments in extents. Therefore, when the existing extents of a segment are full, ORACLE allocates another extent for that segment. Because extents are allocated as needed, the extents of a segment may or may not be contiguous on disk, and may or may not span files.
An Oracle database can use four types of segments:
- Data segment–Stores user data within the database.
- Index segment–Stores indexes.
- Rollback segment–Stores rollback information. This information is used when data must be rolled back.
- Temporary segment–Created when a SQL statement needs a temporary work area; these segments are destroyed when the SQL statement is finished. These segments are used during various database operations, such as sorts.
Schema
The database schema is a collection of logical-structure objects, known as schema objects that define how you see the database’s data. A schema also defines a level of access for the users. All the logical objects in oracle are grouped into a schema.
A scheme is a logical grouping of objects such as:
- Tables
- Clusters
- Indexes
- Views
- Stored procedures
- Triggers
- Sequences
3. What is a tablespace?
A tablespace is a logical storage unit within an Oracle database. It is logical because a tablespace is not visible in the file system of the machine on which the database resides. A tablespace, in turn, consists of at least one datafile which, in turn, are physically located in the file system of the server. Btw, a datafile belongs to exactly one tablespace.
Each table, index and so on that is stored in an Oracle database belongs to a tablespace. The tablespace builds the bridge between the Oracle database and the filesystem in which the table’s or index’ data is stored.
There are three types of tablespaces in Oracle:
- Permanent tablespaces
- Undo tablespaces
- temporary tablespaces
A tablespace is created with the create tablespace sql command.
See adding tablespaces to primary servers if you want to add a tablespace to a primary server in a standby environment.
Dropping a tablespace
Dropping a tablespace is a structural change to the database that requires a backup.
The extent size of the objects in the tablespace are subject to the tablespace’s minimum extent size.
Quotas on tablespaces
Users can have (space) quotas on tablespaces. This is a means to limit how much space a user uses on a tablespace. This quota can be set using alter user quota…
Tablespace groups
This is a feature that is available with Oracle 10g.
Assigning a tablespace to a tablespace group:
alter tablespace ts_user tablespace group ts_grp_user;
Renaming Tablespaces
This is a feature that is available with Oracle 10g and can be useful in transportable tablespace scenarios.
alter tablespace ts_user rename to ts_user_01;
The system and sysaux tablespace cannot be renamed, though. But that will not be much of a problem, will it? However, read only tablespaces can be renamed.
After renaming a tablespace, the controlfiles should immediatly be backed up.
The system tablespace
The system tablespace is always available when a database is open (it cannot be taken offline).
The system tablespace stores the data dictionary (or their base tables, respectively).
The sysaux tablespace
The sysaux tablespace is new with Oracle 10g. It is used to store database components that were stored in the system tablespace in prior releases of the database.
Also, the tablespaces that were needed for RMAN’s recovery catalog, for Ultra Search, for Data Mining, for XDP and for OLAP are going to sysaux with 10g.
Additionally, it is the place where automatic workload repository stores its information.
occupants
An occupant is a set of (application-) tables within the sysaux tablespace. The occupants can be viewed with the v$sysaux_occupants
Transportable tablespaces
See transportable tablespaces
Bigfile tablespaces
This is a feature of Oracle 10g. A bigfile tablespace contains only one datafile (or tempfile) which can be as big as 2^32 (=4GB) blocks.
create bigfile tablespace beeeg_ts data file '/o1/dat/beeeg.dbf' size 2T
Bigfile tablespaces are supported only for locally managed tablespaces with automatic segment-space management (which is the default setting since Oracle 9i). There are two exceptions: locally managed undo and temporary tablespaces can be bigfile tablespaces, even though their segments are manually managed.
The system and sysaux tablespace cannot be created as bigfile tablespace.
Bigfile tablespaces should be used with automatic storage management, or other logical volume managers that support dynamically extensible logical volumes, striping and RAID.
Smallfile tablespaces
A smallfile tablespace is a traditional tablespace that can contain up to 1022 datafiles.
Default tablespaces
An Oracle database can be configured with a default tablespace and a default temporary tablespace. These are used for users who are not configured with default tablespaces or default temporary tablespaces.
Displaying tablespaces
The dba_tablespaces view displays all tablespaces along with the respective relevant information.
Space management
Oracle maintains extents for a tablespace. There are two different methods for Oracle to keep track of free and used (occupied) extents:
- Dictionary managed tablespace
- Locally managed tablespace
It’s not possible to alter the space allocation method of a tablespace after it has been created. (Update 10g R2: it seems that is now possible with dbms_space_admin.)
Dictionary managed tablespaces
Extents are allocated according to the following storage parameters
- initial
- next
- pctincrease
The information about used and free extents is stored in the dictionary.
Locally managed tablespaces
A ‘bitmap’ is stored within the tablespace. Each bit within this bitmap determines if a corresponding extent in the tablespace is free or used.
The extent sizes are either uniform or autoallocate. Hence, the following storage parameters don’t make sense and are not permitted:
- next
- pctincrease
- minextents
- maxextents
- default storage
Locally managed tablespaces have two advantages: recursive space management is avoided and adjacent free space is automatically coalesced.
Tablespace count limits
As per 10g, a database cannot have more than 65536 tablespaces. See also datafile count limits.
A bigfile tablespace can address 4294967296 (or 232) db blocks. A smallfile tablespace can address 4194304 (or 222) db blocks.
State of tablespaces
A tablespace is either online (ready to be accessed) or offline (not accessible).
A tablespace becomes offline either
- when the DBA explicitely makes the tablespace offline, or
- when an error occures while Oracle accesses a datafile within the tablespace.
Offline tablespaces cannot be transported to other databases.
Read Only Tablespaces
Read only tablespaces have the benefit that they need be backed up only once.
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 bjects 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.)
A View in Oracle and in other database systems is simply the representation of a SQL statement that is stored in memory so that it can easily be re-used. For example, if we frequently issue the following query
SELECT customerid, customername FROM customers WHERE countryid='US';
To create a view use the CREATE VIEW command as seen in this example
CREATE VIEW view_uscustomers AS SELECT customerid, customername FROM customers WHERE countryid='US';
This command creates a new view called view_uscustomers. Note that this command does not result in anything being actually stored in the database at all except for a data dictionary entry that defines this view. This means that every time you query this view, Oracle has to go out and execute the view and query the database data. We can query the view like this:
SELECT * FROM view_uscustomers WHERE customerid BETWEEN 100 AND 200;
And Oracle will transform the query into this:
SELECT * FROM (select customerid, customername from customers WHERE countryid='US') WHERE customerid BETWEEN 100 AND 200
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.
