DBMS MCQ Question Answer Interview Most Important

DBMS MCQ

This section of interview questions and answers focuses on "Database Management System". One shall practice these interview questions to improve their concepts for various interviews such as campus interviews, walk-in interviews, company interviews, placements, entrance exams, and other competitive exams.

1) Which of the following is generally used for performing tasks like creating the structure of the relations, and deletrelationstion?

1. DML(Data Manipulation Language)
2. Query
3. Relational Schema
4. DDL(Data Definition Language)

 

Answer: D

Explanation: The term "DDL" stands for Data Definition Language, used to perform all other essential tasks such as deletingrelationsn and related schemas in defining the structure relation.

---

2) Which of the following provides the ability to query information from the database and insert tuples into, delete tuples from, and modify tuples in the database?

1. DML(Data Manipulation Language)
2. DDL(Data Definition Language)
3. Query
4. Relational Schema

 

Answer: A

Explanation: The term "DML" stands for the Data Manipulation Language used to perform the required changes in the relation's values.

---

3) The given Query can also be replaced with_______:

1. SELECT name, course_id  
2. FROM instructor, teaches  
3. WHERE instructor_ID= teaches_ID;  

1. Select name,course_id from teaches,instructor where instructor_id=course_id;
2. Select name, course_id from instructor natural join teaches;
3. Select name, course_id from instructor;
4. Select course_id from instructor join teaches;

 

Answer: B

Explanation: Join clause joins two tables by matching the common column

---

4) Which one of the following given statements possibly contains the error?

1. select * from emp where empid = 10003;
2. select empid from emp where empid = 10006;
3. select empid from emp;
4. select empid where empid = 1009 and Lastname = 'GELLER';

 

Answer: D

Explanation: The Query given in option D does not contain the "from" clause, which specifies the relation from which the values have to be selected or fetched. Therefore the correct answer is D.

---

5) Ready the Query carefully:

1. SELECT emp_name  
2. FROM department  
3. WHERE dept_name LIKE ' _____ Computer Science';  

In the above-given Query, which of the following can be placed in the Query's blank portion to select the "dept_name" that also contains Computer Science as its ending string?

1. &
2. _
3. %
4. $

 

Answer: C

Explanation: In the above-given Query, the "%" (like) operator will be used, which is generally used while searching for a certain pattern in the strings. It represents the single and multiple characters. In this case, it used with "Where "louse to select the "dept_name" that contains the Computer Since as its ending string. To understand it more clearly, consider the following syntax:

Syntax

SELECT column1, column2, ...
FROM table_name
WHERE columnN LIKE pattern;

---

6) What do you mean by one to many relationships?

1. One class may have many teachers
2. One teacher can have many classes
3. Many classes may have many teachers
4. Many teachers may have many classes

 

Answer: B

Explanation: We can understand the "one to many" relationship as a teacher who may have more than one class to attend.

---

7) In the following Query, which of the following can be placed in the Query's blank portion to display the salary from highest to lowest amount, andsortg the ememployeesame alphabetically?

1. SELECT *  
2. FROM instructor  
3. ORDER BY salary ____, name ___;  

1. Ascending, Descending
2. Asc, Desc
3. Desc, Asc
4. All of the above

 

Answer: C

Explanation: To sort the salary from highest to lowest amount and display the employee's name alphabetically, one can use the "Desc and Asc" in the above-given Query.

---

8) The given Query can be replaced with ____________:

1. SELECT name  
2. FROM instructor1  
3. WHERE salary <= 100000 AND salary >= 90000;  

a.

1. SELECT name  
2. FROM instructor1  
3. WHERE salary BETWEEN 100000 AND 90000  

b.

1. SELECT name  
2. FROM instructor|  
3. WHERE salary BETWEEN 90000 AND 100000;  

c.

1. SELECT name  
2. FROM instructor1  
3. were salary BETWEEN 90000 AND 100000;  

d.

1. SELECT name  
2. FROM instructor!  
3. WHERE salary <= 90000 AND salary>=100000;  

 

Answer: C

Explanation: The "SQL" contains a comparison known as the "BETWEEN," which is also used in one of the given queries, as you can see. The "BETWEEN" operator is generally used to simplify the "WHERE" clause that is used to specify that the value is greater than one value or greater than some values, less than one or more values.

---

9) A Database Management System is a type of _________software.

1. It is a type of system software
2. It is a kind of application software
3. It is a kind of general software
4. Both A and C

 

Answer: A

Explanation: The DBMS (or Database Management System) is a kind of system software used for several operations such as creating tables/databases, storing data, managing databases. It also allows modifying the data stored in the database as well.

---

10) The term "FAT" is stands for_____

1. File Allocation Tree
2. File Allocation Table
3. File Allocation Graph
4. All of the above

 

Answer: B

Explanation: The term "FAT" can be described as a file structure (or file architecture). In which all the information about the files where they are stored and where all these files need to be stored or in which directory, all that information generally stored in the file structure. Therefore the Operating system creates a table in which all the files and clusters are stored, known as the file allocation table.

---

11) Which of the following can be considered as the maximum size that is supported by FAT?

1. 8GB
2. 4GB
3. 4TB
4. None of the above

 

Answer: B

Explanation: The files with a size of less than 4 GB or equal to 4GB are easily supported by the FAT. All files with a size greater than the maximum size (or is 4Gb) are not supported by the FAT.

---

12) The term "NTFS" refers to which one of the following?

1. New Technology File System
2. New Tree File System
3. New Table type File System
4. Both A and C

 

Answer: A

Explanation: In the old operating systems, the file structure used to store and manage files is called the FAT 32 ( or File Allocation Table). Later, when the technology eevolvedwith time, a new type of file system is introduced, known as the New Technology File System. It overcomes all the drawbacks, issues that exist in FAT file architecture and has many other new features such as it is fast, it can handle files whose size is even greater than 4 GB.

---

13) Which of the following can be considered as the maximum size that is supported by NTFS?

1. 4GB
2. 16TB
3. 64TB
4. 8TB

 

Answer: A

Explanation: The New Technology File System was introduced in 1993 for the very first time as a part of new Windows operating systems (Windows NT 3.1).

You may be surprised to know that windows are still using it because of its fast speed and can support files whose size is up to 16TB (16 terabytes), which is much more than that file size supported by the old system.

---

14) A huge collection of the information or data accumulated form several different sources is known as ________:

1. Data Management
2. Data Mining
3. Data Warehouse
4. fromBoth B and C

 

Answer: C

Explanation: A huge collection of different types of data/information which collected from many different sources stored in one place is called a data warehouse. It works just like the standard warehouse, which is generally a type of holding area/building to store the different types of different goods in huge quantities until they are further distributed. Therefore the correct answer is Data Warehouse.

---

15) Which of the following can be used to extract or filter the data & information from the data warehouse?

1. Data redundancy
2. Data recovery tool
3. Data mining
4. Both B and C

 

Answer: C

Explanation: The tools used to obtain meaningful information or Data from the large collection of data (or from the Data Warehouses) are known as the Data mining tools. Through data mining, one can easily filter the useful information or data from the massive collection of the data. Therefore the correct answer is Data Mining.

---

16) Which one of the following refers to the copies of the same data (or information) occupying the memory space at multiple places.

1. Data Repository
2. Data Inconsistency
3. Data Mining
4. Data Redundancy

 

Answer: D

Explanation: The data redundancy generally occurs whenever more than one copy of the exact same data exists in several different places. Sometimes it may cause data inconsistency, which can result in an unreliable source of data or information that is not good for anyone.

---

17) Which one of the following refers to the "data about data"?

1. Directory
2. Sub Data
3. Warehouse
4. Meta Data

 

Answer: D

Explanation: The Mata Data refers to the type of data that describes the other data or information.

A general file or document is nothing more than a sequence of bytes that contains information like file name, file size, and date & time when the document was created or modified.

To understand it in more details, consider the following example:

For a common music file, the metadata may include the singer's name, the year it was released, and the lyrics, etc.

---

18) Which of the following refers to the level of data abstraction that describes exactly how the data actually stored?

1. Conceptual Level
2. Physical Level
3. File Level
4. Logical Level

 

Answer: B

Explanation: Data abstraction means displaying or sharing only the data that is needed and hiding from all other data until it is necessary to share it. However, the data abstraction level that describes how the data was actually stored in the user's machine (or system) is known as the Physical level. Therefore the correct answer is B.

---

19) To which of the following the term "DBA" referred?

1. Data Bank Administrator
2. Database Administrator
3. Data Administrator
4. None of the above

 

Answer: B

Explanation: The term "DBA" refers to the Database Administrator that manages the whole database and updates the database on a regular basis. In short, the database administrator has the responsibility for the whole database and to maintain it in an optimal & stable state.

---

20) In general, a file is basically a collection of all related______.

1. Rows & Columns
2. Fields
3. Database
4. Records

 

Answer: D

Explanation: Whenever we have some related data, information or records, we collect all those related data (or records), put them together, store them in one place, and give that collection a name that is known as a file.

---

21) The term "Data" refers to:

1. The electronic representation of the information( or data)
2. Basic information
3. Row Facts and figures
4. Both A and C

 

Answer: C

Explanation: In general, the term "data" refers to the row facts and figure, whereas the information referred to as the data, which is really important for someone or a particular person. Therefore the correct answer is C.

---

22) Rows of a relation are known as the _______.

1. Degree
2. Tuples
3. Entity
4. All of the above

 

Answer: D

Explanation: In SQL, the relation is represented by a table, and a table is a collection of rows and columns. Therefore the collection of rows & columns is called the table, whereas a table is known as the relation in the SQL. So in a relation (or we can say table), rows are called the tuples. So, the correct answer will be tuples.

---

23) Which of the following refers to the number of tuples in a relation?

1. Entity
2. Column
3. Cardinality
4. None of the above

 

Answer: C

Explanation: Cardinality refers to the number of tuples of relation because cardinality represents the number of tuples in a relation.

To understand it in more detail, consider the following given example:

Suppose we have a relation (or table) that contains 30 tuples (or Rows) and four columns, so the cardinality of our relation will be 30.

---

24) Which one of the following is a type of Data Manipulation Command?

1. Create
2. Alter
3. Delete
4. All of the above

 

Answer: C

Explanation: In data manipulation language, the command like select, insert, update, and delete is used to manipulate the information (or data, records), for example create a table, update table delete table, etc. Therefore the correct answer is C.

---

25) Which of the following command is a type of Data Definition language command?

1. Create
2. Update
3. Delete
4. Merge

 

Answer: A

Explanation: The Data definition language is generally used to maintain or define the structure of the database. For example, creating a table, drop table, alter table. In short, we can say that the command of data definition languages is used to work on the table's schemas inside the database.

---

26) Which of the following is a top-down approach in which the entity's higher level can be divided into two lower sub-entities?

1. Aggregation
2. Generalization
3. Specialization
4. All of the above

 

Answer: C

Explanation: In specialization, the top-down approach is used, and it is apposite to the generalization.

In the specialization, the higher-level entity can be divided into sub lower entities. It is generally used for identifying the subset of an entity set which share the distinguishing characteristics.

To understand it more clearly, consider the following example:

Suppose you have an entity, e.g., A vehicle. So through the specialization, you can be divided further into sub-entities like two-wheelers and four-wheelers. Therefore the correct answer is C.

---

27) In which one of the following, the multiple lower entities are grouped (or combined) together to form a single higher-level entity?

1. Specialization
2. Generalization
3. Aggregation
4. None of the above

 

Answer: B

Explanation:

The bottom-up approach is used in the generalization. The several lower-level sub-entities are grouped together to make an individual higher-level entity. In short, we can say that it is totally the opposite of specialization. To understand it more clearly, consider the following example:

Example

Suppose you have several lower entities like bus, car, motorbike etc. So, in order to make a more generalize (or higher level ) entity, you can combine them under a new higher-level entity such as a vehicle.

---

28) In a relation database, every tuples divided into the fields are known as the______.

1. Queries
2. Domains
3. Relations
4. All of the above

 

Answer: B

Explanation: In a database, the number of rows inside a table is known as tuples, and if we further divide those tuples (or rows) into those fields, they become the domains. So the correct answer will be B.

---

29) The term "TCL" stands for_____.

1. Ternary Control Language
2. Transmission Control Language
3. Transaction Central Language
4. Transaction Control Language

 

Answer: D

Explanation: The term "TCL" refers to the Transaction Control Language, which is another language just like the "DDL" and "DML". The commands like commit, save point, rollback come under the TCL used to control the transactions. Therefore the correct answer will be D.

---

30) In the relational table, which of the following can also be represented by the term "attribute"?

1. Entity
2. Row
3. Column
4. Both B &C

 

Answer: C

Explanation: In the database, the number of rows inside a table is called the tuples, and the numbers of columns are known as the attributes. Therefore the correct answer is C.

---

31) Which of the following refers to the number of attributes in a relation?

1. Degree
2. Row
3. Column
4. All of the above

 

Answer: A

Explanation: The column inside the table is called the attribute, and the total number of attributes inside the table is called the degree. So, here as we can see, degree refers to the number of attributes in a relation. Therefore the correct answer is C.

---

32) Which of the following is used in the application programs to request data from the database management system?

1. Data Manipulation language
2. Data Definition Language
3. Data Control Language
4. All of the above

 

Answer: A

Explanation: To fetch data from the database management system, generally, the "select" statement is used. Therefore, the Data Manipulation Language is used in the application programs while requesting the data from the database management system.

---

33) Which one of the following command is used to delete the existing row in a table?

1. Delete
2. Update
3. Insert
4. None of the above

 

Answer: A

Explanation: To delete an existing row in a table the generally, the "delete" command is used. The "delete" command is one of the commands that belong to the Data manipulation language.

---

34) Which of the following commands is used to save any transaction permanently into the database?

1. Commit
2. Rollback
3. Savepoint
4. None of the above

 

Answer: A

Explanation: The Transaction Control Language is used while working with the transactions. To save any transaction permanently into the database generally, the "commit" command is used, by which any changes get reflected permanently into the database, and they become irreversible.

---

35) Which one of the following commands is used for removing (or deleting) a relation forms the SQL database?

1. Delete
2. Drop
3. Remove
4. All of the above

 

Answer: B

Explanation: To removing( or deleting) a relation, the "drop" command is used instead of delete because here we are working on the objects of the database. So, to maintain the objects of a database, the Data definition language is used. Therefore the "drop" command is used to delete a relation form a database instead of using the "delete" command.

---

36) Which one of the following commands is used to restore the database to the last committed state?

1. Savepoint
2. Rollback
3. Commit
4. Both A & B

 

Answer: B

Explanation: As we all know that while working with the Transactions, the Transaction Control Language is used. To restore the database to the last committed state ( or to undo the latest changes), the "rollback" command is used that belongs to the Transaction Control Language.

---

37) The database management system can be considered as the collection of ______ that enables us to create and maintain the database.

1. Translators
2. Programs
3. Keys
4. Language activity

 

Answer: B

Explanation: The Database management system can also considered as the set of programs that enables users to create and maintain the database.

---

38) The term "DFD" stands for?

1. Data file diagram
2. Data flow document
3. Data flow diagram
4. None of the above

 

Answer: C

Explanation: The term"DFD" stands for the Data Flow Diagram, and to represent the flow of the data( or information), the data flow diagram is used. The data flow diagram also helps in understanding how the data is managed in the application. Therefore the correct answer is C.

---

39) Which of the following refers collection of the information stored in a database at a specific time?

1. Independence
2. Instance of the database
3. Schema
4. Data domain

 

Answer: B

Explanation: The collection of data (or information) stored in a database at a particular moment is known as the instance of the database. Therefore the correct answer is B.

---

40) The term "SQL" stands for

1. Standard query language
2. Sequential query language
3. Structured query language
4. Server-side query language

 

Answer: C

Explanation: The term "SQL" stands for the structured query language, which is one of the most widely, used programming languages for relational databases. It can also be used for managing and organizing data in all sorts of systems in which different data relationships exist. Therefore the correct answer is C.

---

41) The term "Data independence" refers to____

1. Data is defined separately and not included in the programs
2. Programs are not dependent on the logical attributes of the data
3. Programs are not dependent on the physical attributes of the data
4. Both B & C

 

Answer:

Explanation: The term "Data independence" refers that the program should not be dependent on either the logical attributes of data or physical attributes of data.

---

42) What is the relation calculus?

1. It is a kind of procedural language
2. It is a non-procedural language
3. It is a high-level language
4. It is Data Definition language

 

Answer: B

Explanation: In database management systems, relation calculus is a type of non-procedural query language that describes what data needs to be retrieved. However, it does not explain how to retrieve the data.

---

43) Which one of the following refers to the total view of the database content?

1. Conceptual view
2. Physical view
3. Internal view
4. External view

 

Answer: A

Explanation: The conceptual view refers to the total view of the content available in the database. Therefore the correct answer is A.

---

44) For what purpose the DML is provided?

1. Addition of new structure in the database
2. Manipulation & processing of the database
3. Definition of the physical structure of the database system
4. All of the above

 

Answer: B

Explanation: The term "DML" refers to the data manipulation language, which is commonly used to perform basic operations such as insert data, delete, and update data in a database. So the correct answer will be B.

---

45) The term "ODBC" stands for_____

1. Oral database connectivity
2. Oracle database connectivity
3. Open database connectivity
4. Object database connectivity

 

Answer: C

Explanation: The term "ODBC" refers to the Open Database Connectivity that is a SQL based programming interface introduced by Microsoft.

---

46) The architecture of a database can be viewed as the ________

1. One level
2. Two-level
3. Three-level
4. Four level

 

Answer: C

Explanation: A database's architecture can be viewed as the three levels of abstraction that are named as the internal level, external level, and the conceptual level. Therefore the correct answer is C.

---

47) In the relation model, the relation are generally termed as ________

1. Tuples
2. Attributes
3. Rows
4. Tables

 

Answer: D

Explanation: In the relation model, the relations are also referred to as the tables because the relations are considered as the technical name of the table. So the correct answer is D.

---

48) The Database Management Query language is generally designed for the _____

1. Support end-users who use English like commands
2. Specifying the structure of the database
3. Support in the development of the complex applications software
4. All of the above

 

Answer: D

Explanation: The database management query language is generally designed by keeping in mind that it must support the end-users who are familiar with the English-like commands. It should also boost the process of development of the complex applications software and helps in specifying the structure of the database.

---

49) Which of the following keys is generally used to represents the relationships between the tables?

1. Primary key
2. Foreign key
3. Secondary key
4. None of the above

 

Answer: B

Explanation: To represent the relationships between the various tables in the database, generally, the foreign key is used. Therefore the correct answer is the foreign key.

---

50) Which of the following levels is considered as the level closed to the end-users?

1. Internal Level
2. External Level
3. Conceptual Level
4. Physical Level

 

Answer: B

Explanation: The database's external level is the one and only level that is considered the closest level to the end-users. So the correct answer will be the B.

---

51) A computer security protocol for logging-in can be considered as the example of the ______ component of an information system.

1. Data
2. Software
3. Procedure
4. Hardware

 

---

52) Which one of the following keyword is used to find out the number of values in a column?

1. TOTAL
2. COUNT
3. SUM
4. ADD

 

Answer: B

Explanation: The "COUNT" keyword is used to find the total number of values inside a column. So whenever a user wants to find the total values in a column, he can use the keyword "COUNT".

---

53) Which one of the following is commonly used to define the overall design of the database?

1. Application program
2. Data definition language
3. Schema
4. Source code

 

Answer: C

Explanation: In order to define the overall design of the database, generally, the schema is used. Therefore the correct answer is C.

---

54) Which one of the following commands is used to modify a column inside a table?

1. Drop
2. Update
3. Alter
4. Set

 

Answer: C

Explanation: To make manipulations in a column of a specific table, the "alter" command is used. To understand it more clearly, consider the following example:

Read More

 

Deadlock in DBMS

A deadlock is a condition where two or more transactions are waiting indefinitely for one another to give up locks. Deadlock is said to be one of the most feared complications in DBMS as no task ever gets finished and is in a waiting state forever.

For example: In the student table, transaction T1 holds a lock on some rows and needs to update some rows in the grade table. Simultaneously, transaction T2 holds locks on some rows in the grade table and needs to update the rows in the Student table held by Transaction T1.

Now, the main problem arises. Now Transaction T1 is waiting for T2 to release its lock and similarly, transaction T2 is waiting for T1 to release its lock. All activities come to a halt state and remain at a standstill. It will remain at a standstill until the DBMS detects the deadlock and aborts one of the transactions.

Deadlock Avoidance

• When a database is stuck in a deadlock state, then it is better to avoid the database rather than abort or restarting the database. This is a waste of time and resources.
• A deadlock avoidance mechanism is used to detect any deadlock situation in advance. A method like "wait for graph" is used for detecting the deadlock situation but this method is suitable only for the smaller database. For the larger database, the deadlock prevention method can be used.

Deadlock Detection

In a database, when a transaction waits indefinitely to obtain a lock, then the DBMS should detect whether the transaction is involved in a deadlock or not. The lock manager maintains a Wait for the graph to detect the deadlock cycle in the database.

Wait for Graph

• This is a suitable method for deadlock detection. In this method, a graph is created based on the transaction and its lock. If the created graph has a cycle or closed loop, then there is a deadlock.
• The wait for the graph is maintained by the system for every transaction which is waiting for some data held by the others. The system keeps checking the graph if there is any cycle in the graph.

The wait for a graph for the above scenario is shown below:

closed-loop

Deadlock Prevention

• The deadlock prevention method is suitable for a large database. If the resources are allocated in such a way that deadlock never occurs, then the deadlock can be prevented.
• The Database management system analyzes the operations of the transaction whether they can create a deadlock situation or not. If they do, then the DBMS never allowed that transaction to be executed.

Wait-Die scheme

In this scheme, if a transaction requests for a resource that is already held with a conflicting lock by another transaction then the DBMS simply checks the timestamp of both transactions. It allows the older transaction to wait until the resource is available for execution.

Let's assume there are two transactions Ti and Tj and let TS(T) is a timestamp of any transaction T. If T2 holds a lock by some other transaction and T1 is requesting resources held by T2 then the following actions are performed by DBMS:

1. Check if TS(Ti) < TS(Tj) - If Ti is the older transaction and Tj has held some resource, then Ti is allowed to wait until the data item is available for execution. That means if the older transaction is waiting for a resource that is locked by the younger transaction, then the older transaction is allowed to wait for the resource until it is available.
2. Check if TS(T_i) < TS(Tj) - If Ti is an older transaction and has held some resource and if Tj is waiting for it, then Tj is killed and restarted later with the random delay but with the same timestamp.

Wound wait scheme

• In the wound wait scheme, if the older transaction requests for a resource that is held by the younger transaction, then the older transaction forces the younger one to kill the transaction and release the resource. After the minute delay, the younger transaction is restarted but with the same timestamp.
• If the older transaction has held a resource that is requested by the Younger transaction, then the younger transaction is asked to wait until the older one releases it.

Read More

 

Checkpoint

• The checkpoint is a type of mechanism where all the previous logs are removed from the system and permanently stored in the storage disk.
• The checkpoint is like a bookmark. While the execution of the transaction, such checkpoints are marked, and the transaction is executed then using the steps of the transaction, the log files will be created.
• When it reaches the checkpoint, then the transaction will be updated in the database, and till that point, the entire log file will be removed from the file. Then the log file is updated with the new step of the transaction till the next checkpoint and so on.
• The checkpoint is used to declare a point before which the DBMS was in a consistent state, and all transactions were committed.

Recovery using Checkpoint

In the following manner, a recovery system recovers the database from this failure:

• The recovery system reads log files from the end to the start. It reads log files from T4 to T1.
• The recovery system maintains two lists, a redo-list, and an undo list.
• The transaction is put into a redo state if the recovery system sees a log with <Tn, Start> and <Tn, Commit> or just <Tn, Commit>. In the redo-list and their previous list, all the transactions are removed and then redone before saving their logs.
• For example: In the log file, transactions T2 and T3 will have <Tn, Start> and <Tn, Commit>. The T1 transaction will have only <Tn, commit> in the log file. That's why the transaction is committed after the checkpoint is crossed. Hence it puts T1, T2, and T3 transactions into the redo list.
• The transaction is put into undo state if the recovery system sees a log with <Tn, Start> but no commit or abort log is found. In the undo list, all the transactions are undone, and their logos are removed.
• For example Transaction, T4 will have <Tn, Start>. So T4 will be put into undo list since this transaction is not yet complete and failed.

Read More

 

Log-Based Recovery

• The log is a sequence of records. The log of each transaction is maintained in some stable storage so that if any failure occurs, then it can be recovered from there.
• If any operation is performed on the database, then it will be recorded in the log.
• But the process of storing the logs should be done before the actual transaction is applied in the database.

Let's assume there is a transaction to modify the City of a student. The following logs are written for this transaction.

• When the transaction is initiated, then it writes the 'start' log.
1. <Tn, Start>  
• When the transaction modifies the City from 'Noida' to 'Bangalore', then another log is written to the file.
1. <Tn, City, 'Noida', 'Bangalore' >  
• When the transaction is finished, then it writes another log to indicate the end of the transaction.
1. <Tn, Commit>  

There are two approaches to modifying the database:

1. Deferred database modification:

• The deferred modification technique occurs if the transaction does not modify the database until it has committed been.
• In this method, all the logs are created and stored in the stable storage, and the database is updated when a transaction commits.

2. Immediate database modification:

• The Immediate modification technique occurs if database modification occurs while the transaction is still active.
• In this technique, the database is modified immediately after every operation. It follows an actual database modification.

Recovery using Log records

When the system is crashed, then the system consults the log to find which transactions need to be undone and which need to be redone.

1. If the log contains the record <Ti, Start> and <Ti, Commit> or <Ti, Commit>, then the Transaction Ti needs to be redone.
2. Iflogthe contains record<T_n, Start> but does not contain the record either <Ti, commit> or <Ti, abort>, then the Transaction Ti needs to be undone.

Read More

 

Failure Classification

To find awhere the problem has occurred, we generalize a failure into the following categories:

1. Transaction failure
2. System crash
3. Disk failure

1. Transaction failure

The transaction failure occurs when it fails to execute or when it reaches a point from where it can't go any further. If a few transaction or process is hurt, then this is called as transaction failure.

Reasons for a transaction failure could be -

1. Logical errors: If a transaction cannot complete due to some code error or an internal error condition, then the logical error occurs.
2. Syntax error: It occurs when the DBMS itself terminates an active transaction because the database system is not able to execute it. For example, The system aborts an active transaction, in case of deadlock or resource unavailability.

2. System Crash

• System failure can occur due to power failure or other hardware or software failure. Example: Operating system error.

  Fail-stop assumption: In the system crash, non-volatile storage is assumed not to be corrupted.

3. Disk Failure

• It occurs wherewhen hard-disk drives or storage drives used to fail frequently. It was a common problem in the early days of technology evolution.
• Disk failure occurs due to the formation of bad sectors, disk head crash, and unreachability to the disk or any other failure, which destroy all or part of disk storage.

Read More

 

Recoverability of Schedule

Sometimes a transaction may not execute completely due to a software issue, system crash or hardware failure. In that case, the failed transaction has to be rollback. But some other transaction may also have used value produced by the failed transaction. So we also have to rollback those transactions.

The above table 1 shows a schedule which has two transactions. T1 reads and writes the value of A and that value is read and written by T2. T2 commits but later on, T1 fails. Due to the failure, we have to rollback T1. T2 should also be rollback because it reads the value written by T1, but T2 can't be rollback because it already committed. So this type of schedule is known as irrecoverable schedule.

Irrecoverable schedule: The schedule will be irrecoverable if Tj reads the updated value of Ti and Tj committed before Ti commit.

The above table 2 shows a schedule with two transactions. Transaction T1 reads and writes A, and that value is read and written by transaction T2. But later on, T1 fails. Due to this, we have to rollback T1. T2 should be rollback because T2 has read the value written by T1. As it has not committed before T1 commits so we can rollback transaction T2 as well. So it is recoverable with cascade rollback.

Recoverable with cascading rollback: The schedule will be recoverable with cascading rollback if Tj reads the updated value of Ti. Commit of Tj is delayed till commit of Ti.

The above Table 3 shows a schedule with two transactions. Transaction T1 reads and write A and commits, and that value is read and written by T2. So this is a cascade less recoverable schedule.

Read More