Database Transactions Overview

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Questions and Answers

What characterizes a phantom read in transactions?

Reading additional rows during repeated execution of a query. (correct)

Allowing transactions to read only committed data.

Preventing all data from being modified by other transactions.

Reading modified but uncommitted data.

Which isolation level allows transactions to read uncommitted data?

SERIALIZABLE

READ COMMITTED

READ UNCOMMITTED (correct)

REPEATABLE READ

What is a key feature of the SERIALIZABLE isolation level?

It offers no protection against dirty reads.

It prevents all types of reads.

It allows phantom reads to occur.

It prohibits other transactions from updating or deleting read data. (correct)

Which statement accurately describes the REPEATABLE READ isolation level?

It permits additional rows to be inserted into the tables during the transaction. Signup and view all the answers

What consequence does READ COMMITTED isolation level primarily address?

Dirty reads are prevented by disallowing reads of uncommitted data. Signup and view all the answers

What does the concept of atomicity in transactions require?

All operations of a transaction should be completed. Signup and view all the answers

Which statement best describes the durability requirement in transactions?

Changes made by a committed transaction must persist despite failures. Signup and view all the answers

What happens during a rollback of a transaction?

The database returns to its last known consistent state. Signup and view all the answers

What is the purpose of the transaction log in a DBMS?

To keep track of all transactions that affect the database. Signup and view all the answers

How does the isolation requirement affect transactions?

Only sequential execution of transactions is allowed until completion. Signup and view all the answers

What does the COMMIT command signify in a transaction?

The transaction has been successfully completed and changes are preserved. Signup and view all the answers

What is the role of the SAVE TRANSACTION command?

It allows for rolling back to a specific point without undoing the entire transaction. Signup and view all the answers

What is implied by the consistency requirement in a transaction?

All data written should meet the database's rules and constraints. Signup and view all the answers

What state is a transaction in when it has executed its final statement but has not yet completed all operations?

Partially Committed State Signup and view all the answers

Which scenario best describes a 'Lost Update'?

Two transactions update the same data element, and one update is lost. Signup and view all the answers

What is the primary purpose of a lock in database transactions?

To reserve exclusive use of a data item for the current transaction. Signup and view all the answers

Which type of lock allows shared access for reading data without restricting other transactions?

Shared Lock Signup and view all the answers

What is 'Deadlock Detection' in database management?

The process of periodically checking for deadlocks in the database. Signup and view all the answers

In which scenario does a 'Dirty Read' occur?

A transaction reads uncommitted data from a concurrent transaction. Signup and view all the answers

What is an effect of using Row-Level Locks compared to Database-Level Locks?

Row-Level Locks are less restrictive and allow more transactions to access data. Signup and view all the answers

Which statement accurately defines 'Consistency' in the context of database transactions?

Transactions lead to a valid state of the database after completion. Signup and view all the answers

Study Notes

Database Transactions

Transaction is a logical unit of work made up of a collection of operations.
Database Request is a single SQL statement within a transaction.
Database Consistent State adheres to the constraints defined in the database schema.
Transaction Log is a record kept by the DBMS of all transactions that modify the database.

Transaction Properties (ACID)

Atomicity ensures all operations within a transaction complete successfully, or none are applied.
Consistency guarantees only valid data, adhering to all rules and constraints, is written to the database.
Isolation ensures data accessed by a transaction is not available to other transactions, until the first completes.
Durability guarantees that once a transaction commits, its changes are permanent and persist despite system failures.

Transaction States

Active State where a transaction performs READ and WRITE operations.
Partially committed state when the final query statement has executed but not acknowledged.
Committed state when all operations complete successfully.
Failed state when an operation within a transaction cannot be completed.
Terminated state when the transaction leaves the system, successfully committed or aborted.

Concurrency Control

Concurrency Control manages simultaneous transactions to preserve data consistency and integrity.
Lost Update occurs when two transactions modify the same data, and one update overwrites the other.
Uncommitted Data occurs when a transaction reads data from another transaction that is not yet committed, and later rolls back.
Inconsistent Retrievals occur when a transaction reads data before and after another transaction modifies the same data, resulting in inconsistent data view.

Locking

Lock restricts access to data to a single transaction.
Database-Level Lock locks the entire database.
Table-Level Lock locks the whole table.
Row-Level Lock locks a specific row.
Binary Lock has two states: locked (1) and unlocked (0).
Exclusive Lock reserves access to the locked object exclusively for the transaction that acquired it.
Shared Lock allows multiple transactions to read data from the object, but only one to write.

Deadlock

Deadlock occurs when two transactions wait indefinitely for each other to release a lock.

Deadlock Handling

Deadlock Prevention actively prevents deadlocks by aborting transactions with potential deadlock issues.
Deadlock Detection periodically checks for deadlocks and resolves them.
Deadlock Avoidance requires transactions to obtain all necessary locks before execution.

Transaction Isolation Levels

Dirty Read allows a transaction to read data that is not yet committed by another transaction.
Non-repeatable Read allows a transaction to read a row multiple times, with different results, due to updates or deletions by other transactions.
Phantom Read allows a transaction to repeat a query and retrieve additional rows because another transaction has inserted new rows in the meantime.
READ UNCOMMITTED is the least restrictive isolation level.
Ignores locks placed by other transactions.
Can read uncommitted, "dirty" data.
READ COMMITTED is the default isolation level for SQL Server.
Prevents dirty reads by disallowing reading data modified but not yet committed by other transactions.
Allows other transactions to modify, insert, or delete data between statements within the current transaction, leading to non-repeatable reads or phantom data.
REPEATABLE READ is more restrictive than READ COMMITTED.
Encompasses READ COMMITTED behavior.
Prevents other transactions from modifying or deleting data that has been read by the current transaction until it commits.
Does not prevent phantom reads because new rows can be inserted by other transactions.
SERIALIZABLE ISOLATION is the most restrictive level.
Prevents any modifications or deletions of data read by a transaction, ensuring data consistency.
Provides a solution to the phantom read problem.

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Description

Explore the fundamental concepts of database transactions, including their properties defined by the ACID principles. This quiz covers transaction states, database requests, and the importance of transaction logs in ensuring consistent database management. Test your knowledge on how these elements interact to maintain data integrity.