Dirty Read and Phantom Read Quiz

A dirty read occurs when a transaction reads data that has been modified by another ongoing transaction but not yet committed. This means the reading transaction may access temporary, unstable data that could be rolled back, leading to potential inconsistencies if the changes are later discarded.

READ COMMITTED and REPEATABLE READ isolation levels prevent dirty reads by ensuring that a transaction only sees committed data. READ COMMITTED allows a transaction to read only committed changes made by other transactions, while REPEATABLE READ guarantees that if a transaction reads a value, it will see the same value for subsequent reads until it completes.

A phantom read occurs in database transactions when a query is executed multiple times and the results differ due to concurrent transactions. This can happen when new rows are added or existing rows are deleted by other transactions between the executions, leading to a situation where rows appear or disappear unexpectedly.

REPEATABLE READ isolation level prevents dirty reads and non-repeatable reads but allows phantom reads. This means that while a transaction can read the same data multiple times and get consistent results, new rows that match the query criteria may be added by other transactions, leading to different results upon subsequent reads within the same transaction.

READ UNCOMMITTED is the lowest isolation level in database transactions, allowing transactions to read data that has been modified but not yet committed by other transactions. This means that a transaction can access uncommitted changes, leading to "dirty reads," where it may retrieve data that could later be rolled back or changed.

A non-repeatable read occurs when a transaction reads the same row multiple times and sees different committed values due to changes made by other transactions in between the reads. This inconsistency can lead to unreliable data retrieval, as the initial state of the data may no longer be valid by the time it is read again.

REPEATABLE READ isolation level ensures that if a transaction reads a row, subsequent reads within the same transaction will return the same data. However, it does not prevent phantom reads, where new rows added by other transactions can appear in subsequent queries of the same transaction, leading to potential inconsistencies.

SERIALIZABLE is the strongest isolation level as it ensures complete isolation from other transactions, preventing phenomena like dirty reads, non-repeatable reads, and phantom reads. By executing transactions in a way that they appear to be processed one after the other, it guarantees the highest level of consistency and integrity in the database.

READ COMMITTED isolation level ensures that a transaction can only read data that has been committed, thereby preventing dirty reads, which occur when a transaction reads uncommitted changes made by another transaction. However, it does not prevent phantom reads, where new rows can appear in a subsequent read due to other transactions committing changes.

In SERIALIZABLE isolation, transactions are executed in a way that ensures complete isolation from one another, making them appear as if they are processed sequentially. This prevents anomalies and ensures data consistency, as no transaction can see the intermediate states of others, effectively simulating a single-threaded execution environment.

READ UNCOMMITTED isolation level allows transactions to read data that has been modified but not yet committed by other transactions. This can lead to dirty reads, where uncommitted changes are visible. Additionally, it can result in non-repeatable reads and phantom reads, as data may change between reads within the same transaction.

In the REPEATABLE READ isolation level, a transaction ensures that any rows it reads will remain consistent throughout its execution. This means that if the same rows are read multiple times, the values will not change, leading to identical results. This guarantees data stability and avoids issues like non-repeatable reads.