Question

A transaction is said to have ____ isolation if it does not overwrite the dirty reads of higher-level transactions. ___ (one) isolation has no lost updates, and ___ isolation has no lost updates and no dirty reads. Finally, ___ isolation (also called true isolation) has, in addition to level 2 properties, repeatable reads.

Answer 1

The levels of transaction isolation in databases are Read Uncommitted, Read Committed, Repeatable Read, and Serializable. Each level provides increasing guarantees against data anomalies, with Serializable being the highest level ensuring true isolation.

Step-by-step explanation:

The question refers to the levels of transaction isolation in database management systems. In terms of transaction isolation, there are typically four levels defined by the ANSI SQL standard:

• Read Uncommitted: A transaction has read uncommitted isolation if it allows dirty reads, which means it can read data from changes that have not yet been committed by other transactions.
• Read Committed: A transaction with read committed isolation has no lost updates. This level prevents a transaction from reading data that has been modified but not committed by another transaction.
• Repeatable Read: Repeatable read isolation has no lost updates and no dirty reads. Once a read is performed, the same data will be returned in subsequent reads within the same transaction, even if other transactions are simultaneously trying to modify it.
• Serializable: Finally, serializable isolation (also called true isolation), in addition to having the properties of level 2 (repeatable reads), ensures that reads are repeatable over multiple transactions. This is the highest level of isolation and prevents phantom reads, where additional rows might be added to the dataset by another transaction after the first read.

To fill in the blanks: A transaction is said to have read uncommitted isolation if it does not overwrite the dirty reads of higher-level transactions. Read committed isolation has no lost updates, and repeatable read isolation has no lost updates and no dirty reads. Finally, serializable isolation has, in addition to level 2 properties, repeatable reads.

Answer 2

A transaction is said to have "Read Uncommitted" isolation if it does not overwrite the dirty reads of higher-level transactions. "Read Committed" isolation has no lost updates, and "Serializable" isolation has no lost updates and no dirty reads. Finally, "Serializable" isolation (also called true isolation) has, in addition to level 2 properties, repeatable reads.

Step-by-step explanation:

Isolation levels in database transactions determine how transactions interact with each other regarding visibility of changes. "Read Uncommitted" allows transactions to read uncommitted changes, risking dirty reads. "Read Committed" ensures no dirty reads but allows lost updates. "Serializable" ensures both no dirty reads and no lost updates, providing the highest level of isolation. Additionally, "Serializable" (true isolation) prevents non-repeatable reads, ensuring that once a transaction reads a piece of data, it remains consistent during the transaction's duration.