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Chapter 17: Problem 12

Describe the four levels of isolation in SQL.

Short Answer

Expert verified
The four levels of isolation in SQL are Read Uncommitted, Read Committed, Repeatable Read, and Serializable. Each level provides a different balance between concurrency and consistency, ranging from highest concurrency and lowest consistency at the Read Uncommitted level to lowest concurrency and highest consistency at the Serializable level.

Step by step solution

01

Isolation Level 1: Read Uncommitted

The Read Uncommitted level is the lowest level of isolation. In this level, a transaction can read data that has been modified (but not yet committed) by another transaction. This may lead to phenomena such as dirty reads, or reading data that could be rolled back by another transaction.
02

Isolation Level 2: Read Committed

In the Read Committed level, a transaction only sees data that has been committed before the transaction began. This prevents dirty reads, but can still lead to non-repeatable reads, where a row can be modified by another transaction between repeated reads within the same transaction.
03

Isolation Level 3: Repeatable Read

The Repeatable Read level ensures that any rows read during a transaction will not be changed by other transactions for the duration of that transaction. This prevents dirty reads and non-repeatable reads, but can still lead to phantom reads, where new rows can be added by another transaction and will be included in subsequent reads within the same transaction.
04

Isolation Level 4: Serializable

The highest level of isolation, Serializable, includes the restrictions of all lower levels and, in addition, ensures that transactions occur in a fully isolated fashion. This means it prevents dirty reads, non-repeatable reads, and phantom reads. However, it also leads to the lowest level of concurrency, as transactions are effectively executed in sequence.

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Most popular questions from this chapter

What is a schedule (history)? Define the concepts of recoverable, cascadeless, and strict schedules, and compare them in terms of their recoverability.

What is a serial schedule? What is a serializable schedule? Why is a serial schedule considered correct? Why is a serializable schedule considered correct?

Which of the following schedules is (conflict) serializable? For each serializable schedule, determine the equivalent serial schedules. a. \(r_{1}(X) ; r_{3}(X) ; w_{1}(X) ; r_{2}(X) ; w_{3}(X)\) b. \(r_{1}(X) ; r_{3}(X) ; w_{3}(X) ; w_{1}(X) ; r_{2}(X)\) c. \(r_{3}(X) ; r_{2}(X) ; w_{3}(X) ; r_{1}(X) ; w_{1}(X)\) d. \(r_{3}(X) ; r_{2}(X) ; r_{1}(X) ; w_{3}(X) ; w_{1}(X)\)

Discuss the different measures of transaction equivalence. What is the difference between conflict equivalence and view equivalence?

What is the difference between the constrained write and the unconstrained write assumptions? Which is more realistic?
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