Transaction isolation levels in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of transaction isolation levels in a database?

Transaction isolation levels control how and when the changes made by one transaction become visible to other concurrent transactions, helping to prevent data inconsistencies.

beginner

Name the four standard transaction isolation levels defined by SQL.

The four standard isolation levels are: Read Uncommitted, Read Committed, Repeatable Read, and Serializable.

intermediate

What phenomenon does the Read Committed isolation level prevent?

Read Committed prevents dirty reads, meaning a transaction cannot read data that another uncommitted transaction has modified.

advanced

Explain the difference between Repeatable Read and Serializable isolation levels.

Repeatable Read ensures that if a transaction reads data twice, it sees the same data both times, preventing non-repeatable reads. Serializable is stricter; it makes transactions appear as if they run one after another, preventing all concurrency anomalies including phantom reads.

intermediate

What is a phantom read in the context of transaction isolation?

A phantom read happens when a transaction reads a set of rows matching a condition, but a subsequent read in the same transaction finds additional rows that were inserted by another committed transaction.

Which isolation level allows dirty reads?

A Repeatable Read

B Read Committed

C Read Uncommitted

D Serializable

Read Uncommitted is the lowest isolation level and allows dirty reads, meaning it can read uncommitted changes from other transactions.

Which isolation level prevents phantom reads in PostgreSQL?

A Read Committed

B Read Uncommitted

C Repeatable Read

D Serializable

Serializable isolation level prevents phantom reads by ensuring transactions behave as if executed sequentially.

In PostgreSQL, what is the default transaction isolation level?

A Read Committed

B Read Uncommitted

C Repeatable Read

D Serializable

PostgreSQL uses Read Committed as the default isolation level.

Which isolation level guarantees that repeated reads within the same transaction see the same data?

A Read Uncommitted

B Repeatable Read

C Serializable

D Read Committed

Repeatable Read ensures that if you read the same data twice in a transaction, it will not change.

What problem does the Serializable isolation level solve that Repeatable Read does not?

A Phantom reads

B Non-repeatable reads

C Dirty reads

D Lost updates

Serializable isolation prevents phantom reads, which can still occur under Repeatable Read.

Describe the four standard transaction isolation levels and what concurrency problems each prevents.

Think about how visible changes from other transactions affect your current transaction.

Explain why choosing the right transaction isolation level is important in database applications.

Consider real-life situations where multiple people access and change shared data.