Read committed behavior in PostgreSQL - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does Read Committed isolation level mean in PostgreSQL?

It means a transaction only sees data committed before the query started or committed during its execution, but it never sees uncommitted changes from other transactions.

intermediate

In Read Committed mode, can a query inside a transaction see changes made by other transactions that commit after the query started?

No, each query sees only data committed before that query began, not changes committed after the query started.

beginner

How does Read Committed behavior prevent dirty reads?

It prevents dirty reads by never showing data from transactions that have not yet committed.

intermediate

Can Read Committed isolation level cause non-repeatable reads?

Yes, because each query sees the latest committed data, data can change between queries in the same transaction.

beginner

What is a real-life example of Read Committed behavior?

Imagine reading a shared notebook: you only see notes that others have finished writing and put away, but if they add new notes later, your next read will see those new notes.

Which of the following best describes Read Committed isolation level?

A Queries see only committed data as of the start of each query

B Queries see all data including uncommitted changes

C Queries see a snapshot of data as of the start of the transaction

D Queries lock all rows they read

Read Committed means each query sees only data committed before that query started.

Can a transaction in Read Committed isolation see changes committed by another transaction after it started but before its next query?

A No, it only sees data committed before the transaction started

B Yes, each query sees the latest committed data at its start

C No, it sees a fixed snapshot throughout the transaction

D Yes, but only if it explicitly refreshes

In Read Committed, each query sees the latest committed data at the time the query starts.

Which problem is prevented by Read Committed isolation?

A Lost updates

B Phantom reads

C Non-repeatable reads

D Dirty reads

Read Committed prevents dirty reads by not showing uncommitted data.

Which problem can still occur under Read Committed isolation?

A Dirty reads

B Write skew

C Non-repeatable reads

D Reading uncommitted data

Non-repeatable reads can happen because data can change between queries in the same transaction.

In PostgreSQL, what is the default transaction isolation level?

A Read Committed

B Repeatable Read

C Serializable

D Read Uncommitted

PostgreSQL uses Read Committed as the default isolation level.

Explain how Read Committed isolation level controls visibility of data changes during a transaction.

Think about when data becomes visible to each query.

Describe a real-life analogy that helps understand Read Committed behavior.

Imagine reading notes others write and finish at different times.