What levels of transaction isolation exist?

There are four main levels of transaction isolation defined by the SQL standard:

1. Read Uncommitted

   • A transaction can see changes made by other transactions even if they are not yet committed.
   • Dirty reads are possible.

2. Read Committed

   • A transaction only sees data that has been committed.
   • Dirty reads are prevented, but non-repeatable reads are possible.

3. Repeatable Read

   • Guarantees that data read during a transaction will not change until the transaction completes.
   • Prevents dirty and non-repeatable reads, but phantom reads are possible.

4. Serializable

   • The strictest level, transactions are executed as if they were sequential.
   • All types of read anomalies are prevented.

The choice of isolation level depends on the requirements for consistency and performance. For example, PostgreSQL uses Read Committed by default.