Can you explain the different levels of transaction isolation in database management systems?

Transaction isolation levels determine how much a transaction can see the changes made by other transactions and help manage concurrency issues.

The main isolation levels (according to the SQL standard):

1. Read Uncommitted

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

2. Read Committed

   • A transaction only sees data committed by other transactions.
   • Prevents dirty reads but non-repeatable reads are possible.

3. Repeatable Read

   • Ensures that data read in a transaction does not change until it completes.
   • Prevents 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.

Example: At the Read Committed level, a transaction can see changes made by others after it started, which can lead to non-repeatable reads.

In PostgreSQL, the default level is Read Committed, but a stricter level can be set to ensure greater consistency.