What are transaction isolation levels?

Transaction isolation levels determine the degree to which the changes made by one transaction are visible to other concurrently executing transactions. They help control concurrency and prevent data anomalies such as dirty reads, non-repeatable reads, and phantom reads.

The main isolation levels defined by the SQL standard include:

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 committed data.
   • Dirty reads are prevented, but non-repeatable reads can occur.

3. Repeatable Read

   • Guarantees that if a transaction reads data, subsequent reads of the same data will return the same values.
   • Prevents dirty and non-repeatable reads, but phantom reads can occur.

4. Serializable

   • The strictest isolation level.
   • Transactions are executed as if they were sequential, one after another.
   • All types of anomalies, including phantom reads, are prevented.

Choosing an isolation level is a trade-off between performance and data integrity. Higher levels of isolation reduce concurrency and can lead to locking, but provide greater consistency.