Can you describe the main issues related to different levels of transaction isolation?

The main issues related to transaction isolation levels are associated with possible anomalies during concurrent data access:

• Read Uncommitted: a transaction can see changes made by other transactions that are not yet committed (dirty read). This can lead to reading inconsistent data.

• Read Committed: a transaction only sees committed changes, but non-repeatable reads are possible — if data changes between two reads within the same transaction.

• Repeatable Read: guarantees that data read in a transaction will not change until the transaction ends. However, phantom reads are possible — the appearance of new rows matching the query condition.

• Serializable: the strictest level, which prevents all the above anomalies, ensuring full transaction isolation but may reduce performance due to locking.

Choosing an isolation level is a compromise between data integrity and performance.