What are the main difficulties that can be encountered when performing parallel transactions?

The main difficulties in executing parallel transactions are:

• Race conditions: when multiple transactions try to modify the same data simultaneously.
• Deadlocks: situations where transactions wait for each other, and none can proceed.
• Lost updates: when changes made by one transaction are overwritten by another.
• Violation of isolation: reading 'dirty' data, non-repeatable reads, or phantom reads.

To address these issues, transaction isolation levels, locking mechanisms, and optimistic/pessimistic concurrency control are used.