What are the main difficulties that arise when executing transactions simultaneously in databases?

The main difficulties in executing transactions simultaneously in databases are related to ensuring data integrity and correctness of results:

• Race conditions: when multiple transactions try to modify the same data simultaneously, conflicts can occur.
• Lost updates: changes made by one transaction may be overwritten by another without acknowledgment.
• Dirty reads: one transaction reads data that has not yet been committed by another transaction, which may be rolled back.
• Non-repeatable reads: data read in one transaction can be changed by another transaction before the first transaction completes.
• Phantom reads: when executing the same query again in a transaction, new rows added by another transaction appear.

To combat these issues, transaction isolation levels and locking mechanisms are used, as well as optimistic and pessimistic concurrency control strategies.