How to choose the level of transaction isolation and what are the overheads?

The transaction isolation level determines how isolated a transaction is from modifications made by other transactions and affects possible read anomalies (dirty read, non-repeatable read, phantom read).

Main isolation levels:

• Read Uncommitted — the lowest level, allows reading uncommitted data from other transactions (dirty read). Minimal overhead but low reliability.
• Read Committed — allows reading only committed data, prevents dirty reads but permits non-repeatable reads.
• Repeatable Read — guarantees that data read in a transaction will not change until it completes, preventing non-repeatable reads, but phantom reads are possible.
• Serializable — the strictest level, provides full isolation, preventing all anomalies, but the overhead for locking and reduced concurrency is maximal.

How to choose:

• If maximum performance is important and some anomalies are acceptable — Read Committed.
• If data consistency within a transaction is required — Repeatable Read.
• If full isolation and integrity are critical — Serializable.

Overheads are associated with locking, waiting, and reduced concurrency. The higher the isolation level, the greater the likelihood of locks and delays, which can lead to decreased system throughput.