What are transaction isolation levels?

Transaction isolation levels determine how much a transaction can see of the intermediate (uncommitted) data changes made by other concurrently executing transactions. They help manage conflicts during parallel data access.

The SQL standard defines four levels:

1. Read Uncommitted: The lowest level. A transaction can see data that another transaction has not yet committed (dirty read).

2. Read Committed: A transaction only sees data that was committed before it started, as well as changes it made itself. It prevents dirty reads but can still encounter non-repeatable reads and phantom reads.

3. Repeatable Read: Guarantees that if a transaction reads a row multiple times, it always gets the same value for that row. It prevents non-repeatable reads but can still encounter phantom reads.

4. Serializable: The highest level of isolation. It guarantees complete isolation of transactions as if they were executed sequentially. It prevents all three issues: dirty reads, non-repeatable reads, and phantom reads. Usually implemented with locking or multi-version concurrency control (MVCC).

Types of problems during parallel data access:

• Dirty Read: Reading data modified by another transaction that has not yet committed. If that transaction rolls back, the read data becomes invalid.
• Non-repeatable Read: Reading the same row twice within a transaction, but another transaction modifies and commits that row between the reads. The second read returns a different value.
• Phantom Read: Executing two identical queries within a transaction that return a set of rows. Between the queries, another transaction adds or deletes rows matching the query condition. The second query returns a different set of rows (new or disappeared rows).

Choosing an isolation level is a trade-off between data integrity and performance. Higher levels provide better integrity but may reduce concurrency due to locking or version management overhead. In Python, libraries like psycopg2 for PostgreSQL or sqlite3 for SQLite often allow setting the isolation level for a connection or transaction.