PostgreSQL supports four isolation levels defined by the SQL standard:
1. Read Uncommitted
2. Read Committed
3. Repeatable Read
4. SerializableThe lower isolation levels offer higher performance but less transactional safety, while the higher ones offer less performance but stronger guarantees.
1. **Read Uncommitted**: This isolation level allows a transaction to see uncommitted changes from other transactions. This means that dirty reads are possible, in which an uncommitted change can be read by another transaction, leading to inconsistent data. This is the lowest level of isolation and is not recommended for most applications.
2. **Read Committed**: In this isolation level, a transaction can only see committed changes from other transactions. A dirty read is not possible, as a transaction will only see changes that have been committed. However, non-repeatable reads may occur, in which a query may yield different results if it is run multiple times within the same transaction due to other transactions committing changes in the meantime.
3. **Repeatable Read**: In this isolation level, a transaction will only see changes that were committed before it started. This means that a transaction will not see changes made by other transactions during its execution. This provides stronger guarantees than read committed, but can lead to phantom reads, in which a query yields different results when run multiple times within the same transaction due to other transactions inserting or deleting rows.
4. **Serializable**: This is the strongest isolation level, in which a transaction enforces that its execution should not conflict with the execution of any other transaction. This guarantees that a transaction will appear to be executing alone, and will not see changes made by other transactions or allow other transactions to see its changes until it commits. Serializable can lead to serialization failures, in which two transactions cannot execute concurrently due to conflicts, and one of them must be aborted and retried to avoid deadlocks.
Performance-wise, the lower isolation levels have lower overhead and are generally faster, as they allow for more concurrency between transactions. Serializable, on the other hand, has higher overhead due to the need for conflict detection and may lead to performance degradation under high concurrency. It is important to choose an appropriate isolation level based on the consistency and safety requirements of your application, balancing it with the desired performance characteristics.
In PostgreSQL, you can set the isolation level using the ‘SET TRANSACTION‘ command, for example:
connection.setTransactionIsolation(Connection.TRANSACTION_READ_COMMITTED);