What transaction isolation levels are available in PostgreSQL?

PostgreSQL supports four standard transaction isolation levels defined by the SQL standard:

1. Read Uncommitted — the lowest isolation level. Allows reading data that has not yet been committed (dirty reads). In PostgreSQL, it effectively behaves like Read Committed, as dirty reads are not supported.

2. Read Committed — the default level. A transaction sees only data that was committed at the moment of each individual query. Changes made by other transactions may appear between queries within the same transaction.

3. Repeatable Read — the transaction sees a snapshot of the data at the start of the transaction. All queries within the transaction see the same data, even if other transactions modify and commit changes. It prevents non-repeatable reads but allows phantom reads.

4. Serializable — the strictest level. Transactions are executed as if they were run sequentially, one after another. It prevents phantom reads and provides full isolation but may lead to transaction rollbacks in case of conflicts.

Example of setting the isolation level:

BEGIN TRANSACTION ISOLATION LEVEL SERIALIZABLE;
-- operations
COMMIT;

Thus, the choice of isolation level affects the balance between performance and data consistency.