presto-hive/src/main/java/io/prestosql/plugin/hive/HiveConnector.java/HiveConnector/beginTransaction
class HiveConnector:    @Override
    public ConnectorTransactionHandle beginTransaction(IsolationLevel isolationLevel, boolean readOnly)
    {
        checkConnectorSupports(READ_UNCOMMITTED, isolationLevel);
        ConnectorTransactionHandle transaction = new HiveTransactionHandle();
        try (ThreadContextClassLoader ignored = new ThreadContextClassLoader(classLoader)) {
            transactionManager.put(transaction, metadataFactory.get());
        }
        return transaction;
    }