presto-cassandra/src/main/java/io/prestosql/plugin/cassandra/CassandraConnector.java/CassandraConnector/beginTransaction
class CassandraConnector:    @Override
    public ConnectorTransactionHandle beginTransaction(IsolationLevel isolationLevel, boolean readOnly)
    {
        checkConnectorSupports(READ_UNCOMMITTED, isolationLevel);
        return CassandraTransactionHandle.INSTANCE;
    }