presto-main/src/main/java/io/prestosql/transaction/InMemoryTransactionManager.java/InMemoryTransactionManager/beginTransaction
class InMemoryTransactionManager:        private ConnectorTransactionHandle beginTransaction(Connector connector)
        {
            if (connector instanceof InternalConnector) {
                return ((InternalConnector) connector).beginTransaction(transactionId, isolationLevel, readOnly);
            }
            else {
                return connector.beginTransaction(isolationLevel, readOnly);
            }
        }