presto-elasticsearch/src/main/java/io/prestosql/elasticsearch/ElasticsearchConnector.java/ElasticsearchConnector/beginTransaction
class ElasticsearchConnector:    @Override
    public ConnectorTransactionHandle beginTransaction(IsolationLevel isolationLevel, boolean readOnly)
    {
        checkConnectorSupports(READ_COMMITTED, isolationLevel);
        return ElasticsearchTransactionHandle.INSTANCE;
    }