presto-mongodb/src/main/java/io/prestosql/plugin/mongodb/MongoConnector.java/MongoConnector/beginTransaction
class MongoConnector:    @Override
    public ConnectorTransactionHandle beginTransaction(IsolationLevel isolationLevel, boolean readOnly)
    {
        checkConnectorSupports(READ_UNCOMMITTED, isolationLevel);
        MongoTransactionHandle transaction = new MongoTransactionHandle();
        transactions.put(transaction, new MongoMetadata(mongoSession));
        return transaction;
    }