presto-base-jdbc/src/main/java/io/prestosql/plugin/jdbc/JdbcConnector.java/JdbcConnector/beginTransaction
class JdbcConnector:    @Override
    public ConnectorTransactionHandle beginTransaction(IsolationLevel isolationLevel, boolean readOnly)
    {
        checkConnectorSupports(READ_COMMITTED, isolationLevel);
        JdbcTransactionHandle transaction = new JdbcTransactionHandle();
        transactions.put(transaction, jdbcMetadataFactory.create());
        return transaction;
    }