presto-raptor-legacy/src/main/java/io/prestosql/plugin/raptor/legacy/RaptorConnector.java/RaptorConnector/beginTransaction
class RaptorConnector:    @Override
    public ConnectorTransactionHandle beginTransaction(IsolationLevel isolationLevel, boolean readOnly)
    {
        checkConnectorSupports(READ_COMMITTED, isolationLevel);
        RaptorTransactionHandle transaction = new RaptorTransactionHandle();
        transactions.put(transaction, metadataFactory.create(tableId -> beginDelete(tableId, transaction.getUuid())));
        return transaction;
    }