presto-mongodb/src/main/java/io/prestosql/plugin/mongodb/MongoPageSource.java/MongoPageSource/getNextPage
class MongoPageSource:    @Override
    public Page getNextPage()
    {
        verify(pageBuilder.isEmpty());
        count = 0;
        for (int i = 0; i < ROWS_PER_REQUEST; i++) {
            if (!cursor.hasNext()) {
                finished = true;
                break;
            }
            currentDoc = cursor.next();
            count++;

            pageBuilder.declarePosition();
            for (int column = 0; column < columnTypes.size(); column++) {
                BlockBuilder output = pageBuilder.getBlockBuilder(column);
                appendTo(columnTypes.get(column), currentDoc.get(columnNames.get(column)), output);
            }
        }

        Page page = pageBuilder.build();
        pageBuilder.reset();
        return page;
    }