presto-orc/src/main/java/io/prestosql/orc/StripeReader.java/StripeReader/createRowGroup
class StripeReader:    private static RowGroup createRowGroup(int groupId, int rowOffset, int rowCount, long minAverageRowBytes, Map<StreamId, ValueInputStream<?>> valueStreams, Map<StreamId, StreamCheckpoint> checkpoints)
    {
        ImmutableMap.Builder<StreamId, InputStreamSource<?>> builder = ImmutableMap.builder();
        for (Entry<StreamId, StreamCheckpoint> entry : checkpoints.entrySet()) {
            StreamId streamId = entry.getKey();
            StreamCheckpoint checkpoint = entry.getValue();

            // skip streams without data
            ValueInputStream<?> valueStream = valueStreams.get(streamId);
            if (valueStream == null) {
                continue;
            }

            builder.put(streamId, createCheckpointStreamSource(valueStream, checkpoint));
        }
        InputStreamSources rowGroupStreams = new InputStreamSources(builder.build());
        return new RowGroup(groupId, rowOffset, rowCount, minAverageRowBytes, rowGroupStreams);
    }