Main.java

import io.reactivex.Flowable;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import org.immutables.value.Value;

public class Main {

  @Value.Immutable
  interface Row {
    String id();
    List<Integer> values();
  }

  @Value.Immutable
  interface ResultSet {
    List<Row> rows();
  }

  public static class Document {
    public final String id;
    public final List<Row> rows;

    private Document(String id, List<Row> rows) {
      this.id = id;
      this.rows = rows;
    }
  }

  public static class Accumulator {
    public final String id;
    public final List<Row> rows;

    private Accumulator(String id) {
      this.id = id;
      this.rows = new ArrayList<>();
    }

    private Document toDoc() {
      return new Document(id, rows);
    }
  }

  public static void main(String args[]) {
    Flowable<ResultSet> results = Flowable.just(
        ImmutableResultSet.builder().rows(Arrays.asList(
            ImmutableRow.builder().id("a").values(Arrays.asList(1, 2)).build(),
            ImmutableRow.builder().id("a").values(Arrays.asList(7, 8)).build(),
            ImmutableRow.builder().id("b").values(Arrays.asList(1, 2)).build()
        )).build(),
        ImmutableResultSet.builder().rows(Arrays.asList(
            ImmutableRow.builder().id("a").values(Arrays.asList(3, 4)).build(),
            ImmutableRow.builder().id("b").values(Arrays.asList(3, 4)).build()
        )).build(),
        ImmutableResultSet.builder().rows(Arrays.asList(
            ImmutableRow.builder().id("a").values(Arrays.asList(5, 6)).build(),
            ImmutableRow.builder().id("b").values(Arrays.asList(5, 6)).build()
        )).build());

    results
        .collect(HashMap<String, Accumulator>::new, (map, rs) -> {
          for (Row row : rs.rows()) {
            map.computeIfAbsent(row.id(), Accumulator::new).rows.add(row);
          }
        })
        .map(HashMap::values)
        .flattenAsFlowable(a -> a)
        .map(Accumulator::toDoc)
        .subscribe(d -> {
          System.out.println(d.id);
          System.out.println(d.rows);
        }).dispose();
 }
}