Implementation of breadth first in aggregation in ES

{
  "size": 0,
  "aggs": {
    "all_speaker": {
      "terms": {
        "field": "speaker",
        "size": 5
      },
      "aggs": {
        "avg_speech": {
          "avg": {
            "field": "speech_number"
          }
        }
      }
    }
  }
}

First step ： First of all, I will use org.elasticsearch.search.aggregations.LeafBucketCollector#collect Methods collect all all_speaker The data in the bucket

        return new LeafBucketCollector() {
            int lastDoc = 0;

            @Override
            public void collect(int doc, long bucket) throws IOException {
                if (context == null) {
                    context = ctx;
                    docDeltasBuilder = PackedLongValues.packedBuilder(PackedInts.DEFAULT);
                    bucketsBuilder = PackedLongValues.packedBuilder(PackedInts.DEFAULT);
                }
                docDeltasBuilder.add(doc - lastDoc);
                bucketsBuilder.add(bucket);
                lastDoc = doc;
                maxBucket = Math.max(maxBucket, bucket);
            }
        };

The second step ： Replay collected data , Only deal with the barrels that survived the last round  selectedBuckets

    /**
     * Replay the wrapped collector, but only on a selection of buckets.
     */
    @Override
    public void prepareSelectedBuckets(long... selectedBuckets) throws IOException {
        if (finished == false) {
            throw new IllegalStateException("Cannot replay yet, collection is not finished: postCollect() has not been called");
        }
        if (this.selectedBuckets != null) {
            throw new IllegalStateException("Already been replayed");
        }

        final LongHash hash = new LongHash(selectedBuckets.length, BigArrays.NON_RECYCLING_INSTANCE);
        for (long bucket : selectedBuckets) {
            hash.add(bucket);
        }
        this.selectedBuckets = hash;

        boolean needsScores = scoreMode().needsScores();
        Weight weight = null;
        if (needsScores) {
            Query query = isGlobal ? new MatchAllDocsQuery() : searchContext.query();
            weight = searchContext.searcher().createWeight(searchContext.searcher().rewrite(query), ScoreMode.COMPLETE, 1f);
        }

        for (Entry entry : entries) {
            assert entry.docDeltas.size() > 0 : "segment should have at least one document to replay, got 0";
            try {
                final LeafBucketCollector leafCollector = collector.getLeafCollector(entry.context);
                DocIdSetIterator scoreIt = null;
                if (needsScores) {
                    Scorer scorer = weight.scorer(entry.context);
                    // We don't need to check if the scorer is null
                    // since we are sure that there are documents to replay (entry.docDeltas it not empty).
                    scoreIt = scorer.iterator();
                    leafCollector.setScorer(scorer);
                }
                final PackedLongValues.Iterator docDeltaIterator = entry.docDeltas.iterator();
                final PackedLongValues.Iterator buckets = entry.buckets.iterator();
                int doc = 0;
                for (long i = 0, end = entry.docDeltas.size(); i < end; ++i) {
                    doc += docDeltaIterator.next();
                    final long bucket = buckets.next();
                    final long rebasedBucket = hash.find(bucket);
                    if (rebasedBucket != -1) {
                        if (needsScores) {
                            if (scoreIt.docID() < doc) {
                                scoreIt.advance(doc);
                            }
                            // aggregations should only be replayed on matching documents
                            assert scoreIt.docID() == doc;
                        }
                        leafCollector.collect(doc, rebasedBucket);
                    }
                }
            } catch (CollectionTerminatedException e) {
                // collection was terminated prematurely
                // continue with the following leaf
            }
        }
        collector.postCollection();
    }