protected ExplainResponse shardOperation(ExplainRequest request, int shardId)
      throws ElasticSearchException {
    IndexService indexService = indicesService.indexService(request.index());
    IndexShard indexShard = indexService.shardSafe(shardId);
    Term uidTerm =
        new Term(UidFieldMapper.NAME, Uid.createUidAsBytes(request.getType(), request.getId()));
    Engine.GetResult result = indexShard.get(new Engine.Get(false, uidTerm));
    if (!result.exists()) {
      return new ExplainResponse(false);
    }

    SearchContext context =
        new SearchContext(
            0,
            new ShardSearchRequest()
                .types(new String[] {request.getType()})
                .filteringAliases(request.getFilteringAlias()),
            null,
            result.searcher(),
            indexService,
            indexShard,
            scriptService);
    SearchContext.setCurrent(context);

    try {
      context.parsedQuery(parseQuery(request, indexService));
      context.preProcess();
      int topLevelDocId = result.docIdAndVersion().docId + result.docIdAndVersion().reader.docBase;
      Explanation explanation;
      if (context.rescore() != null) {
        RescoreSearchContext ctx = context.rescore();
        Rescorer rescorer = ctx.rescorer();
        explanation = rescorer.explain(topLevelDocId, context, ctx);
      } else {
        explanation = context.searcher().explain(context.query(), topLevelDocId);
      }
      if (request.getFields() != null) {
        if (request.getFields().length == 1 && "_source".equals(request.getFields()[0])) {
          request.setFields(null); // Load the _source field
        }
        // Advantage is that we're not opening a second searcher to retrieve the _source. Also
        // because we are working in the same searcher in engineGetResult we can be sure that a
        // doc isn't deleted between the initial get and this call.
        GetResult getResult =
            indexShard
                .getService()
                .get(result, request.getId(), request.getType(), request.getFields());
        return new ExplainResponse(true, explanation, getResult);
      } else {
        return new ExplainResponse(true, explanation);
      }
    } catch (IOException e) {
      throw new ElasticSearchException("Could not explain", e);
    } finally {
      context.release();
      SearchContext.removeCurrent();
    }
  }
  @Override
  protected ExplainResponse shardOperation(ExplainRequest request, ShardId shardId)
      throws ElasticsearchException {
    IndexService indexService = indicesService.indexServiceSafe(shardId.getIndex());
    IndexShard indexShard = indexService.shardSafe(shardId.id());
    Term uidTerm =
        new Term(UidFieldMapper.NAME, Uid.createUidAsBytes(request.type(), request.id()));
    Engine.GetResult result = indexShard.get(new Engine.Get(false, uidTerm));
    if (!result.exists()) {
      return new ExplainResponse(shardId.getIndex(), request.type(), request.id(), false);
    }

    SearchContext context =
        new DefaultSearchContext(
            0,
            new ShardSearchRequest(request)
                .types(new String[] {request.type()})
                .filteringAliases(request.filteringAlias())
                .nowInMillis(request.nowInMillis),
            null,
            result.searcher(),
            indexService,
            indexShard,
            scriptService,
            pageCacheRecycler,
            bigArrays,
            threadPool.estimatedTimeInMillisCounter());
    SearchContext.setCurrent(context);

    try {
      context.parsedQuery(indexService.queryParserService().parseQuery(request.source()));
      context.preProcess();
      int topLevelDocId = result.docIdAndVersion().docId + result.docIdAndVersion().context.docBase;
      Explanation explanation = context.searcher().explain(context.query(), topLevelDocId);
      for (RescoreSearchContext ctx : context.rescore()) {
        Rescorer rescorer = ctx.rescorer();
        explanation = rescorer.explain(topLevelDocId, context, ctx, explanation);
      }
      if (request.fields() != null
          || (request.fetchSourceContext() != null && request.fetchSourceContext().fetchSource())) {
        // Advantage is that we're not opening a second searcher to retrieve the _source. Also
        // because we are working in the same searcher in engineGetResult we can be sure that a
        // doc isn't deleted between the initial get and this call.
        GetResult getResult =
            indexShard
                .getService()
                .get(
                    result,
                    request.id(),
                    request.type(),
                    request.fields(),
                    request.fetchSourceContext(),
                    false);
        return new ExplainResponse(
            shardId.getIndex(), request.type(), request.id(), true, explanation, getResult);
      } else {
        return new ExplainResponse(
            shardId.getIndex(), request.type(), request.id(), true, explanation);
      }
    } catch (IOException e) {
      throw new ElasticsearchException("Could not explain", e);
    } finally {
      context.close();
      SearchContext.removeCurrent();
    }
  }