1、什么是聚合?

聚合是对文档数据的统计、分析、计算

聚合的常见种类有哪些?
        桶(Bucket)聚合:用来对文档做分组
                TermAggregation:按照文档字段值分组

                Date Histogram:按照日期阶梯分组，例如一周为一组，或者一月为一组

        度量(Metric)聚合:用以计算一些值，比如:最大值、最小值、平均值等
                AVg:求平均值
                Max:求最大值
                Min:求最小值
                Stats:同时求max、min、avg、sum等

        Pipeline:基于其它聚合结果再做聚合

参与聚合的字段类型必须是:
        keyword
        数值
        日期
        布尔

2、DSL实现Bucket聚合

        

        可以新增在aggs内order字段以自定义排序规则，也可以在最外层新增query来限定聚合范围

aggs代表聚合，与query同级，此时query的作用是?

        限定聚合的的文档范围

聚合必须的三要素:
        聚合名称
        聚合类型
        聚合字段

聚合可配置属性有:
        size:指定聚合结果数量
        order:指定聚合结果排序方式
        field:指定聚合字段

3、DSL实现Metrics聚合

        在聚合类型同级多加一层聚合即可，如图

4、RestClient实现聚合

@Test
    void testAggregation() throws IOException {
        //1、准备Request
        SearchRequest request = new SearchRequest("hotel");
        //2、准备dsl
        request.source().size(0);
        request.source().aggregation(AggregationBuilders
                .terms("brandAgg")
                .field("brand")
                .size(10)
        );
        //3、发出请求
        SearchResponse response = client.search(request, RequestOptions.DEFAULT);
        //4、解析结果
        Aggregations aggregations = response.getAggregations();
        Terms brandTerms = aggregations.get("brandAgg");
        List<? extends Terms.Bucket> buckets = brandTerms.getBuckets();
        for (Terms.Bucket bucket : buckets) {
            String keyAsString = bucket.getKeyAsString();
            System.out.println(keyAsString);
        }
    }

        封装请求信息逐层封装，解析相应结果逐层解析即可

        注意返回值类型与方法传参，ctrl+p可以提示方法所需形参类型

        多条件聚合只需增加aggregations，并在取出结果时逐一拆分即可

    @Override
    public Map<String, List<String>> filters() {
        try {
            SearchRequest request = new SearchRequest("hotel");
            Map<String, List<String>> result = new HashMap<>();
            buildAggregation(request);
            SearchResponse response = client.search(request, RequestOptions.DEFAULT);
            Aggregations aggregations = response.getAggregations();
            List<String> brandlist = getAggByName(aggregations,"brandAgg");
            result.put("品牌",brandlist);
            result.put("星级",getAggByName(aggregations,"starNameAgg"));
            result.put("城市",getAggByName(aggregations,"cityAgg"));
            return result;
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
    }

    private static List<String> getAggByName(Aggregations aggregations,String aggName) {
        Terms brandterms = aggregations.get(aggName);
        List<String> brandlist = new ArrayList<>();
        List<? extends Terms.Bucket> buckets = brandterms.getBuckets();
        for (Terms.Bucket bucket : buckets) {
            brandlist.add(bucket.getKeyAsString());
        }
        return brandlist;
    }

    private static void buildAggregation(SearchRequest request) {
        request.source().aggregation(AggregationBuilders
                .terms("brandAgg")
                .size(100)
                .field("brand"));
        request.source().aggregation(AggregationBuilders
                .terms("cityAgg")
                .size(100)
                .field("city"));
        request.source().aggregation(AggregationBuilders
                .terms("starNameAgg")
                .size(100)
                .field("starName"));
    }