链接地址：
https://www.elastic.co/guide/en/elasticsearch/client/java-api/2.4/_bucket_aggregations.html
聚合主要有三大类：
key
and a document criterion. When the aggregation is executed, all the buckets criteria are evaluated on every document in the context and when a criterion matches, the document is considered to “fall in” the relevant bucket. By the end of the aggregation process, we’ll end up with a list of buckets – each one with a set of documents that “belong” to it.
数学
运算相关，比如：求取平均值
SearchResponse sr = node.client().prepareSearch()
.addAggregation(
AggregationBuilders.terms("by_country").field("country")
.subAggregation(AggregationBuilders.dateHistogram("by_year")
.field("dateOfBirth")
.interval((DateHistogramInterval.YEAR)
.subAggregation(AggregationBuilders.avg("avg_children").field("children"))
)
)
.execute().actionGet();-
Terms aggregation (bucket)
Date Histogram aggregation (bucket)
Average aggregation (metric)
metric Aggregation
几具体操作：
max、min、sun、avg
MetricsAggregationBuilder aggregation =
AggregationBuilders
.avg("agg")
.field("height");取结果
Avg agg = sr.getAggregations().get("agg");
double value = agg.getValue();stats包括上述的外加一个count
而extende stats 不仅包括stats还包括平法差一类的，更全面
还有百分之几，范围什么的，可以根据自己的需求再看看这一方面的介绍，metric Aggregation
Bucket aggregations
global 全局的，filter 、filters、missing 没有值或者是null的统计、terms 分类、range、date range、histogram 直方图、date histogram 等等，具体的运用看Bucket aggregation的介绍。