以球员信息为例，player索引的player type包含5个字段，姓名，年龄，薪水，球队，场上位置。
index的mapping为：

"mappings": {

	"player": {

		"properties": {

			"name": {

				"index": "not_analyzed",

				"type": "string"

			},

			"age": {

				"type": "integer"

			},

			"salary": {

				"type": "integer"

			},

			"team": {

				"index": "not_analyzed",

				"type": "string"

			},

			"position": {

				"index": "not_analyzed",

				"type": "string"

			}

		},

		"_all": {

			"enabled": false

		}

	}

}

索引中的全部数据：


 
首先，初始化Builder：

SearchRequestBuilder sbuilder = client.prepareSearch("player").setTypes("player");

接下来举例说明各种聚合操作的实现方法，因为在es的api中，多字段上的聚合操作需要用到子聚合(subAggregation)，初学者可能找不到方法（网上资料比较少，笔者在这个问题上折腾了两天，最后度了源码才彻底搞清楚T_T），后边会特意说明多字段聚合的实现方法。另外，聚合后的排序也会单独说明。

• group by/count

例如要计算每个球队的球员数，如果使用SQL语句，应表达如下：

select team, count(*) as player_count from player group by team;

ES的java api：

TermsBuilder teamAgg= AggregationBuilders.terms("player_count ").field("team");

sbuilder.addAggregation(teamAgg);

SearchResponse response = sbuilder.execute().actionGet();

 

• group by多个field

例如要计算每个球队每个位置的球员数，如果使用SQL语句，应表达如下：

select team, position, count(*) as pos_count from player group by team, position;

ES的java api：

TermsBuilder teamAgg= AggregationBuilders.terms("player_count ").field("team");

TermsBuilder posAgg= AggregationBuilders.terms("pos_count").field("position");

sbuilder.addAggregation(teamAgg.subAggregation(posAgg));

SearchResponse response = sbuilder.execute().actionGet();

 

• max/min/sum/avg

例如要计算每个球队年龄最大/最小/总/平均的球员年龄，如果使用SQL语句，应表达如下：

select team, max(age) as max_age from player group by team;

ES的java api：

TermsBuilder teamAgg= AggregationBuilders.terms("player_count ").field("team");

MaxBuilder ageAgg= AggregationBuilders.max("max_age").field("age");

sbuilder.addAggregation(teamAgg.subAggregation(ageAgg));

SearchResponse response = sbuilder.execute().actionGet();

• 对多个field求max/min/sum/avg

例如要计算每个球队球员的平均年龄，同时又要计算总年薪，如果使用SQL语句，应表达如下：

select team, avg(age)as avg_age, sum(salary) as total_salary from player group by team;

ES的java api：

TermsBuilder teamAgg= AggregationBuilders.terms("team");

AvgBuilder ageAgg= AggregationBuilders.avg("avg_age").field("age");

SumBuilder salaryAgg= AggregationBuilders.avg("total_salary ").field("salary");

sbuilder.addAggregation(teamAgg.subAggregation(ageAgg).subAggregation(salaryAgg));

SearchResponse response = sbuilder.execute().actionGet();

 

• 聚合后对Aggregation结果排序

例如要计算每个球队总年薪，并按照总年薪倒序排列，如果使用SQL语句，应表达如下：

select team, sum(salary) as total_salary from player group by team order by total_salary desc;

ES的java api：

TermsBuilder teamAgg= AggregationBuilders.terms("team").order(Order.aggregation("total_salary ", false);

SumBuilder salaryAgg= AggregationBuilders.avg("total_salary ").field("salary");

sbuilder.addAggregation(teamAgg.subAggregation(salaryAgg));

SearchResponse response = sbuilder.execute().actionGet();

需要特别注意的是，排序是在TermAggregation处执行的，Order.aggregation函数的第一个参数是aggregation的名字，第二个参数是boolean型，true表示正序，false表示倒序。 

• Aggregation结果条数的问题

默认情况下，search执行后，仅返回10条聚合结果，如果想反悔更多的结果，需要在构建TermsBuilder 时指定size：

TermsBuilder teamAgg= AggregationBuilders.terms("team").size(15);

 

• Aggregation结果的解析/输出

得到response后：

Map<String, Aggregation> aggMap = response.getAggregations().asMap();

StringTerms teamAgg= (StringTerms) aggMap.get("keywordAgg");

Iterator<Bucket> teamBucketIt = teamAgg.getBuckets().iterator();

while (teamBucketIt .hasNext()) {

Bucket buck = teamBucketIt .next();

//球队名

String team = buck.getKey();

//记录数

long count = buck.getDocCount();

//得到所有子聚合

Map subaggmap = buck.getAggregations().asMap();

//avg值获取方法

double avg_age= ((InternalAvg) subaggmap.get("avg_age")).getValue();

//sum值获取方法

double total_salary = ((InternalSum) subaggmap.get("total_salary")).getValue();

//...

//max/min以此类推

}

 

• 总结

综上，聚合操作主要是调用了SearchRequestBuilder的addAggregation方法，通常是传入一个TermsBuilder，子聚合调用TermsBuilder的subAggregation方法，可以添加的子聚合有TermsBuilder、SumBuilder、AvgBuilder、MaxBuilder、MinBuilder等常见的聚合操作。
 
从实现上来讲，SearchRequestBuilder在内部保持了一个私有的 SearchSourceBuilder实例， SearchSourceBuilder内部包含一个List<AbstractAggregationBuilder>，每次调用addAggregation时会调用 SearchSourceBuilder实例，添加一个AggregationBuilder。
同样的，TermsBuilder也在内部保持了一个List<AbstractAggregationBuilder>，调用addAggregation方法（来自父类addAggregation）时会添加一个AggregationBuilder。有兴趣的读者也可以阅读源码的实现。
 
如果有什么问题，欢迎一起讨论，如果文中有什么错误，欢迎批评指正。
 
注：文中使用的Elastic Search API版本为2.3.2