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。