聚合提供了从数据中分组和提取统计信息的功能。考虑聚合的最简单方法是将其大致等同于SQL GROUP BY和SQL聚合函数。在Elasticsearch中,您可以执行返回匹配的搜索,同时在一个响应中返回与命中相关的聚合结果。这是非常强大和高效的,因为您可以运行查询和多个聚合,并一次性获取两个(或任一)操作的结果,避免使用简洁和简化的API进行网络传输。
首先,此示例按状态对所有帐户进行分组,然后返回按计数降序排序的前10个(默认)状态(也是默认值):
GET /bank/_search
{
"size": 0,
"aggs": {
"group_by_state": {
"terms": {
"field": "state.keyword"
}
}
}
}
在SQL中,上述聚合在概念上类似于:
SELECT state, COUNT(*) FROM bank GROUP BY state ORDER BY COUNT(*) DESC LIMIT 10;
响应(部分显示):
{
"took": 29,
"timed_out": false,
"_shards": {
"total": 5,
"successful": 5,
"skipped" : 0,
"failed": 0
},
"hits" : {
"total" : {
"value": 1000,
"relation": "eq"
},
"max_score" : null,
"hits" : [ ]
},
"aggregations" : {
"group_by_state" : {
"doc_count_error_upper_bound": 20,
"sum_other_doc_count": 770,
"buckets" : [ {
"key" : "ID",
"doc_count" : 27
}, {
"key" : "TX",
"doc_count" : 27
}, {
"key" : "AL",
"doc_count" : 25
}, {
"key" : "MD",
"doc_count" : 25
}, {
"key" : "TN",
"doc_count" : 23
}, {
"key" : "MA",
"doc_count" : 21
}, {
"key" : "NC",
"doc_count" : 21
}, {
"key" : "ND",
"doc_count" : 21
}, {
"key" : "ME",
"doc_count" : 20
}, {
"key" : "MO",
"doc_count" : 20
} ]
}
}
}
我们可以看到ID(爱达荷州)有27个账户,其次是TX(德克萨斯州)的27个账户,其次是AL(阿拉巴马州)的25个账户,依此类推。
在前一个聚合的基础上,此示例按州计算平均帐户余额(同样仅针对按降序排序的前10个州):
GET /bank/_search
{
"size": 0,
"aggs": {
"group_by_state": {
"terms": {
"field": "state.keyword"
},
"aggs": {
"average_balance": {
"avg": {
"field": "balance"
}
}
}
}
}
}
请注意我们如何嵌套group_by_state聚合中的average_balance聚合。这是所有聚合的常见模式。您可以在聚合中任意嵌套聚合,以从数据中提取所需的结果集。
在前一个聚合的基础上,我们现在按降序排列平均余额:
GET /bank/_search
{
"size": 0,
"aggs": {
"group_by_state": {
"terms": {
"field": "state.keyword",
"order": {
"average_balance": "desc"
}
},
"aggs": {
"average_balance": {
"avg": {
"field": "balance"
}
}
}
}
}
}
此示例演示了我们如何按年龄段(20-29岁,30-39岁和40-49岁)进行分组,然后按性别进行分组,最后得到每个年龄段的平均帐户余额:
GET /bank/_search
{
"size": 0,
"aggs": {
"group_by_age": {
"range": {
"field": "age",
"ranges": [
{
"from": 20,
"to": 30
},
{
"from": 30,
"to": 40
},
{
"from": 40,
"to": 50
}
]
},
"aggs": {
"group_by_gender": {
"terms": {
"field": "gender.keyword"
},
"aggs": {
"average_balance": {
"avg": {
"field": "balance"
}
}
}
}
}
}
}
}
还有许多其他聚合功能,我们在此不再详述。如果您想进行进一步的实验,聚合参考指南是一个很好的起点。