Elasticsearch如何实现SQL语句中 Group By 和 Limit 的功能

有 SQL 背景的同学在学习 Elasticsearch 时，面对一个查询需求，不由自主地会先思考如何用 SQL 来实现，然后再去想 Elasticsearch 的 Query DSL 如何实现。那么本篇就给大家讲一条常见的 SQL 语句如何用 Elasticsearch 的查询语言实现。

1. SQL语句

假设我们有一个汽车的数据集，每个汽车都有车型、颜色等字段，我希望获取颜色种类大于1个的前2车型。假设汽车的数据模型如下：

{
    "model":"modelA",
    "color":"red"
}

假设我们有一个 cars 表，通过如下语句创建测试数据。

INSERT INTO cars (model,color) VALUES ('A','red'); 
INSERT INTO cars (model,color) VALUES ('A','white'); 
INSERT INTO cars (model,color) VALUES ('A','black'); 
INSERT INTO cars (model,color) VALUES ('A','yellow'); 
INSERT INTO cars (model,color) VALUES ('B','red'); 
INSERT INTO cars (model,color) VALUES ('B','white'); 
INSERT INTO cars (model,color) VALUES ('C','black'); 
INSERT INTO cars (model,color) VALUES ('C','red'); 
INSERT INTO cars (model,color) VALUES ('C','white'); 
INSERT INTO cars (model,color) VALUES ('C','yellow'); 
INSERT INTO cars (model,color) VALUES ('C','blue'); 
INSERT INTO cars (model,color) VALUES ('D','red');
INSERT INTO cars (model,color) VALUES ('A','red'); 

那么实现我们需求的 SQL 语句也比较简单，实现如下：

SELECT model,COUNT(DISTINCT color) color_count FROM cars GROUP BY model HAVING color_count > 1 ORDER BY color_count desc LIMIT 2;

这条查询语句中 Group By 是按照 model 做分组， Having color_count>1 限定了车型颜色种类大于1，ORDER BY color_count desc 限定结果按照颜色种类倒序排列，而 LIMIT 2 限定只返回前3条数据。

那么在 Elasticsearch 中如何实现这个需求呢？

2. 在 Elasticsearch 模拟测试数据

首先我们需要先在 elasticsearch 中插入测试的数据，这里我们使用 bulk 接口 ，如下所示：

POST _bulk
{"index":{"_index":"cars","_type":"doc","_id":"1"}}
{"model":"A","color":"red"}
{"index":{"_index":"cars","_type":"doc","_id":"2"}}
{"model":"A","color":"white"}
{"index":{"_index":"cars","_type":"doc","_id":"3"}}
{"model":"A","color":"black"}
{"index":{"_index":"cars","_type":"doc","_id":"4"}}
{"model":"A","color":"yellow"}
{"index":{"_index":"cars","_type":"doc","_id":"5"}}
{"model":"B","color":"red"}
{"index":{"_index":"cars","_type":"doc","_id":"6"}}
{"model":"B","color":"white"}
{"index":{"_index":"cars","_type":"doc","_id":"7"}}
{"model":"C","color":"black"}
{"index":{"_index":"cars","_type":"doc","_id":"8"}}
{"model":"C","color":"red"}
{"index":{"_index":"cars","_type":"doc","_id":"9"}}
{"model":"C","color":"white"}
{"index":{"_index":"cars","_type":"doc","_id":"10"}}
{"model":"C","color":"yellow"}
{"index":{"_index":"cars","_type":"doc","_id":"11"}}
{"model":"C","color":"blue"}
{"index":{"_index":"cars","_type":"doc","_id":"12"}}
{"model":"D","color":"red"}
{"index":{"_index":"cars","_type":"doc","_id":"13"}}
{"model":"A","color":"red"}

其中 index 为 cars，type 为 doc，所有数据与mysql 数据保持一致。大家可以在 Kibana 的 Dev Tools 中执行上面的命令，然后执行下面的查询语句验证数据是否已经成功存入。

GET cars/_search

3. Group By VS Terms/Metric Aggregation

SQL 中 Group By 语句在 Elasticsearch 中对应的是 Terms Aggregation，即分桶聚合，对应 Group By color 的语句如下所示：

GET cars/_search
{
  "size":0,
  "aggs":{
    "models":{
      "terms":{
        "field":"model.keyword"
      }
    }
  }
}

结果如下：

{
  "took": 161,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": 13,
    "max_score": 0,
    "hits": []
  },
  "aggregations": {
    "models": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "key": "A",
          "doc_count": 5
        },
        {
          "key": "C",
          "doc_count": 5
        },
        {
          "key": "B",
          "doc_count": 2
        },
        {
          "key": "D",
          "doc_count": 1
        }
      ]
    }
  }
}

我们看 aggregations 这个 key 下面的即为返回结果。

SQL 语句中还有一项是 COUNT(DISTINCT color) color_count 用于计算每个 model 的颜色数，在 Elasticsearch 中我们需要使用一个指标类聚合 Cardinality ，进行不同值计数。语句如下：

GET cars/_search
{
  "size": 0,
  "aggs": {
    "models": {
      "terms": {
        "field": "model.keyword"
      },
      "aggs": {
        "color_count": {
          "cardinality": {
            "field": "color.keyword"
          }
        }
      }
    }
  }
}

其返回结果如下：

{
  "took": 74,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": 13,
    "max_score": 0,
    "hits": []
  },
  "aggregations": {
    "models": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "key": "A",
          "doc_count": 5,
          "color_count": {
            "value": 4
          }
        },
        {
          "key": "C",
          "doc_count": 5,
          "color_count": {
            "value": 5
          }
        },
        {
          "key": "B",
          "doc_count": 2,
          "color_count": {
            "value": 2
          }
        },
        {
          "key": "D",
          "doc_count": 1,
          "color_count": {
            "value": 1
          }
        }
      ]
    }
  }
}

结果中 color_count 即为每个 model 的颜色数，但这里所有的模型都返回了，我们只想要颜色数大于1的模型，因此这里还要加一个过滤条件。

4. Having Condition VS Bucket Filter Aggregation

Having color_count > 1 在 Elasticsearch 中对应的是 Bucket Filter 聚合，语句如下所示：

GET cars/_search
{
  "size": 0,
  "aggs": {
    "models": {
      "terms": {
        "field": "model.keyword"
      },
      "aggs": {
        "color_count": {
          "cardinality": {
            "field": "color.keyword"
          }
        },
        "color_count_filter": {
          "bucket_selector": {
            "buckets_path": {
              "colorCount": "color_count"
            },
            "script": "params.colorCount>1"
          }
        }
      }
    }
  }
}

返回结果如下：

{
  "took": 39,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": 13,
    "max_score": 0,
    "hits": []
  },
  "aggregations": {
    "models": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "key": "A",
          "doc_count": 5,
          "color_count": {
            "value": 4
          }
        },
        {
          "key": "C",
          "doc_count": 5,
          "color_count": {
            "value": 5
          }
        },
        {
          "key": "B",
          "doc_count": 2,
          "color_count": {
            "value": 2
          }
        }
      ]
    }
  }
}

此时返回结果只包含颜色数大于1的模型，但大家会发现颜色数多的 C 不是在第一个位置，我们还需要做排序处理。

5. Order By Limit VS Bucket Sort Aggregation

ORDER BY color_count desc LIMIT 3 在 Elasticsearch 中可以使用 Bucket Sort 聚合实现，语句如下所示：

GET cars/_search
{
  "size": 0,
  "aggs": {
    "models": {
      "terms": {
        "field": "model.keyword"
      },
      "aggs": {
        "color_count": {
          "cardinality": {
            "field": "color.keyword"
          }
        },
        "color_count_filter": {
          "bucket_selector": {
            "buckets_path": {
              "colorCount": "color_count"
            },
            "script": "params.colorCount>1"
          }
        },
        "color_count_sort": {
          "bucket_sort": {
            "sort": {
              "color_count": "desc"
            },
            "size": 2
          }
        }
      }
    }
  }
}

返回结果如下：

{
  "took": 32,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "skipped": 0,
    "failed": 0
  },
  "hits": {
    "total": 13,
    "max_score": 0,
    "hits": []
  },
  "aggregations": {
    "models": {
      "doc_count_error_upper_bound": 0,
      "sum_other_doc_count": 0,
      "buckets": [
        {
          "key": "C",
          "doc_count": 5,
          "color_count": {
            "value": 5
          }
        },
        {
          "key": "A",
          "doc_count": 5,
          "color_count": {
            "value": 4
          }
        }
      ]
    }
  }
}

至此我们便将 SQL 语句实现的功能用 Elasticsearch 查询语句实现了。对比 SQL 语句与 Elasticsearch 的查询语句，大家会发现后者复杂了很多，但并非无章可循，随着大家对常见语法越来越熟悉，相信一定会越写越得心应手！