一、功能场景

在mysql数据库中查询数据时，我们可以采用dinstinct关键字去重。那么，在ES中如何实现查询结果去重呢？

---

二、原理分析

DISTINCT关键字去重的sql语句等价于对需要去重的字段进行GROUP BY。

以下2个sql都能实现对name,age字段查询结果的去重。

SELECT DISTINCT name,age FROM distinct_index

SELECT name,age FROM distinct_index GROUP BY name,age

三、方案汇总

根据上面的分析，总结出以下2种实现方案：
1、使用GROUP BY的SQL查询
2、使用Aggregation(聚合)查询
说明：
ES6.3之后的版本以及支持SQL查询

四、数据准备

## 删除索引
## DELETE distinct_index
## 新建索引
PUT distinct_index
{
  "mappings": {
    "properties": {
      "name": {
        "type": "keyword"
      },
      "age": {
        "type": "integer"
      }
    }
  }
}
## 添加数据
POST distinct_index/_bulk?refresh
{ "create": { } }
{ "name": "小天", "age": 25}
{ "create": { } }
{ "name": "小天", "age": 25}
{ "create": { } }
{ "name": "老万", "age": 35}
{ "create": { } }
{ "name": "老王", "age": 45}
{ "create": { } }
{ "name": "小明", "age": 15}
{ "create": { } }
{ "name": "小明", "age": 15}
{ "create": { } }
{ "name": "小红", "age": 12}
{ "create": { } }
{ "name": "乐乐", "age": 18}

五、方案实战

1、SQL查询方案

##可以通过format参数控制返回结果的格式，txt表示文本格式，看起来更直观点，默认为json格式。
POST _sql?format=txt
{
  "query": "SELECT name,age FROM distinct_index group by name,age"
}

查询结果：

和预期相符，查询结果达到去重的效果。

SQL语句转DSL：

POST /_sql/translate
{
  "query": "SELECT name,age FROM distinct_index group by name,age"
}

结果：

{
  "size" : 0,
  "_source" : false,
  "stored_fields" : "_none_",
  "aggregations" : {
    "groupby" : {
      "composite" : {
        "size" : 1000,
        "sources" : [
          {
            "f5b401c4" : {
              "terms" : {
                "field" : "name",
                "missing_bucket" : true,
                "order" : "asc"
              }
            }
          },
          {
            "f557e07b" : {
              "terms" : {
                "field" : "age",
                "missing_bucket" : true,
                "order" : "asc"
              }
            }
          }
        ]
      }
    }
  }
}

说明：
通过将sql语句转为dsl语句可以发现，sql语句中的group by查询底层原理是转化成了dsl中的aggregations聚合查询。

2、aggregations聚合查询

POST distinct_index/_search
{
	"from": 0,
	"size": 0,
	"aggregations": {
		"name": {
			"terms": {
				"field": "name",
				"size": 2147483647
			},
			"aggregations": {
				"age": {
					"terms": {
						"field": "age",
						"size": 2147483647
					}
				}
			}
		}
	}
}

查询结果：

{
  "took" : 3,
  "timed_out" : false,
  "_shards" : {
    "total" : 1,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 0
  },
  "hits" : {
    "total" : {
      "value" : 8,
      "relation" : "eq"
    },
    "max_score" : null,
    "hits" : [ ]
  },
  "aggregations" : {
    "name" : {
      "doc_count_error_upper_bound" : 0,
      "sum_other_doc_count" : 0,
      "buckets" : [
        {
          "key" : "小天",
          "doc_count" : 2,
          "age" : {
            "doc_count_error_upper_bound" : 0,
            "sum_other_doc_count" : 0,
            "buckets" : [
              {
                "key" : 25,
                "doc_count" : 2
              }
            ]
          }
        },
        {
          "key" : "小明",
          "doc_count" : 2,
          "age" : {
            "doc_count_error_upper_bound" : 0,
            "sum_other_doc_count" : 0,
            "buckets" : [
              {
                "key" : 15,
                "doc_count" : 2
              }
            ]
          }
        },
        {
          "key" : "乐乐",
          "doc_count" : 1,
          "age" : {
            "doc_count_error_upper_bound" : 0,
            "sum_other_doc_count" : 0,
            "buckets" : [
              {
                "key" : 18,
                "doc_count" : 1
              }
            ]
          }
        },
        {
          "key" : "小红",
          "doc_count" : 1,
          "age" : {
            "doc_count_error_upper_bound" : 0,
            "sum_other_doc_count" : 0,
            "buckets" : [
              {
                "key" : 12,
                "doc_count" : 1
              }
            ]
          }
        },
        {
          "key" : "老万",
          "doc_count" : 1,
          "age" : {
            "doc_count_error_upper_bound" : 0,
            "sum_other_doc_count" : 0,
            "buckets" : [
              {
                "key" : 35,
                "doc_count" : 1
              }
            ]
          }
        },
        {
          "key" : "老王",
          "doc_count" : 1,
          "age" : {
            "doc_count_error_upper_bound" : 0,
            "sum_other_doc_count" : 0,
            "buckets" : [
              {
                "key" : 45,
                "doc_count" : 1
              }
            ]
          }
        }
      ]
    }
  }
}

六、异常记录

1、ES中的sql查询不支持DISTINCT关键字

POST _sql?format=txt
{
  "query": "SELECT DISTINCT name,age FROM distinct_index"
}

报错：

{
  "error" : {
    "root_cause" : [
      {
        "type" : "verification_exception",
        "reason" : "Found 1 problem\nline 1:8: SELECT DISTINCT is not yet supported"
      }
    ],
    "type" : "verification_exception",
    "reason" : "Found 1 problem\nline 1:8: SELECT DISTINCT is not yet supported"
  },
  "status" : 400
}

2、ES的sql查询中表名中不能有中划线，比如my-index-000001

PUT my-index-000001
{
  "mappings": {
    "properties": {
      "name": {
        "type": "keyword"
      },
      "age": {
        "type": "integer"
      }
    }
  }
}

POST _sql?format=txt
{
  "query": "SELECT name FROM my-index-000001"
}

报错：

{
  "error" : {
    "root_cause" : [
      {
        "type" : "parsing_exception",
        "reason" : "line 1:20: extraneous input '-' expecting {<EOF>, ',', 'ANALYZE', 'ANALYZED', 'AS', 'CATALOGS', 
        ……

遇到这种情况，最简单方法是给索引添加别名。

POST /_aliases
{
  "actions" : [
    { "add" : { "index" : "my-index-000001", "alias" : "my_index" } }
  ]
}
POST _sql?format=txt
{
  "query": "SELECT name FROM my_index"
}

---

总结

本文主要介绍了ES中如何实现类似dinstinct的数据去重功能。
1、首先通过通过dinstinct和group by的等价sql语句，说明可以通过分组函数实现数据去重。
2、分别介绍了ES中通过sql语句查询和aggregations聚合查询实现对查询结果的去重。
3、ES中的sql查询不支持DISTINCT关键字
4、ES中的sql查询中表名不能包含中划线，比如my-index-000001