1、演算法介紹

relevance score演算法，簡單來說，就是計算出，一個索引中的文字，與搜尋文字，他們之間的關聯匹配程度

Elasticsearch使用的是 term frequency / inverse document frequency演算法，簡稱為TF/IDF演算法

Term frequency：搜尋文字中的各個詞條在field文字中出現了多少次，出現次數越多，就越相關
搜尋請求：hello world
doc1：hello you, and world is very good
doc2：hello, how are you

Inverse document frequency：搜尋文字中的各個詞條在整個索引的所有文件中出現了多少次，出現的次數越多，就越不相關
搜尋請求：hello world
doc1：hello, today is very good
doc2：hi world, how are you
比如說，在index中有1萬條document，hello這個單詞在所有的document中，一共出現了1000次；world這個單詞在所有的document中，一共出現了100次
doc2更相關

Field-length norm：field長度，field越長，相關度越弱
搜尋請求：hello world
doc1：{ “title”: “hello article”, “content”: “babaaba 1萬個單詞” }
doc2：{ “title”: “my article”, “content”: “blablabala 1萬個單詞，hi world” }
hello world在整個index中出現的次數是一樣多的
doc1更相關，title field更短

2、_score是如何被計算出來的

可以使用關鍵字具體檢視

{
  "took": 4,
  "timed_out": false,
  "_shards": {
    "total": 5,
    "successful": 5,
    "failed": 0
  },
  "hits": {
    "total": 3,
    "max_score": 0.51623213,
    "hits": [
      {
        "_shard": "[website][3]",
        "_node": "iYthfUSYTs-1R27gsnmCpA",
        "_index": "website",
        "_type": "article",
        "_id": "1",
        "_score": 0.51623213,
        "_source": {
          "title": "one article",
          "content": "this is my one article",
          "post_date": "2017-01-01",
          "author_id": 111
        },
        "_explanation": {
          "value": 0.51623213,
          "description": "sum of:",
          "details": [
            {
              "value": 0.51623213,
              "description": "sum of:",
              "details": [
                {
                  "value": 0.25811607,
                  "description": "weight(title:one in 0) [PerFieldSimilarity], result of:",
                  "details": [
                    {
                      "value": 0.25811607,
                      "description": "score(doc=0,freq=1.0 = termFreq=1.0\n), product of:",
                      "details": [
                        {
                          "value": 0.2876821,
                          "description": "idf, computed as log(1 + (docCount - docFreq + 0.5) / (docFreq + 0.5)) from:",
                          "details": [
                            {
                              "value": 1,
                              "description": "docFreq",
                              "details": []
                            },
                            {
                              "value": 1,
                              "description": "docCount",
                              "details": []
                            }
                          ]
                        },
                        {
                          "value": 0.89722675,
                          "description": "tfNorm, computed as (freq * (k1 + 1)) / (freq + k1 * (1 - b + b * fieldLength / avgFieldLength)) from:",
                          "details": [
                            {
                              "value": 1,
                              "description": "termFreq=1.0",
                              "details": []
                            },
                            {
                              "value": 1.2,
                              "description": "parameter k1",
                              "details": []
                            },
                            {
                              "value": 0.75,
                              "description": "parameter b",
                              "details": []
                            },
                            {
                              "value": 2,
                              "description": "avgFieldLength",
                              "details": []
                            },
                            {
                              "value": 2.56,
                              "description": "fieldLength",
                              "details": []
                            }
                          ]
                        }
                      ]
                    }
                  ]
                },
                {
                  "value": 0.25811607,
                  "description": "weight(title:article in 0) [PerFieldSimilarity], result of:",
                  "details": [
                    {
                      "value": 0.25811607,
                      "description": "score(doc=0,freq=1.0 = termFreq=1.0\n), product of:",
                      "details": [
                        {
                          "value": 0.2876821,
                          "description": "idf, computed as log(1 + (docCount - docFreq + 0.5) / (docFreq + 0.5)) from:",
                          "details": [
                            {
                              "value": 1,
                              "description": "docFreq",
                              "details": []
                            },
                            {
                              "value": 1,
                              "description": "docCount",
                              "details": []
                            }
                          ]
                        },
                        {
                          "value": 0.89722675,
                          "description": "tfNorm, computed as (freq * (k1 + 1)) / (freq + k1 * (1 - b + b * fieldLength / avgFieldLength)) from:",
                          "details": [
                            {
                              "value": 1,
                              "description": "termFreq=1.0",
                              "details": []
                            },
                            {
                              "value": 1.2,
                              "description": "parameter k1",
                              "details": []
                            },
                            {
                              "value": 0.75,
                              "description": "parameter b",
                              "details": []
                            },
                            {
                              "value": 2,
                              "description": "avgFieldLength",
                              "details": []
                            },
                            {
                              "value": 2.56,
                              "description": "fieldLength",
                              "details": []
                            }
                          ]
                        }
                      ]
                    }
                  ]
                }
              ]
            },
            {
              "value": 0,
              "description": "match on required clause, product of:",
              "details": [
                {
                  "value": 0,
                  "description": "# clause",
                  "details": []
                },
                {
                  "value": 1,
                  "description": "*:*, product of:",
                  "details": [
                    {
                      "value": 1,
                      "description": "boost",
                      "details": []
                    },
                    {
                      "value": 1,
                      "description": "queryNorm",
                      "details": []
                    }
                  ]
                }
              ]
            }
          ]
        }
      },
      {
        "_shard": "[website][2]",
        "_node": "iYthfUSYTs-1R27gsnmCpA",
        "_index": "website",
        "_type": "article",
        "_id": "2",
        "_score": 0.25811607,
        "_source": {
          "title": "two article",
          "content": "this is my two article",
          "post_date": "2017-02-02",
          "author_id": 222
        },
        "_explanation": {
          "value": 0.25811607,
          "description": "sum of:",
          "details": [
            {
              "value": 0.25811607,
              "description": "sum of:",
              "details": [
                {
                  "value": 0.25811607,
                  "description": "weight(title:article in 0) [PerFieldSimilarity], result of:",
                  "details": [
                    {
                      "value": 0.25811607,
                      "description": "score(doc=0,freq=1.0 = termFreq=1.0\n), product of:",
                      "details": [
                        {
                          "value": 0.2876821,
                          "description": "idf, computed as log(1 + (docCount - docFreq + 0.5) / (docFreq + 0.5)) from:",
                          "details": [
                            {
                              "value": 1,
                              "description": "docFreq",
                              "details": []
                            },
                            {
                              "value": 1,
                              "description": "docCount",
                              "details": []
                            }
                          ]
                        },
                        {
                          "value": 0.89722675,
                          "description": "tfNorm, computed as (freq * (k1 + 1)) / (freq + k1 * (1 - b + b * fieldLength / avgFieldLength)) from:",
                          "details": [
                            {
                              "value": 1,
                              "description": "termFreq=1.0",
                              "details": []
                            },
                            {
                              "value": 1.2,
                              "description": "parameter k1",
                              "details": []
                            },
                            {
                              "value": 0.75,
                              "description": "parameter b",
                              "details": []
                            },
                            {
                              "value": 2,
                              "description": "avgFieldLength",
                              "details": []
                            },
                            {
                              "value": 2.56,
                              "description": "fieldLength",
                              "details": []
                            }
                          ]
                        }
                      ]
                    }
                  ]
                }
              ]
            },
            {
              "value": 0,
              "description": "match on required clause, product of:",
              "details": [
                {
                  "value": 0,
                  "description": "# clause",
                  "details": []
                },
                {
                  "value": 1,
                  "description": "*:*, product of:",
                  "details": [
                    {
                      "value": 1,
                      "description": "boost",
                      "details": []
                    },
                    {
                      "value": 1,
                      "description": "queryNorm",
                      "details": []
                    }
                  ]
                }
              ]
            }
          ]
        }
      },
      {
        "_shard": "[website][4]",
        "_node": "iYthfUSYTs-1R27gsnmCpA",
        "_index": "website",
        "_type": "article",
        "_id": "3",
        "_score": 0.25811607,
        "_source": {
          "title": "three article",
          "content": "this is my three article",
          "post_date": "2017-03-03",
          "author_id": 333
        },
        "_explanation": {
          "value": 0.25811607,
          "description": "sum of:",
          "details": [
            {
              "value": 0.25811607,
              "description": "sum of:",
              "details": [
                {
                  "value": 0.25811607,
                  "description": "weight(title:article in 0) [PerFieldSimilarity], result of:",
                  "details": [
                    {
                      "value": 0.25811607,
                      "description": "score(doc=0,freq=1.0 = termFreq=1.0\n), product of:",
                      "details": [
                        {
                          "value": 0.2876821,
                          "description": "idf, computed as log(1 + (docCount - docFreq + 0.5) / (docFreq + 0.5)) from:",
                          "details": [
                            {
                              "value": 1,
                              "description": "docFreq",
                              "details": []
                            },
                            {
                              "value": 1,
                              "description": "docCount",
                              "details": []
                            }
                          ]
                        },
                        {
                          "value": 0.89722675,
                          "description": "tfNorm, computed as (freq * (k1 + 1)) / (freq + k1 * (1 - b + b * fieldLength / avgFieldLength)) from:",
                          "details": [
                            {
                              "value": 1,
                              "description": "termFreq=1.0",
                              "details": []
                            },
                            {
                              "value": 1.2,
                              "description": "parameter k1",
                              "details": []
                            },
                            {
                              "value": 0.75,
                              "description": "parameter b",
                              "details": []
                            },
                            {
                              "value": 2,
                              "description": "avgFieldLength",
                              "details": []
                            },
                            {
                              "value": 2.56,
                              "description": "fieldLength",
                              "details": []
                            }
                          ]
                        }
                      ]
                    }
                  ]
                }
              ]
            },
            {
              "value": 0,
              "description": "match on required clause, product of:",
              "details": [
                {
                  "value": 0,
                  "description": "# clause",
                  "details": []
                },
                {
                  "value": 1,
                  "description": "*:*, product of:",
                  "details": [
                    {
                      "value": 1,
                      "description": "boost",
                      "details": []
                    },
                    {
                      "value": 1,
                      "description": "queryNorm",
                      "details": []
                    }
                  ]
                }
              ]
            }
          ]
        }
      }
    ]
  }
}
 

3、分析一個document是如何被匹配上的

{
  "_index": "website",
  "_type": "article",
  "_id": "1",
  "matched": true,
  "explanation": {
    "value": 0.51623213,
    "description": "sum of:",
    "details": [
      {
        "value": 0.51623213,
        "description": "sum of:",
        "details": [
          {
            "value": 0.25811607,
            "description": "weight(title:one in 0) [PerFieldSimilarity], result of:",
            "details": [
              {
                "value": 0.25811607,
                "description": "score(doc=0,freq=1.0 = termFreq=1.0\n), product of:",
                "details": [
                  {
                    "value": 0.2876821,
                    "description": "idf, computed as log(1 + (docCount - docFreq + 0.5) / (docFreq + 0.5)) from:",
                    "details": [
                      {
                        "value": 1,
                        "description": "docFreq",
                        "details": []
                      },
                      {
                        "value": 1,
                        "description": "docCount",
                        "details": []
                      }
                    ]
                  },
                  {
                    "value": 0.89722675,
                    "description": "tfNorm, computed as (freq * (k1 + 1)) / (freq + k1 * (1 - b + b * fieldLength / avgFieldLength)) from:",
                    "details": [
                      {
                        "value": 1,
                        "description": "termFreq=1.0",
                        "details": []
                      },
                      {
                        "value": 1.2,
                        "description": "parameter k1",
                        "details": []
                      },
                      {
                        "value": 0.75,
                        "description": "parameter b",
                        "details": []
                      },
                      {
                        "value": 2,
                        "description": "avgFieldLength",
                        "details": []
                      },
                      {
                        "value": 2.56,
                        "description": "fieldLength",
                        "details": []
                      }
                    ]
                  }
                ]
              }
            ]
          },
          {
            "value": 0.25811607,
            "description": "weight(title:article in 0) [PerFieldSimilarity], result of:",
            "details": [
              {
                "value": 0.25811607,
                "description": "score(doc=0,freq=1.0 = termFreq=1.0\n), product of:",
                "details": [
                  {
                    "value": 0.2876821,
                    "description": "idf, computed as log(1 + (docCount - docFreq + 0.5) / (docFreq + 0.5)) from:",
                    "details": [
                      {
                        "value": 1,
                        "description": "docFreq",
                        "details": []
                      },
                      {
                        "value": 1,
                        "description": "docCount",
                        "details": []
                      }
                    ]
                  },
                  {
                    "value": 0.89722675,
                    "description": "tfNorm, computed as (freq * (k1 + 1)) / (freq + k1 * (1 - b + b * fieldLength / avgFieldLength)) from:",
                    "details": [
                      {
                        "value": 1,
                        "description": "termFreq=1.0",
                        "details": []
                      },
                      {
                        "value": 1.2,
                        "description": "parameter k1",
                        "details": []
                      },
                      {
                        "value": 0.75,
                        "description": "parameter b",
                        "details": []
                      },
                      {
                        "value": 2,
                        "description": "avgFieldLength",
                        "details": []
                      },
                      {
                        "value": 2.56,
                        "description": "fieldLength",
                        "details": []
                      }
                    ]
                  }
                ]
              }
            ]
          }
        ]
      },
      {
        "value": 0,
        "description": "match on required clause, product of:",
        "details": [
          {
            "value": 0,
            "description": "# clause",
            "details": []
          },
          {
            "value": 1,
            "description": "*:*, product of:",
            "details": [
              {
                "value": 1,
                "description": "boost",
                "details": []
              },
              {
                "value": 1,
                "description": "queryNorm",
                "details": []
              }
            ]
          }
        ]
      }
    ]
  }
}