【转载】Elasticsearch 5.x 字段折叠的使用，广度搜索
https://elasticsearch.cn/article/132html
备注，分组字段只能是 keyword或num类型，不能是text类型
在 Elasticsearch 5.x 有一个字段折叠（Field Collapsing， #22337）的功能很是有意思，在这里分享一下，
字段折叠是一个颇有历史的需求了，能够看这个 issue，编号 #256，最初是2010年7月提的issue，也是讨论最多的帖子之一（240+评论），熬了6年才支持的特性，你说牛不牛，哈哈。
目测该特性将于5.3发布，尝鲜地址： Elasticsearch-5.3.0-SNAPSHOT，文档地址： search-request-collapse。
So，什么是字段折叠，能够理解就是按特定字段进行合并去重，好比咱们有一个菜谱搜索，我但愿按菜谱的“菜系”字段进行折叠，即返回结果每一个菜系都返回一个结果，也就是按菜系去重，我搜索关键字“鱼”，要去返回的结果里面各类菜系都有，有湘菜，有粤菜，有中餐，有西餐，别全是湘菜，就是这个意思，经过按特定字段折叠以后，来丰富搜索结果的多样性。
说到这里，有人确定会想到，使用 term agg+ top hits agg 来实现啊，这种组合两种聚和的方式能够实现上面的功能，不过也有一些局限性，好比，不能分页， #4915；结果不够精确（top term+top hits，es 的聚合实现选择了牺牲精度来提升速度）；数据量大的状况下，聚合比较慢，影响搜索体验。
而新的的字段折叠的方式是怎么实现的的呢，有这些要点：
- 折叠+取 inner_hits 分两阶段执行（组合聚合的方式只有一个阶段），因此 top hits 永远是精确的。
- 字段折叠只在 top hits 层执行，不须要每次都在完整的结果集上对为每一个折叠主键计算实际的 doc values 值，只对 top hits 这小部分数据操做就能够，和 term agg 相比要节省不少内存。
- 由于只在 top hits 上进行折叠，因此相比组合聚合的方式，速度要快不少。
- 折叠 top docs 不须要使用全局序列（global ordinals）来转换 string，相比 agg 这也节省了不少内存。
- 分页成为可能，和常规搜索同样，具备相同的局限，先获取 from+size 的内容，再合并。
- search_after 和 scroll 暂未实现，不过具有可行性。
- 折叠只影响搜索结果，不影响聚合，搜索结果的 total 是全部的命中纪录数，去重的结果数未知（没法计算）。
下面来看看具体的例子，就知道怎么回事了，使用起来很简单。
- 先准备索引和数据，这里以菜谱为例，name：菜谱名，type 为菜系，rating 为用户的累积平均评分
DELETE recipes
PUT recipes
POST recipes/type/_mapping
{
"properties": {
"name":{
"type": "text"
},
"rating":{
"type": "float"
},"type":{
"type": "keyword"
}
}
}
POST recipes/type/
{
"name":"清蒸鱼头","rating":1,"type":"湘菜"
}
POST recipes/type/
{
"name":"剁椒鱼头","rating":2,"type":"湘菜"
}
POST recipes/type/
{
"name":"红烧鲫鱼","rating":3,"type":"湘菜"
}
POST recipes/type/
{
"name":"鲫鱼汤（辣）","rating":3,"type":"湘菜"
}
POST recipes/type/
{
"name":"鲫鱼汤（微辣）","rating":4,"type":"湘菜"
}
POST recipes/type/
{
"name":"鲫鱼汤（变态辣）","rating":5,"type":"湘菜"
}
POST recipes/type/
{
"name":"广式鲫鱼汤","rating":5,"type":"粤菜"
}
POST recipes/type/
{
"name":"鱼香肉丝","rating":2,"type":"川菜"
}
POST recipes/type/
{
"name":"奶油鲍鱼汤","rating":2,"type":"西菜"
}
- 如今咱们看看普通的查询效果是怎么样的，搜索关键字带“鱼”的菜，返回3条数据
POST recipes/type/_search
{
"query": {"match": {
"name": "鱼"
}},"size": 3
} 全是湘菜，个人天，最近上火不想吃辣，这个第一页的结果对我来讲就是垃圾，以下：
{
"took": 2,
"timed_out": false,
"_shards": {
"total": 5,
"successful": 5,
"failed": 0
},
"hits": {
"total": 9,
"max_score": 0.26742277,
"hits": [
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHYF_OA-dG63Txsd",
"_score": 0.26742277,
"_source": {
"name": "鲫鱼汤（变态辣）",
"rating": 5,
"type": "湘菜"
}
},
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHXO_OA-dG63Txsa",
"_score": 0.19100356,
"_source": {
"name": "红烧鲫鱼",
"rating": 3,
"type": "湘菜"
}
},
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHWy_OA-dG63TxsZ",
"_score": 0.19100356,
"_source": {
"name": "剁椒鱼头",
"rating": 2,
"type": "湘菜"
}
}
]
}
}咱们再看看，此次我想加个评分排序，你们都喜欢的是那些，看看有没有喜欢吃的，执行查询：
POST recipes/type/_search
{
"query": {"match": {
"name": "鱼"
}},"sort": [
{
"rating": {
"order": "desc"
}
}
],"size": 3
} 结果稍微好点了，不过3个里面2个是湘菜，仍是有点不合适，结果以下：
{
"took": 1,
"timed_out": false,
"_shards": {
"total": 5,
"successful": 5,
"failed": 0
},
"hits": {
"total": 9,
"max_score": null,
"hits": [
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHYF_OA-dG63Txsd",
"_score": null,
"_source": {
"name": "鲫鱼汤（变态辣）",
"rating": 5,
"type": "湘菜"
},
"sort": [
5
]
},
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHYW_OA-dG63Txse",
"_score": null,
"_source": {
"name": "广式鲫鱼汤",
"rating": 5,
"type": "粤菜"
},
"sort": [
5
]
},
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHX7_OA-dG63Txsc",
"_score": null,
"_source": {
"name": "鲫鱼汤（微辣）",
"rating": 4,
"type": "湘菜"
},
"sort": [
4
]
}
]
}
}
如今我知道了，我要看看其余菜系，这家不是还有西餐、广东菜等各类菜系的么，来来，帮我每一个菜系来一个菜看看，换 terms agg 先获得惟一的 term 的 bucket，再组合 top_hits agg，返回按评分排序的第一个 top hits，有点复杂，不要紧，看下面的查询就知道了：
GET recipes/type/_search
{
"query": {
"match": {
"name": "鱼"
}
},
"sort": [
{
"rating": {
"order": "desc"
}
}
],"aggs": {
"type": {
"terms": {
"field": "type",
"size": 10
},"aggs": {
"rated": {
"top_hits": {
"sort": [{
"rating": {"order": "desc"}
}],
"size": 1
}
}
}
}
},
"size": 0,
"from": 0
} 看下面的结果，虽然 json 结构有点复杂，不过总算是咱们想要的结果了，湘菜、粤菜、川菜、西菜都出来了，每样一个，不重样：
{
"took": 4,
"timed_out": false,
"_shards": {
"total": 5,
"successful": 5,
"failed": 0
},
"hits": {
"total": 9,
"max_score": 0,
"hits": []
},
"aggregations": {
"type": {
"doc_count_error_upper_bound": 0,
"sum_other_doc_count": 0,
"buckets": [
{
"key": "湘菜",
"doc_count": 6,
"rated": {
"hits": {
"total": 6,
"max_score": null,
"hits": [
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHYF_OA-dG63Txsd",
"_score": null,
"_source": {
"name": "鲫鱼汤（变态辣）",
"rating": 5,
"type": "湘菜"
},
"sort": [
5
]
}
]
}
}
},
{
"key": "川菜",
"doc_count": 1,
"rated": {
"hits": {
"total": 1,
"max_score": null,
"hits": [
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHYr_OA-dG63Txsf",
"_score": null,
"_source": {
"name": "鱼香肉丝",
"rating": 2,
"type": "川菜"
},
"sort": [
2
]
}
]
}
}
},
{
"key": "粤菜",
"doc_count": 1,
"rated": {
"hits": {
"total": 1,
"max_score": null,
"hits": [
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHYW_OA-dG63Txse",
"_score": null,
"_source": {
"name": "广式鲫鱼汤",
"rating": 5,
"type": "粤菜"
},
"sort": [
5
]
}
]
}
}
},
{
"key": "西菜",
"doc_count": 1,
"rated": {
"hits": {
"total": 1,
"max_score": null,
"hits": [
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHY3_OA-dG63Txsg",
"_score": null,
"_source": {
"name": "奶油鲍鱼汤",
"rating": 2,
"type": "西菜"
},
"sort": [
2
]
}
]
}
}
}
]
}
}
}
上面的实现方法，前面已经说了，能够作，有局限性，那看看新的字段折叠法如何作到呢，查询以下，加一个 collapse 参数，指定对那个字段去重就好了，这里固然对菜系“type”字段进行去重了：
GET recipes/type/_search
{
"query": {
"match": {
"name": "鱼"
}
},
"collapse": {
"field": "type"
},
"size": 3,
"from": 0
}结果很理想嘛，命中结果仍是熟悉的那个味道（和查询结果长的同样嘛），以下：
{
"took": 1,
"timed_out": false,
"_shards": {
"total": 5,
"successful": 5,
"failed": 0
},
"hits": {
"total": 9,
"max_score": null,
"hits": [
{
"_index": "recipes",
"_type": "type",
"_id": "AVoDNlRJ_OA-dG63TxpW",
"_score": 0.018980097,
"_source": {
"name": "鲫鱼汤（微辣）",
"rating": 4,
"type": "湘菜"
},
"fields": {
"type": [
"湘菜"
]
}
},
{
"_index": "recipes",
"_type": "type",
"_id": "AVoDNlRk_OA-dG63TxpZ",
"_score": 0.013813315,
"_source": {
"name": "鱼香肉丝",
"rating": 2,
"type": "川菜"
},
"fields": {
"type": [
"川菜"
]
}
},
{
"_index": "recipes",
"_type": "type",
"_id": "AVoDNlRb_OA-dG63TxpY",
"_score": 0.0125863515,
"_source": {
"name": "广式鲫鱼汤",
"rating": 5,
"type": "粤菜"
},
"fields": {
"type": [
"粤菜"
]
}
}
]
}
}我再试试翻页，把 from 改一下，如今返回了3条数据，from 改为3，新的查询以下：
{
"took": 1,
"timed_out": false,
"_shards": {
"total": 5,
"successful": 5,
"failed": 0
},
"hits": {
"total": 9,
"max_score": null,
"hits": [
{
"_index": "recipes",
"_type": "type",
"_id": "AVoDNlRw_OA-dG63Txpa",
"_score": 0.012546891,
"_source": {
"name": "奶油鲍鱼汤",
"rating": 2,
"type": "西菜"
},
"fields": {
"type": [
"西菜"
]
}
}
]
}
}上面的结果只有一条了，去重以后原本就只有4条数据，上面的工做正常，每一个菜系只有一个菜啊，那我不乐意了，帮我每一个菜系里面多返回几条，我好选菜啊，加上参数 inner_hits 来控制返回的条数，这里返回2条，按 rating 也排个序，新的查询构造以下：
GET recipes/type/_search
{
"query": {
"match": {
"name": "鱼"
}
},
"collapse": {
"field": "type",
"inner_hits": {
"name": "top_rated",
"size": 2,
"sort": [
{
"rating": "desc"
}
]
}
},
"sort": [
{
"rating": {
"order": "desc"
}
}
],
"size": 2,
"from": 0
}查询结果以下，完美：
{
"took": 1,
"timed_out": false,
"_shards": {
"total": 5,
"successful": 5,
"failed": 0
},
"hits": {
"total": 9,
"max_score": null,
"hits": [
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHYF_OA-dG63Txsd",
"_score": null,
"_source": {
"name": "鲫鱼汤（变态辣）",
"rating": 5,
"type": "湘菜"
},
"fields": {
"type": [
"湘菜"
]
},
"sort": [
5
],
"inner_hits": {
"top_rated": {
"hits": {
"total": 6,
"max_score": null,
"hits": [
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHYF_OA-dG63Txsd",
"_score": null,
"_source": {
"name": "鲫鱼汤（变态辣）",
"rating": 5,
"type": "湘菜"
},
"sort": [
5
]
},
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHX7_OA-dG63Txsc",
"_score": null,
"_source": {
"name": "鲫鱼汤（微辣）",
"rating": 4,
"type": "湘菜"
},
"sort": [
4
]
}
]
}
}
}
},
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHYW_OA-dG63Txse",
"_score": null,
"_source": {
"name": "广式鲫鱼汤",
"rating": 5,
"type": "粤菜"
},
"fields": {
"type": [
"粤菜"
]
},
"sort": [
5
],
"inner_hits": {
"top_rated": {
"hits": {
"total": 1,
"max_score": null,
"hits": [
{
"_index": "recipes",
"_type": "type",
"_id": "AVoESHYW_OA-dG63Txse",
"_score": null,
"_source": {
"name": "广式鲫鱼汤",
"rating": 5,
"type": "粤菜"
},
"sort": [
5
]
}
]
}
}
}
}
]
}
}好了，字段折叠介绍就到这里。
[尊重社区原创，转载请保留或注明出处]
本文地址：http://elasticsearch.cn/article/132
本文地址：http://elasticsearch.cn/article/132
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