ES Field Collapsing 字段折叠使用详解
在 Elasticsearch 5.x 有一个字段折叠(Field Collapsing,#22337)的功能非常有意思,在这里分享一下,
字段折叠是一个很有历史的需求了,可以看这个 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,可以理解就是按特定字段进行合并去重,比如我们有一个菜谱搜索,我希望按菜谱的“菜系”字段进行折叠,即返回结果每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种菜系都有,有湘菜,有粤菜,有中餐,有西餐,别全是湘菜,就是这个意思,通过按特定字段折叠之后,来丰富搜索结果的多样性。
说到这里,有人肯定会想到,使用 term agg+ top hits agg 来实现啊,这种组合两种聚和的方式可以实现上面的功能,不过也有一些局限性,比如,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高速度);数据量大的情况下,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),所以 top hits 永远是精确的。
- 字段折叠只在 top hits 层执行,不需要每次都在完整的结果集上对为每个折叠主键计算实际的 doc values 值,只对 top hits 这小部分数据操作就可以,和 term agg 相比要节省很多内存。
- 因为只在 top hits 上进行折叠,所以相比组合聚合的方式,速度要快很多。
- 折叠 top docs 不需要使用全局序列(global ordinals)来转换 string,相比 agg 这也节省了很多内存。
- 分页成为可能,和常规搜索一样,具有相同的局限,先获取 from+size 的内容,再合并。
- search_after 和 scroll 暂未实现,不过具备可行性。
- 折叠只影响搜索结果,不影响聚合,搜索结果的 total 是所有的命中纪录数,去重的结果数未知(无法计算)。
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"
}
- 现在我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,我的天,最近上火不想吃辣,这个第一页的结果对我来说就是垃圾,如下:
{"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"}}]}
}
我们再看看,这次我想加个评分排序,大家都喜欢的是那些,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,还是有点不合适,结果如下:
{"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]}]}
}
现在我知道了,我要看看其他菜系,这家不是还有西餐、广东菜等各种菜系的么,来来,帮我每个菜系来一个菜看看,换 terms agg 先得到唯一的 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,没关系,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,不过总算是我们想要的结果了,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,可以做,有局限性,那看看新的字段折叠法如何做到呢,查询如下,加一个 collapse 参数,指定对那个字段去重就行了,这里当然对菜系“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,命中结果还是熟悉的那个味道(和查询结果长的一样嘛),如下:
{"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,现在返回了3条数据,from 改成3,新的查询如下:
{"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,去重之后本来就只有4条数据,上面的工作正常,每个菜系只有一个菜啊,那我不乐意了,帮我每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,新的查询构造如下:
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
}
查询结果如下,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。
转自:https://elasticsearch.cn/article/132
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