hierarchical faceting with Elasticsearch

Question

I'm using elasticsearch and need to implement facet search for hierarchical object as follow:

• category 1 (10)
  • subcategory 1 (4)
  • subcategory 2 (6)
• category 2 (X)
  • ...

So I need to get facets for two related objects. Documentation says that it's possible to get such kind of facets for numeric value, but I need it for strings http://www.elasticsearch.org/guide/en/elasticsearch/reference/current/search-facets-terms-stats-facet.html

Here is another interesting topic, unfortunately it's old: http://elasticsearch-users.115913.n3.nabble.com/Pivot-facets-td2981519.html

Does it possible with elastic search? If so, how can I do that?

Answer 1

Currently, elasticsearch does not support hierarchical facetting out-of-the-box. But the upcoming 1.0 release features a new aggregations module, that can be used to get these kind of facets (which are more like pivot-facets rather than hierarchical facets). Version 1.0 is currently in beta, you can download the second beta and test out aggregatins by yourself. Your example might look like

curl -XPOST 'localhost:9200/_search?pretty' -d '
{
   "aggregations": {
      "main category": {
         "terms": {
            "field": "cat_1",
            "order": {"_term": "asc"}
         },
         "aggregations": {
            "sub category": {
               "terms": {
                  "field": "cat_2",
                  "order": {"_term": "asc"}
               }
            }
         }
      }
   }
}'

The idea is, to have a different field for each level of facetting and bucket your facets based on the terms of the first level (cat_1). These aggregations then would have sub-buckets, based on the terms of the second level (cat_2). The result may look like

{
  "aggregations" : {
    "main category" : {
      "buckets" : [ {
        "key" : "category 1",
        "doc_count" : 10,
        "sub category" : {
          "buckets" : [ {
            "key" : "subcategory 1",
            "doc_count" : 4
          }, {
            "key" : "subcategory 2",
            "doc_count" : 6
          } ]
        }
      }, {
        "key" : "category 2",
        "doc_count" : 7,
        "sub category" : {
          "buckets" : [ {
            "key" : "subcategory 1",
            "doc_count" : 3
          }, {
            "key" : "subcategory 2",
            "doc_count" : 4
          } ]
        }
      } ]
    }
  }
}