Elasticsearch data aggregation with faceting on arrays

I think it is because filters are not analyzed. AT is stopword, so it is not indexed. You can check it using _analyze API: http://localhost:9200/test/_analyze?text=AT&field=countries.

You can check non stopword, for example CN, but this is lowercased http://localhost:9200/test/_analyze?text=CN&field=countries. So cn (which is in fact stored in index) doesn't match with CN in you facet filter.

You can try to modify your search to lowercased country abbreviation:

curl -XGET 'http://localhost:9200/test/company/_search?pretty=true' -d '
{"query"  : { "match_all" : {} },
 "facets" : {
    "foo" : {
      "filter" : {
        "term" : { "countries" : "cn" }
      }
    }
  }
}'

to get

"facets" : {
    "foo" : {
      "_type" : "filter",
      "count" : 15
    }
  }

But I think you should define mapping for countries to "index":"not_analyzed" to avoid this (both stopwords and lowercasing)

# Delete index
#
curl -XDELETE 'http://localhost:9200/test'

# Create with mapping
#
curl -XPUT 'http://localhost:9200/test/' -d '{
  "mappings": {
    "company": {
      "properties": {
        "countries": { "type": "string", "index" : "not_analyzed"  }
      }
    }
  }
}'


# Index documents
#
curl -XPUT 'http://localhost:9200/test/company/10' -d '{"countries" : ["CH", "CN"], "name" : "company10"}'
curl -XPUT 'http://localhost:9200/test/company/11' -d '{"countries" : ["AT", "CH", "CN", "DE", "EN", "FR"], "name" : "company11"}'
curl -XPUT 'http://localhost:9200/test/company/12' -d '{"countries" : ["AT", "CN", "EN", "FR"], "name" : "company12"}'
curl -XPUT 'http://localhost:9200/test/company/13' -d '{"countries" : ["CH", "CN", "HU"], "name" : "company13"}'
curl -XPUT 'http://localhost:9200/test/company/14' -d '{"countries" : ["CH", "CN", "EN", "FR"], "name" : "company14"}'
curl -XPUT 'http://localhost:9200/test/company/15' -d '{"countries" : ["AT", "CN", "DE", "EN", "FR", "HU"], "name" : "company15"}'
curl -XPUT 'http://localhost:9200/test/company/16' -d '{"countries" : ["AT", "BE", "CH", "DE", "EN", "FR", "HU"], "name" : "company16"}'
curl -XPUT 'http://localhost:9200/test/company/17' -d '{"countries" : ["BE", "CN", "EN"], "name" : "company17"}'
curl -XPUT 'http://localhost:9200/test/company/18' -d '{"countries" : ["AT", "CH", "CN", "DE"], "name" : "company18"}'
curl -XPUT 'http://localhost:9200/test/company/19' -d '{"countries" : ["AT", "CH", "CN", "DE", "EN", "FR", "HU"], "name" : "company19"}'
curl -XPUT 'http://localhost:9200/test/company/20' -d '{"countries" : ["EN", "FR"], "name" : "company20"}'
curl -XPUT 'http://localhost:9200/test/company/21' -d '{"countries" : ["AT", "BE", "DE", "FR", "HU"], "name" : "company21"}'
curl -XPUT 'http://localhost:9200/test/company/22' -d '{"countries" : ["AT", "BE", "CH", "DE", "EN", "FR", "HU"], "name" : "company22"}'
curl -XPUT 'http://localhost:9200/test/company/23' -d '{"countries" : ["AT", "BE", "CH", "CN", "DE", "EN", "HU"], "name" : "company23"}'
curl -XPUT 'http://localhost:9200/test/company/24' -d '{"countries" : ["AT", "BE", "CH", "CN", "DE", "EN", "FR"], "name" : "company24"}'
curl -XPUT 'http://localhost:9200/test/company/25' -d '{"countries" : ["AT", "BE", "CH", "DE", "EN", "FR"], "name" : "company25"}'
curl -XPUT 'http://localhost:9200/test/company/26' -d '{"countries" : ["AT", "BE", "CH", "CN", "DE", "EN", "FR", "HU"], "name" : "company26"}'
curl -XPUT 'http://localhost:9200/test/company/27' -d '{"countries" : ["AT", "EN", "FR"], "name" : "company27"}'
curl -XPUT 'http://localhost:9200/test/company/28' -d '{"countries" : ["CN"], "name" : "company28"}'
curl -XPUT 'http://localhost:9200/test/company/29' -d '{"countries" : ["BE", "CH", "CN", "EN", "FR"], "name" : "company29"}'
curl -XPUT 'http://localhost:9200/test/company/30' -d '{"countries" : ["CN"], "name" : "company30"}'

# Refresh index
#
curl -XPOST 'http://localhost:9200/test/_refresh'

# Search
#
curl -XGET 'http://localhost:9200/test/company/_search?pretty=true' -d '
{"query"  : { "match_all" : {} },
 "facets" : {
    "foo" : {
      "filter" : {
        "term" : { "countries" : "AT" }
      }
    }
  }
}
'