GET /_search
{
    "query": {...},
    "aggs" : {
        "categories" : {
            "terms" : { "field" : "category_name" }
        }
    }
}

{
    ...
    "aggregations" : {
        "categories" : {
            "doc_count_error_upper_bound": 0,
            "sum_other_doc_count": 0,
            "buckets" : [
                {
                    "key" : "Mobile phones",
                    "doc_count" : 6
                },
                {
                    "key" : "Batteries for phones",
                    "doc_count" : 3
                },
                {
                    "key" : "Cases for phones",
                    "doc_count" : 2
                }
            ]
        }
    }
}