GET /some-index/_search
{
  "size": 0,
  "track_total_hits": false,
  "aggs": {
    "my_buckets": {
      "composite": {
        "sources": [
          { "user_name": { "terms": { "field": "user_name" } } },
          { "date": { "date_histogram": { "field": "timestamp", "calendar_interval": "1d", "order": "desc" } } }
        ]
      }
    }
  }
}