1GET /_search
2{
3  "size": 0, // don't return any documents, just the aggregation
4  "aggs" : {
5    "distinctValues" : { // this can be any label you want
6      "terms" : {
7        "field" : "path.to.field", // field to be aggregated
8        "size" : 500 // how many unique values to return
9      }
10    }
11  }
12}