ElasticSearch对字段去重统计技术，实现SQL中的count(distinct(*)) 效果

public long countDistinctField(String esIndex, String countField, SearchSourceBuilder sourceBuilder) {
         long count = 0;
         if (StringUtils.isBlank(esIndex) || StringUtils.isBlank(countField)) {
             return count;
         }
 
         SearchRequest searchRequest = new SearchRequest();
         searchRequest.indices(esIndex);
         String identifier = UUID.randomUUID().toString();
         AggregationBuilder aggregationBuilder = AggregationBuilders.cardinality(identifier).field(countField);
         sourceBuilder.aggregation(aggregationBuilder);
         sourceBuilder.size(0);
         searchRequest.source(sourceBuilder);
         try {
             SearchResponse result = client.search(searchRequest, RequestOptions.DEFAULT);
             Histogram histogram = (Histogram) result.getAggregations().asMap().get(countField);
             long total_value = 0;
             for (Histogram.Bucket t : histogram.getBuckets()) {
                 Cardinality cardinality = t.getAggregations().get(identifier);
                 long value = cardinality.getValue();
                 total_value = total_value + value;
             }
             return total_value;
         } catch (Exception e) {
             log.error("Count field failed!", e);
         }
         return 0;
     }