elasticsearch _field_stats 源码分析

_field_stats 实现的功能：https://www.elastic.co/guide/en/elasticsearch/reference/5.6/search-field-stats.html

获取索引下字段的统计信息，如下表，同时还可以针对这些统计值进行过滤：

Field statistics

The field stats api is supported on string based, number based and date based fields and can return the following statistics per field:

max_doc	The total number of documents.
doc_count	The number of documents that have at least one term for this field, or -1 if this measurement isn’t available on one or more shards.
density	The percentage of documents that have at least one value for this field. This is a derived statistic and is based on the max_doc and doc_count.
sum_doc_freq	The sum of each term’s document frequency in this field, or -1 if this measurement isn’t available on one or more shards. Document frequency is the number of documents containing a particular term.
sum_total_term_freq	The sum of the term frequencies of all terms in this field across all documents, or -1 if this measurement isn’t available on one or more shards. Term frequency is the total number of occurrences of a term in a particular document and field.

Field stats index constraints ——kibana里按照时间范围进行绘图就是用到这个。

Field stats index constraints allows to omit all field stats for indices that don’t match with the constraint. An index constraint can exclude indices' field stats based on the min_value and max_value statistic. This option is only useful if the level option is set to indices. Fields that are not indexed (not searchable) are always omitted when an index constraint is defined.

For example index constraints can be useful to find out the min and max value of a particular property of your data in a time based scenario. The following request only returns field stats for the answer_count property for indices holding questions created in the year 2014:

POST _field_stats?level=indices
{
   "fields" : ["answer_count"], 

   "index_constraints" : { 

      "creation_date" : { 

         "max_value" : { 

            "gte" : "2014-01-01T00:00:00.000Z"
         },
         "min_value" : { 

            "lt" : "2015-01-01T00:00:00.000Z"
         }
      }
   }
}

对应ES5.5的源码部分：elasticsearch/search/lookup/IndexField.java

import org.apache.lucene.search.CollectionStatistics;
import org.elasticsearch.common.util.MinimalMap;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

/**
 * Script interface to all information regarding a field.
 * */
public class IndexField extends MinimalMap<String, IndexFieldTerm> {

    /*
     * TermsInfo Objects that represent the Terms are stored in this map when
     * requested. Information such as frequency, doc frequency and positions
     * information can be retrieved from the TermInfo objects in this map.
     */
    private final Map<String, IndexFieldTerm> terms = new HashMap<>();

    // the name of this field
    private final String fieldName;

    /*
     * The holds the current reader. We need it to populate the field
     * statistics. We just delegate all requests there
     */
    private final LeafIndexLookup indexLookup;

    /*
     * General field statistics such as number of documents containing the
     * field.
     */
    private final CollectionStatistics fieldStats;
    public IndexField(String fieldName, LeafIndexLookup indexLookup) throws IOException {

        assert fieldName != null;
        this.fieldName = fieldName;

        assert indexLookup != null;
        this.indexLookup = indexLookup;

        fieldStats = this.indexLookup.getIndexSearcher().collectionStatistics(fieldName);
    }

    /* get number of documents containing the field */
    public long docCount() throws IOException {
        return fieldStats.docCount();
    }

    /* get sum of the number of words over all documents that were indexed */
    public long sumttf() throws IOException {
        return fieldStats.sumTotalTermFreq();
    }

    /*
     * get the sum of doc frequencies over all words that appear in any document
     * that has the field.
     */
    public long sumdf() throws IOException {
        return fieldStats.sumDocFreq();
    }
   // 。。。。。。。
}