Elasticsearch: The Definitive Guide

Chapter 8. Sorting and Relevance

By default, results are returned sorted by relevance—with the most relevant docs first. Later in this chapter, we explain what we mean by relevance and how it is calculated, but let’s start by looking at the sort parameter and how to use it.

Sorting

In order to sort by relevance, we need to represent relevance as a value. In Elasticsearch, the relevance score is represented by the floating-point number returned in the search results as the _score, so the default sort order is _score descending.

Sometimes, though, you don’t have a meaningful relevance score. For instance, the following query just returns all tweets whose user_id field has the value 1:

GET /_search
{
    "query" : {
        "filtered" : {
            "filter" : {
                "term" : {
                    "user_id" : 1
                }
            }
        }
    }
}

Filters have no bearing on _score, and the missing-but-implied match_all query just sets the _score to a neutral value of 1 for all documents. In other words, all documents are considered to be equally relevant.

Sorting by Field Values

In this case, it probably makes sense to sort tweets by recency, with the most recent tweets first. We can do this with the sort parameter:

GET /_search
{
    "query" : {
        "filtered" : {
            "filter" : { "term" : { "user_id" : 1 }}
        }
    },
    "sort": { "date": { "order": "desc" }}
}

You will notice two differences in the results:

"hits" : {
    "total" :           6,
    "max_score" :       null, 
    "hits" : [ {
        "_index" ...