Generally the total hit count can’t be computed accurately without visiting all matches, which is costly for queries that match lots of documents. The track_total_hits parameter allows you to control how the total number of hits should be tracked. Given that it is often enough to have a lower bound of the number of hits, such as "there are at least 10000 hits", the default is set to 10,000. This means that requests will count the total hit accurately up to 10,000 hits. It’s is a good trade off to speed up searches if you don’t need the accurate number of hits after a certain threshold.

When set to true the search response will always track the number of hits that match the query accurately (e.g. total.relation will always be equal to "eq" when track_total_hits is set to true). Otherwise the "total.relation" returned in the "total" object in the search response determines how the "total.value" should be interpreted. A value of "gte" means that the "total.value" is a lower bound of the total hits that match the query and a value of "eq" indicates that "total.value" is the accurate count.

GET twitter/_search
{
    "track_total_hits": true,
     "query": {
        "match" : {
            "message" : "Elasticsearch"
        }
     }
}

... returns:

{
    "_shards": ...
    "timed_out": false,
    "took": 100,
    "hits": {
        "max_score": 1.0,
        "total" : {
            "value": 2048,    
            "relation": "eq"  
        },
        "hits": ...
    }
}

It is also possible to set track_total_hits to an integer. For instance the following query will accurately track the total hit count that match the query up to 100 documents:

GET twitter/_search
{
    "track_total_hits": 100,
     "query": {
        "match" : {
            "message" : "Elasticsearch"
        }
     }
}

The hits.total.relation in the response will indicate if the value returned in hits.total.value is accurate ("eq") or a lower bound of the total ("gte").

For instance the following response:

{
    "_shards": ...
    "timed_out": false,
    "took": 30,
    "hits" : {
        "max_score": 1.0,
        "total" : {
            "value": 42,         
            "relation": "eq"     
        },
        "hits": ...
    }
}

... indicates that the number of hits returned in the total is accurate.

If the total number of his that match the query is greater than the value set in track_total_hits, the total hits in the response will indicate that the returned value is a lower bound:

{
    "_shards": ...
    "hits" : {
        "max_score": 1.0,
        "total" : {
            "value": 100,         
            "relation": "gte"     
        },
        "hits": ...
    }
}

If you don’t need to track the total number of hits at all you can improve query times by setting this option to false:

GET twitter/_search
{
    "track_total_hits": false,
     "query": {
        "match" : {
            "message" : "Elasticsearch"
        }
     }
}

... returns:

{
    "_shards": ...
    "timed_out": false,
    "took": 10,
    "hits" : { 
        "max_score": 1.0,
        "hits": ...
    }
}

Finally you can force an accurate count by setting "track_total_hits" to true in the request.