Returning Empty Buckets

Notice something odd about that last response?

Yep, that’s right. We are missing a few months! By default, the date_histogram (and histogram too) returns only buckets that have a nonzero document count.

This means your histogram will be a minimal response. Often, this is not the behavior you want. For many applications, you would like to dump the response directly into a graphing library without doing any post-processing.

Essentially, we want buckets even if they have a count of zero. We can set two additional parameters that will provide this behavior:

GET /cars/transactions/_search
{
   "size" : 0,
   "aggs": {
      "sales": {
         "date_histogram": {
            "field": "sold",
            "interval": "month",
            "format": "yyyy-MM-dd",
            "min_doc_count" : 0, 
            "extended_bounds" : { 
                "min" : "2014-01-01",
                "max" : "2014-12-31"
            }
         }
      }
   }
}

The two additional parameters will force the response to return all months in the year, regardless of their doc count. The min_doc_count is very understandable: it forces buckets to be returned even if they are empty.

The extended_bounds parameter requires a little explanation. The min_doc_count parameter forces empty buckets to be returned, but by default Elasticsearch will return only buckets that are between the minimum and maximum value in your data.

So if your data falls between April and July, you’ll have buckets representing only those months (empty or otherwise). To get the full year, we need to tell Elasticsearch that we want buckets even if they fall before the minimum value or after the maximum value.

The extended_bounds parameter does just that. Once you add those two settings, you’ll get a response that is easy to plug straight into your graphing libraries and give you a graph like Figure 37, “Cars sold over time”.

Figure 37. Cars sold over time