Elasticsearch: The Definitive Guide

Chapter 25. High-Level Concepts

Like the query DSL, aggregations have a composable syntax: independent units of functionality can be mixed and matched to provide the custom behavior that you need. This means that there are only a few basic concepts to learn, but nearly limitless combinations of those basic components.

To master aggregations, you need to understand only two main concepts:

Buckets

Collections of documents that meet a criterion

Metrics

Statistics calculated on the documents in a bucket

That’s it! Every aggregation is simply a combination of one or more buckets and zero or more metrics. To translate into rough SQL terms:

SELECT COUNT(color) 
FROM table
GROUP BY color 

COUNT(color) is equivalent to a metric.

GROUP BY color is equivalent to a bucket.

Buckets are conceptually similar to grouping in SQL, while metrics are similar to COUNT(), SUM(), MAX(), and so forth.

Let’s dig into both of these concepts and see what they entail.

Buckets

A bucket is simply a collection of documents that meet a certain criteria:

• An employee would land in either the male or female ...