Aggregations

Aggregations grew out of the facets module and the long experience of how users use it (and would like to use it) for real-time data analytics purposes. As such, it serves as the next generation replacement for the functionality we currently refer to as "faceting".

Facets provide a great way to aggregate data within a document set context. This context is defined by the executed query in combination with the different levels of filters that can be defined (filtered queries, top-level filters, and facet level filters). While powerful, their implementation is not designed from the ground up to support complex aggregations and is thus limited.

The aggregations module breaks the barriers the current facet implementation put in place. The new name ("Aggregations") also indicates the intention here - a generic yet extremely powerful framework for building aggregations - any types of aggregations.

An aggregation can be seen as a unit-of-work that builds analytic information over a set of documents. The context of the execution defines what this document set is (e.g. a top-level aggregation executes within the context of the executed query/filters of the search request).

There are many different types of aggregations, each with its own purpose and output. To better understand these types, it is often easier to break them into two main families:

The interesting part comes next. Since each bucket effectively defines a document set (all documents belonging to the bucket), one can potentially associate aggregations on the bucket level, and those will execute within the context of that bucket. This is where the real power of aggregations kicks in: aggregations can be nested!

The following snippet captures the basic structure of aggregations:

"aggregations" : {
    "<aggregation_name>" : {
        "<aggregation_type>" : {
            <aggregation_body>
        }
        [,"aggregations" : { [<sub_aggregation>]+ } ]?
    }
    [,"<aggregation_name_2>" : { ... } ]*
}

The aggregations object (the key aggs can also be used) in the JSON holds the aggregations to be computed. Each aggregation is associated with a logical name that the user defines (e.g. if the aggregation computes the average price, then it would make sense to name it avg_price). These logical names will also be used to uniquely identify the aggregations in the response. Each aggregation has a specific type (<aggregation_type> in the above snippet) and is typically the first key within the named aggregation body. Each type of aggregation defines its own body, depending on the nature of the aggregation (e.g. an avg aggregation on a specific field will define the field on which the average will be calculated). At the same level of the aggregation type definition, one can optionally define a set of additional aggregations, though this only makes sense if the aggregation you defined is of a bucketing nature. In this scenario, the sub-aggregations you define on the bucketing aggregation level will be computed for all the buckets built by the bucketing aggregation. For example, if you define a set of aggregations under the range aggregation, the sub-aggregations will be computed for the range buckets that are defined.

Some aggregations work on values extracted from the aggregated documents. Typically, the values will be extracted from a specific document field which is set using the field key for the aggregations. It is also possible to define a script which will generate the values (per document).

When both field and script settings are configured for the aggregation, the script will be treated as a value script. While normal scripts are evaluated on a document level (i.e. the script has access to all the data associated with the document), value scripts are evaluated on the value level. In this mode, the values are extracted from the configured field and the script is used to apply a "transformation" over these value/s.

Scripts can generate a single value or multiple values per document. When generating multiple values, one can use the script_values_sorted settings to indicate whether these values are sorted or not. Internally, Elasticsearch can perform optimizations when dealing with sorted values (for example, with the min aggregations, knowing the values are sorted, Elasticsearch will skip the iterations over all the values and rely on the first value in the list to be the minimum value among all other values associated with the same document).

The aggregations in this family compute metrics based on values extracted in one way or another from the documents that are being aggregated. The values are typically extracted from the fields of the document (using the field data), but can also be generated using scripts.

Numeric metrics aggregations are a special type of metrics aggregation which output numeric values. Some aggregations output a single numeric metric (e.g. avg) and are called single-value numeric metrics aggregation, others generate multiple metrics (e.g. stats) and are called multi-value numeric metrics aggregation. The distinction between single-value and multi-value numeric metrics aggregations plays a role when these aggregations serve as direct sub-aggregations of some bucket aggregations (some bucket aggregations enable you to sort the returned buckets based on the numeric metrics in each bucket).

Bucket aggregations don’t calculate metrics over fields like the metrics aggregations do, but instead, they create buckets of documents. Each bucket is associated with a criterion (depending on the aggregation type) which determines whether or not a document in the current context "falls" into it. In other words, the buckets effectively define document sets. In addition to the buckets themselves, the bucket aggregations also compute and return the number of documents that "fell in" to each bucket.

Bucket aggregations, as opposed to metrics aggregations, can hold sub-aggregations. These sub-aggregations will be aggregated for the buckets created by their "parent" bucket aggregation.

There are different bucket aggregators, each with a different "bucketing" strategy. Some define a single bucket, some define fixed number of multiple buckets, and others dynamically create the buckets during the aggregation process.