[[painless-metric-agg-reduce-context]]
=== Metric aggregation reduce context

Use a Painless script to
{es_version}/search-aggregations-metrics-scripted-metric-aggregation.html[reduce]
values to produce the result of a scripted metric aggregation. A reduce script
is run once on the coordinating node following a
<<painless-metric-agg-combine-context, combine script>> (or a
<<painless-metric-agg-map-context, map script>> if no combine script is
specified) and is optional as part of a full metric aggregation.

*Variables*

`params` (`Map`, read-only)::
        User-defined parameters passed in as part of the query.

`params['_aggs']` (`Map`)::
        `Map` with values available from the prior combine script (or a map
        script if no combine script is specified).

*Return*

`List`, `Map`, `String`, or primitive::
        A value used as the result.

*API*

The standard <<painless-api-reference, Painless API>> is available.
