OpenSearch/docs/painless/painless-contexts/painless-metric-agg-combine...

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[[painless-metric-agg-combine-context]]
=== Metric aggregation combine context
Use a Painless script to
{es_version}/search-aggregations-metrics-scripted-metric-aggregation.html[combine]
values for use in a scripted metric aggregation. A combine script is run once
per shard following a <<painless-metric-agg-map-context, map script>> 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['_agg']` (`Map`)::
`Map` with values available from the prior map script.
*Return*
`List`, `Map`, `String`, or primitive::
A value collected for use in a
<<painless-metric-agg-reduce-context, reduce script>>. If no reduce
script is specified, the value is used as part of the result.
*API*
The standard <<painless-api-reference, Painless API>> is available.