[role="xpack"]
[testenv="basic"]
[[preview-transform]]
=== Preview {transforms} API

[subs="attributes"]
++++
<titleabbrev>Preview {transforms}</titleabbrev>
++++

Previews a {transform}.

beta[]

[[preview-transform-request]]
==== {api-request-title}

`POST _transform/_preview`

[[preview-transform-prereq]]
==== {api-prereq-title}

* If the {es} {security-features} are enabled, you must have
`manage_data_frame_transforms` cluster privileges to use this API. The built-in
`data_frame_transforms_admin` role has these privileges. You must also have
`read` and `view_index_metadata` privileges on the source index for the
{transform}. For more information,
see <<security-privileges>> and <<built-in-roles>>.

[[preview-transform-desc]]
==== {api-description-title}

This API generates a preview of the results that you will get when you run the
<<put-transform,create {transforms} API>> with the same
configuration. It returns a maximum of 100 results. The calculations are based
on all the current data in the source index. 

[[preview-transform-request-body]]
==== {api-request-body-title}

`source`::
  (Required, object) The source configuration, which has the following
  properties:
  
  `index`:::
    (Required, string or array) The _source indices_ for the
    {transform}. It can be a single index, an index pattern (for
    example, `"myindex*"`), or an array of indices (for example,
    `["index1", "index2"]`).

  `query`:::
    (Optional, object) A query clause that retrieves a subset of data from the
    source index. See <<query-dsl>>.

`pivot`::
  (Required, object) Defines the pivot function `group by` fields and the
  aggregation to reduce the data. See <<transform-pivot>>.
  
[[preview-transform-response]]
==== {api-response-body-title}

`preview`::
  (array) An array of documents. In particular, they are the JSON
  representation of the documents that would be created in the destination index
  by the {transform}. 

==== {api-examples-title}

[source,console]
--------------------------------------------------
POST _transform/_preview
{
  "source": {
    "index": "kibana_sample_data_ecommerce"
  },
  "pivot": {
    "group_by": {
      "customer_id": {
        "terms": {
          "field": "customer_id"
        }
      }
    },
    "aggregations": {
      "max_price": {
        "max": {
          "field": "taxful_total_price"
        }
      }
    }
  }
}
--------------------------------------------------
// TEST[skip:set up sample data]

The data that is returned for this example is as follows:
[source,js]
----
{
  "preview" : [
    {
      "max_price" : 171.0,
      "customer_id" : "10"
    },
    {
      "max_price" : 233.0,
      "customer_id" : "11"
    },
    {
      "max_price" : 200.0,
      "customer_id" : "12"
    }
    ...
  ],
  "mappings": {
    "properties": {
      "max_price": {
        "type": "double"
      },
      "customer_id": {
        "type": "keyword"
      }
    }
  }
}
----
// NOTCONSOLE