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

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

Previews a {transform}.

[[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 the following 
privileges:

* `manage_transform` 
* source index: `read`, `view_index_metadata`

The built-in `transform_admin` role has the `manage_transform` privilege.

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.

It also generates a list of mappings and settings for the destination index.
If the destination index does not exist when you start a {transform}, these are
the mappings and settings that are used. These values are determined based on
the field types of the source index and the {transform} aggregations.

TIP: There are some <<transform-aggresponse-limitations,limitations>> that
might result in poor mappings. As a work-around, create the destination index
or an index template with your preferred mappings before you start the
{transform}.

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


`description`::
(Optional, string) Free text description of the {transform}.

//Begin dest
`dest`::
(Optional, object)
include::{docdir}/rest-api/common-parms.asciidoc[tag=dest]
+
.Properties of `dest`
[%collapsible%open]
====
`index`:::
(Optional, string)
include::{docdir}/rest-api/common-parms.asciidoc[tag=dest-index]

`pipeline`:::
(Optional, string)
include::{docdir}/rest-api/common-parms.asciidoc[tag=dest-pipeline]
====
//End dest

`frequency`::
(Optional, <<time-units, time units>>)
include::{docdir}/rest-api/common-parms.asciidoc[tag=frequency]

//Begin pivot
`pivot`::
(Required, object)
include::{docdir}/rest-api/common-parms.asciidoc[tag=pivot]
+
.Properties of `pivot`
[%collapsible%open]
====

`aggregations` or `aggs`:::
(Required, object)
include::{docdir}/rest-api/common-parms.asciidoc[tag=pivot-aggs]

`group_by`:::
(Required, object)
include::{docdir}/rest-api/common-parms.asciidoc[tag=pivot-group-by]

`max_page_search_size`:::
(Optional, integer)
include::{docdir}/rest-api/common-parms.asciidoc[tag=pivot-max-page-search-size]
====
//End pivot

//Begin source
`source`::
(Required, object)
include::{docdir}/rest-api/common-parms.asciidoc[tag=source-transforms]
+
.Properties of `source`
[%collapsible%open]
====

`index`:::
(Required, string or array)
include::{docdir}/rest-api/common-parms.asciidoc[tag=source-index-transforms]

`query`:::
(Optional, object)
include::{docdir}/rest-api/common-parms.asciidoc[tag=source-query-transforms]
====
//End source

//Begin sync
`sync`::
(Optional, object)
include::{docdir}/rest-api/common-parms.asciidoc[tag=sync]
+
.Properties of `sync`
[%collapsible%open]
====
//Begin sync.time

`time`:::
(Optional, object)
include::{docdir}/rest-api/common-parms.asciidoc[tag=sync-time]
+
.Properties of `analysis_config`
[%collapsible%open]
=====

`delay`::::
(Optional, <<time-units, time units>>)
include::{docdir}/rest-api/common-parms.asciidoc[tag=sync-time-delay]
    
`field`::::
(Optional, string)
include::{docdir}/rest-api/common-parms.asciidoc[tag=sync-time-field]
=====
//End sync.time
====
//End sync

[role="child_attributes"]
[[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}.

//Begin generated_dest_index  
`generated_dest_index`::
(object) Contains details about the destination index.
+
.Properties of `generated_dest_index`
[%collapsible%open]
====

`aliases`:::
(object) The aliases for the destination index.

`mappings`:::
(object) The <<mapping,mappings>> for each document in the destination index. 

`settings`:::
(object) The <<index-modules-settings,index settings>> for the destination index.
====
//End generated_dest_index

==== {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"
    }
    ...
  ],
  "generated_dest_index" : {
    "mappings" : {
      "_meta" : {
        "_transform" : {
          "transform" : "transform-preview",
          "version" : {
            "created" : "7.7.0"
          },
          "creation_date_in_millis" : 1584738236757
        },
        "created_by" : "transform"
      },
      "properties" : {
        "max_price" : {
          "type" : "half_float"
        },
        "customer_id" : {
          "type" : "keyword"
        }
      }
    },
    "settings" : {
      "index" : {
        "number_of_shards" : "1",
        "auto_expand_replicas" : "0-1"
      }
    },
    "aliases" : { }
  }
}
----
// NOTCONSOLE