The Index Stats API provides index statistics. For data streams, the API provides statistics for the stream's backing indexes. By default, the returned statistics are index level. To receive shard-level statistics, set the `level` parameter to `shards`.
When a shard moves to a different node, the shard-level statistics for the shard are cleared. Although the shard is no longer part of the node, the node preserves any node-level statistics to which the shard contributed.
{: .note}
## Path and HTTP methods
```json
GET /_stats
GET /<index_ids>/_stats
GET /<index_ids>/_stats/<metric>
```
## Path parameters
The following table lists the available path parameters. All path parameters are optional.
| Parameter | Data type | Description |
| :--- | :--- | :--- |
| `<index_ids>` | String | A comma-separated list of indexes, data streams, or index aliases used to filter results. Supports wildcard expressions. Defaults to `_all` (`*`).
`<metric>` | String | A comma-separated list of metric groups that will be included in the response. For valid values, see [Metrics](#metrics). Defaults to all metrics. |
### Metrics
The following table lists all available metric groups.
Metric | Description
:--- |:----
`_all` | Return all statistics.
`completion` | Completion suggester statistics.
`docs` | Returns the number of documents and the number of deleted documents that have not yet been merged. Index refresh operations can affect this statistic.
`fielddata` | Field data statistics.
`flush` | Flush statistics.
`get` | Get statistics, including missing stats.
`indexing` | Indexing statistics.
`merge` | Merge statistics.
`query_cache` | Query cache statistics.
`refresh` | Refresh statistics.
`request_cache` | Shard request cache statistics.
`search` | Search statistics, including suggest operation statistics. Search operations can be associated with one or more groups. You can include statistics for custom groups by providing a `groups` parameter, which accepts a comma-separated list of group names. To return statistics for all groups, use `_all`.
`segments` | Statistics about memory use of all open segments. If the `include_segment_file_sizes` parameter is `true`, this metric includes the aggregated disk usage of each Lucene index file.
`store` | Size of the index in byte units.
`translog` | Translog statistics.
`warmer` | Warmer statistics.
## Query parameters
The following table lists the available query parameters. All query parameters are optional.
Parameter | Data type | Description
:--- | :--- | :---
`expand_wildcards` | String | Specifies the type of indexes to which wildcard expressions can expand. Supports comma-separated values. Valid values are: <br> - `all`: Expand to all open and closed indexes, including hidden indexes. <br> - `open`: Expand to open indexes. <br> - `closed`: Expand to closed indexes. <br> - `hidden`: Include hidden indexes when expanding. Must be combined with `open`, `closed`, or both. <br> - `none`: Do not accept wildcard expressions. <br> Default is `open`.
`fields` | String | A comma-separated list or a wildcard expression specifying fields to include in the statistics. Specifies the default field list if neither `completion_fields` nor `fielddata_fields` is provided.
`completion_fields` | String | A comma-separated list or wildcard expression specifying fields to include in field-level `completion` statistics.
`fielddata_fields` | String | A comma-separated list or wildcard expression specifying fields to include in field-level `fielddata` statistics.
`forbid_closed_indices` | Boolean | Specifies not to collect statistics for closed indexes. Default is `true`.
`groups` | String | A comma-separated list of search groups to include in the `search` statistics.
`level` | String | Specifies the level used to aggregate statistics. Valid values are: <br> - `cluster`: Cluster-level statistics. <br> - `indices`: Index-level statistics. <br> - `shards`: Shard-level statistics. <br> Default is `indices`.
`include_segment_file_sizes` | Boolean | Specifies whether to report the aggregated disk usage of each Lucene index file. Only applies to `segments` statistics. Default is `false`.
`include_unloaded_segments` | Boolean | Specifies whether to include information from segments that are not loaded into memory. Default is `false`.
#### Example request: One index
```json
GET /testindex/_stats
```
{% include copy-curl.html %}
#### Example response
By default, the returned statistics are aggregated in the `primaries` and `total` aggregations. The `primaries` aggregation contains statistics for the primary shards. The `total` aggregation contains statistics for both primary and replica shards. The following is an example Index Stats API response:
Starting in OpenSearch 2.10, [concurrent segment search]({{site.url}}{{site.baseurl}}/search-plugins/concurrent-segment-search/) allows each shard-level request to search segments in parallel during the query phase. If you [enable the experimental concurrent segment search feature flag]({{site.url}}{{site.baseurl}}/search-plugins/concurrent-segment-search#enabling-the-feature-flag), the Index Stats API response will contain several additional fields with statistics about slices (units of work executed by a thread). These fields will be provided whether or not the cluster and index settings for concurrent segment search are enabled. For more information about slices, see [Concurrent segment search]({{site.url}}{{site.baseurl}}/search-plugins/concurrent-segment-search#searching-segments-concurrently).
The following table provides information about the added response fields.
|Response field | Description |
|:--- |:--- |
|`search.concurrent_avg_slice_count` |The average slice count of all search requests. This is computed as the total slice count divided by the total number of concurrent search requests. |
|`search.concurrent_query_total` |The total number of query operations that use concurrent segment search. |
|`search.concurrent_query_time_in_millis` |The total amount of time taken by all query operations that use concurrent segment search, in milliseconds. |
|`search.concurrent_query_current` |The number of currently running query operations that use concurrent segment search. |
