Add documentation for star tree index feature

_field-types/supported-field-types/index.md

@@ -30,6 +30,7 @@
 k-NN vector | [`knn_vector`]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/knn-vector/): Allows indexing a k-NN vector into OpenSearch and performing different kinds of k-NN search.
 Percolator | [`percolator`]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/percolator/): Specifies to treat this field as a query. 
 Derived | [`derived`]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/derived/): Creates new fields dynamically by executing scripts on existing fields.
+Star-tree | [`star_tree`]({{site.url}}{{site.baseurl}}/field-types/supported-field-types/star-tree/): Precomputes aggregations and stores them in a [star-tree index](https://docs.pinot.apache.org/basics/indexing/star-tree-index), accelerating the performance of aggregation queries.
 
 ## Arrays
 

_field-types/supported-field-types/star-tree.md

@@ -0,0 +1,199 @@
+---
+layout: default
+title: Star-tree
+nav_order: 61
+parent: Supported field types
+---
+
+# Star-tree field type
+
+This is an experimental feature and is not recommended for use in a production environment. For updates on the progress of the feature or if you want to leave feedback, join the discussion on the [OpenSearch forum](https://forum.opensearch.org/).    
+{: .warning}
+
+A [star-tree index](https://docs.pinot.apache.org/basics/indexing/star-tree-index) precomputes aggregations, accelerating the performance of aggregation queries. 
+If a star-tree index is configured as part of an index mapping, the star-tree index is created and maintained as data is ingested in real time.
+
+OpenSearch will automatically use the star-tree index to optimize aggregations if the queried fields are part of star-tree index dimension fields and the aggregations are on star-tree index metric fields. No changes are required in the query syntax or the request parameters.
+
+For more information, see [Star-tree index]({{site.url}}{{site.baseurl}}/search-plugins/star-tree-index/).
+
+## Prerequisites
+
+To use a star-tree index, follow the instructions in [Enabling a star-tree index]({{site.url}}{{site.baseurl}}/search-plugins/star-tree-index#enabling-a-star-tree-index).
+
+## Limitations
+
+The star-tree index feature has the following limitations:
+
+- A star-tree index should only be enabled on indexes whose data is not updated or deleted because standard updates and deletions are not accounted for in a star-tree index.
+- Currently, only `one` star-tree index can be created per index. Support for multiple star-trees will be added in a future version.
+
+## Examples
+
+The following examples show how to use a star-tree index.
+
+### Star-tree index mapping
+
+Define star-tree mapping in the `composite` section in `mappings`. 
+
+The following example API request creates a corresponding star-tree index for all `request_aggs`. To compute metric aggregations for `request_size` and `latency` fields with queries on `port` and `status` fields, configure the following mappings:
+
+```json
+PUT logs
+{
+  "settings": {
+    "index.number_of_shards": 1,
+    "index.number_of_replicas": 0,
+    "index.composite_index": true
+  },
+  "mappings": {
+    "composite": {
+      "request_aggs": {
+        "type": "star_tree",
+        "config": {
+          "max_leaf_docs": 10000,
+          "skip_star_node_creation_for_dimensions": [
+            "port"
+          ],
+          "ordered_dimensions": [
+            {
+              "name": "status"
+            },
+            {
+              "name": "port"
+            }
+          ],
+          "metrics": [
+            {
+              "name": "request_size",
+              "stats": [
+                "sum",
+                "value_count",
+                "min",
+                "max"
+              ]
+            },
+            {
+              "name": "latency",
+              "stats": [
+                "sum",
+                "value_count",
+                "min",
+                "max"
+              ]
+            }
+          ]
+        }
+      }
+    },
+    "properties": {
+      "status": {
+        "type": "integer"
+      },
+      "port": {
+        "type": "integer"
+      },
+      "request_size": {
+        "type": "integer"
+      },
+      "latency": {
+        "type": "scaled_float",
+        "scaling_factor": 10
+      }
+    }
+  }
+}
+```
+
+
+
+## Star-tree mapping parameters
+
+Specify any star-tree configuration mapping options in the `config` section. Parameters cannot be modified without reindexing documents.
+
+The star-tree `config` section supports the following property.
+
+| Parameter  | Required/Optional | Description  | 
+| :--- | :--- | :--- |
+| `name` | Required | The name of the field. The field name should be present in the `properties` section as part of the index `mapping`. Ensure that the `doc_values` setting is `enabled` for any associated fields.
+
+### Ordered dimensions
+
+The `ordered_dimensions` parameter contains fields based on which metrics will be aggregated in a star-tree index. The star-tree index will be selected for querying only if all the fields in the query are part of the `ordered_dimensions`. 
+
+When using the `ordered_dimesions` parameter, follow these best practices:
+
+- The order of dimensions matters. You can define the dimensions ordered from the highest cardinality to the lowest cardinality for efficient storage and query pruning. 
+- Avoid using high-cardinality fields as dimensions. High-cardinality fields adversely affect storage space, indexing throughput, and query performance.
+- Currently, fields supported by the `ordered_dimensions` parameter are all [numeric field types](https://opensearch.org/docs/latest/field-types/supported-field-types/numeric/), with the exception of `unsigned_long`. For more information, see [GitHub issue #15231](https://github.com/opensearch-project/OpenSearch/issues/15231). 
+- Support for other field types, such as `keyword` and `ip`, will be added in future versions. For more information, see [GitHub issue #16232](https://github.com/opensearch-project/OpenSearch/issues/16232).
+- A minimum of `2` and a maximum of `10` dimensions are supported per star-tree index.
+
+The `ordered_dimensions` parameter supports the following property.
+
+| Parameter  | Required/Optional | Description  | 
+| :--- | :--- | :--- |
+| `name` | Required | The name of the field. The field name should be present in the `properties` section as part of the index `mapping`. Ensure that the `doc_values` setting is `enabled` for any associated fields. |
+
+
+### Metrics
+
+Configure any metric fields on which you need to perform aggregations. `Metrics` are required as part of a star-tree configuration.
+
+When using `metrics`, follow these best practices: 
+
+- Currently, fields supported by `metrics` are all [numeric field types](https://opensearch.org/docs/latest/field-types/supported-field-types/numeric/), with the exception of `unsigned_long`. For more information, see [GitHub issue #15231](https://github.com/opensearch-project/OpenSearch/issues/15231). 
+- Supported metric aggregations include `Min`, `Max`, `Sum`, `Avg`, and `Value_count`. 
+    - `Avg` is a derived metric based on `Sum` and `Value_count` and is not indexed when a query is run. The remaining base metrics are indexed.
+- A maximum of `100` base metrics are supported per star-tree index.
+
+If `Min`, `Max`, `Sum`, and `Value_count` are defined as `metrics` for each field, then up to 25 such fields can be configured, as shown in the following example:
+
+```json
+{
+  "metrics": [
+    {
+      "name": "field1",
+      "stats": [
+        "sum",
+        "value_count",
+        "min",
+        "max"
+      ],
+      ...,
+      ...,
+      "name": "field25",
+      "stats": [
+        "sum",
+        "value_count",
+        "min",
+        "max"
+      ]
+    }
+  ]
+}
+```
+
+
+#### Properties
+
+The `metrics` parameter supports the following properties.
+
+| Parameter   | Required/Optional | Description  | 
+| :--- | :--- | :--- |
+| `name` | Required | The name of the field. The field name should be present in the `properties` section as part of the index `mapping`. Ensure that the `doc_values` setting is `enabled` for any associated fields. |
+| `stats` | Optional | A list of metric aggregations computed for each field. You can choose between `Min`, `Max`, `Sum`, `Avg`, and `Value Count`.<br/>Default is `Sum` and `Value_count`.<br/>`Avg` is a derived metric statistic that will automatically be supported in queries if `Sum` and `Value_Count` are present as part of metric `stats`.
+
+### Star-tree configuration parameters
+
+The following parameters are optional and cannot be modified following index creation.
+
+| Parameter  | Description   | 
+| :--- | :--- |
+| `max_leaf_docs` | The maximum number of star-tree documents that a leaf node can point to. After the maximum number of documents is reached, the nodes will be split based on the value of the next dimension. Default is `10000`. A lower value will use more storage but result in faster query performance. Inversely, a higher value will use less storage but result in slower query performance. For more information, see [Star-tree indexing structure]({{site.url}}{{site.baseurl}}/search-plugins/star-tree-index/#star-tree-index-structure).  |
+| `skip_star_node_creation_for_dimensions`  | A list of dimensions for which a star-tree index will skip star node creation. When `true`, this reduces storage size at the expense of query performance. Default is `false`. For more information about star nodes, see [Star-tree indexing structure]({{site.url}}{{site.baseurl}}/search-plugins/star-tree-index/#star-tree-index-structure). |
+
+## Supported queries and aggregations
+
+For more information about supported queries and aggregations, see [Supported queries and aggregations for a star-tree index]({{site.url}}{{site.baseurl}}/search-plugins/star-tree-index/#supported-queries-and-aggregations).
+

_search-plugins/improving-search-performance.md

@@ -11,4 +11,6 @@ OpenSearch offers several ways to improve search performance:
 
 - Run resource-intensive queries asynchronously with [asynchronous search]({{site.url}}{{site.baseurl}}/search-plugins/async/).
 
-- Search segments concurrently using [concurrent segment search]({{site.url}}{{site.baseurl}}/search-plugins/concurrent-segment-search/).
+- Search segments concurrently using [concurrent segment search]({{site.url}}{{site.baseurl}}/search-plugins/concurrent-segment-search/).
+
+- Improve aggregation performance using a [star-tree index]({{site.url}}{{site.baseurl}}/search-plugins/star-tree-index/).