Adding documentation for filter search in OpenSearch (#7900) (#8545)

_search-plugins/filter-search.md

@@ -0,0 +1,151 @@
+---
+layout: default
+title: Filter search results
+nav_order: 36
+---
+
+# Filter search results
+
+You can filter searches using different methods, each suited to specific scenarios. You can apply filters at the query level, using `boolean` query clauses and `post_filter` and `aggregation` level filters, as follows:
+
+- **Query-level filtering:** Apply `boolean` query filter clauses to filter search hits and aggregations, such as to narrow results to specific categories or brands.
+- **Post-filter filtering:** Use `post_filter` to refine search hits based on user selections while preserving all aggregation options.
+- **Aggregation-level filtering:** Adjust specific aggregations based on selected filters without impacting other aggregations.
+
+## Query-level filtering with Boolean queries
+
+Use a `boolean` query with a filter clause to apply filters to both search hits and aggregations. For example, if a shopper searches for `smartphones` from `BrandA`, a Boolean query can restrict results to only those smartphones from `BrandA`. The following steps guide you through query-level filtering. 
+
+1. Create an index `electronics` and provide the mapping using the following request:
+
+```json
+PUT /electronics
+{
+  "mappings": {
+    "properties": {
+      "brand": { "type": "keyword" },
+      "category": { "type": "keyword" },
+      "price": { "type": "float" },
+      "features": { "type": "keyword" }
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+2. Add documents to the `electronics` index using the following request:
+
+```json
+PUT /electronics/_doc/1?refresh
+{
+  "brand": "BrandA",
+  "category": "Smartphone",
+  "price": 699.99,
+  "features": ["5G", "Dual Camera"]
+}
+PUT /electronics/_doc/2?refresh
+{
+  "brand": "BrandA",
+  "category": "Laptop",
+  "price": 1199.99,
+  "features": ["Touchscreen", "16GB RAM"]
+}
+PUT /electronics/_doc/3?refresh
+{
+  "brand": "BrandB",
+  "category": "Smartphone",
+  "price": 799.99,
+  "features": ["5G", "Triple Camera"]
+}
+```
+{% include copy-curl.html %}
+
+3. Apply a `boolean` filter query to display only `smartphones` from `BrandA` using the following request:
+
+```json
+GET /electronics/_search
+{
+  "query": {
+    "bool": {
+      "filter": [
+        { "term": { "brand": "BrandA" }},
+        { "term": { "category": "Smartphone" }}
+      ]
+    }
+  }
+}
+```
+{% include copy-curl.html %}
+
+## Narrowing results using `post-filter` while preserving aggregation visibility
+
+Use `post_filter` to limit search hits while preserving all aggregation options. For example, if a shopper selects `BrandA`, results are filtered to show only `BrandA` products while maintaining the visibility of all brand options in the aggregations, as shown in the following example request:
+
+```json
+GET /electronics/_search
+{
+  "query": {
+    "bool": {
+      "filter": { "term": { "category": "Smartphone" }}
+    }
+  },
+  "aggs": {
+    "brands": {
+      "terms": { "field": "brand" }
+    }
+  },
+  "post_filter": {
+    "term": { "brand": "BrandA" }
+  }
+}
+```
+{% include copy-curl.html %}
+
+The result should show `BrandA` smartphones in the search hits and all brands in the aggregations.
+
+## Refining aggregations with aggregation-level filtering
+
+You can use aggregation-level filtering to apply filters to specific aggregations without affecting the main aggregation to which they belong. 
+
+For example, you can use aggregation-level filtering to filter the `price_ranges` aggregation based on selected brands, `BrandA` and `BrandB`, without affecting the main `price_ranges` aggregation, as shown in the following example request. This displays price ranges relevant to the selected brands while also displaying overall price ranges for all products.
+
+```json
+GET /electronics/_search
+{
+  "query": {
+    "bool": {
+      "filter": { "term": { "category": "Smartphone" }}
+    }
+  },
+  "aggs": {
+    "price_ranges": {
+      "range": {
+        "field": "price",
+        "ranges": [
+          { "to": 500 },
+          { "from": 500, "to": 1000 },
+          { "from": 1000 }
+        ]
+      }
+    },
+    "filtered_brands": {
+      "filter": {
+        "terms": { "brand": ["BrandA", "BrandB"] }
+      },
+      "aggs": {
+        "price_ranges": {
+          "range": {
+            "field": "price",
+            "ranges": [
+              { "to": 500 },
+              { "from": 500, "to": 1000 },
+              { "from": 1000 }
+            ]
+          }
+        }
+      }
+    }
+  }
+}
+```
+{% include copy-curl.html %}