Adding documentation for filter search in OpenSearch

_search-plugins/filter-search.md

@@ -0,0 +1,146 @@
+---
+layout: default
+title: Filter search results
+nav_order: 36
+---
+
+# Filter search results
+
+You can filter search using different methods, each suited to specific scenarios. You can apply filters at the query level, using `boolean` query clauses, as well as by using `post_filter` and `aggregation` level filters.
+
+## 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.
+
+Create an index `electronics` and provide the mapping:
+
+```
+PUT /electronics
+{
+  "mappings": {
+    "properties": {
+      "brand": { "type": "keyword" },
+      "category": { "type": "keyword" },
+      "price": { "type": "float" },
+      "features": { "type": "keyword" }
+    }
+  }
+}
+```
+
+Add documents to the `electronics` index:
+
+```
+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"]
+}
+```
+
+Apply a `boolean` filter query to display only `smartphones` from `BrandA`:
+
+```
+GET /electronics/_search
+{
+  "query": {
+    "bool": {
+      "filter": [
+        { "term": { "brand": "BrandA" }},
+        { "term": { "category": "Smartphone" }}
+      ]
+    }
+  }
+}
+```
+
+## Using `post-filter` to narrow results without affecting aggregations
+
+Use `post_filter` to limit search hits, while maintaining all aggregation options. For example, if a shopper selects `BrandA`, you can filter results to show only `BrandA` products while keeping all brands visible in the aggregation.
+
+```
+GET /electronics/_search
+{
+  "query": {
+    "bool": {
+      "filter": { "term": { "category": "Smartphone" }}
+    }
+  },
+  "aggs": {
+    "brands": {
+      "terms": { "field": "brand" }
+    }
+  },
+  "post_filter": {
+    "term": { "brand": "BrandA" }
+  }
+}
+```
+This shows `BrandA` smartphones in the search hits while still displaying all brands in the aggregations.
+
+## Aggregation-level filtering to refine aggregations
+We can use aggregation-level filtering to apply filters to specific aggregations, without affecting the overall aggregation.
+
+Use aggregation-level filtering to filter the `price_ranges` aggregation based on selected brands, `BrandA` and `BrandB`, without affecting the main `price_ranges` aggregation. This allows you to display price ranges relevant to the selected brands while still showing overall price ranges for all products.
+
+```
+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 }
+            ]
+          }
+        }
+      }
+    }
+  }
+}
+```
+
+## Summary of filters used for filtering the search results:
+1. Query-level filtering: Apply `boolean` query filter clauses to filter both search hits and aggregations, such as narrowing results to specific categories or brands.
+2. Post-filtering: Use `post_filter` to refine search hits based on user selections, while keeping all aggregation options, like brands, visible and unaffected.
+3. Aggregation-level filtering: Adjust specific aggregations, like price ranges, based on selected filters without impacting other aggregations, such as the brand list.
+