Merge pull request #6350 from EnterpriseDB/DOCS-1142

Added documentation for sub-JSON accessing feature in Mongo FDW Key Features

Author: jpe442

product_docs/docs/mongo_data_adapter/5/06_features_of_mongo_fdw.mdx

@@ -96,3 +96,61 @@ Steps for retrieving the document:
     { "_id" : { "$oid" : "58a1ebbaf543ec0b9054585a" }, "warehouse_id" : 2, "warehouse_name" : "Laptop", "warehouse_created" : { "$date" : 1447229590000 } }
     (2 rows)
     ```
+
+## Accessing nested fields
+
+MongoDB Foreign Data Wrapper allows you to access individual fields within nested JSON documents by mapping the nested structure to columns in a foreign table. 
+This works by mapping the nested structure of the MongoDB document to relational columns in the foreign table definition, using dot notation (key2.subkey21) to reference nested fields.
+You can retrieve these fields from a collection as shown in the following example:
+
+### Example
+
+```text
+db1> db.test_sub_json.find()
+[
+  {
+    _id: ObjectId('658040214890799d6e0173d0'),
+    key1: 'hello',
+    key2: {
+      subkey21: 'hello-sub1',
+      subkey22: 'hello-sub2',
+      subtstmp: ISODate('2022-12-16T19:16:17.801Z')
+    }
+  }
+]
+```
+
+Steps for retrieving sub-fields from the document:
+
+1.  Create a foreign table. To access a sub-field use the dot (".") in the column name as shown below:
+
+    ```sql
+    CREATE FOREIGN TABLE ft_nested_json_test(
+        _id NAME,
+        key1 varchar,
+        "key2.subkey21"   varchar,
+        "key2.subkey22"   varchar,
+        "key2.subtstmp"   timestamp
+    )SERVER mongo_server
+    OPTIONS (database 'db1', collection 'test_sub_json');
+    ```
+
+1. Retrieve the document with sub-fields:
+
+    ```sql
+    SELECT * FROM ft_nested_json_test;
+    __OUTPUT__
+               _id            | key1  | key2.subkey21 | key2.subkey22 |     key2.subtstmp      
+    --------------------------+-------+---------------+---------------+------------------------
+     658040214890799d6e0173d0 | hello | hello-sub1    | hello-sub2    | 16-DEC-22 19:16:17.801
+    ```
+
+1. Retrieve an individual field:
+
+    ```sql
+    SELECT "key2.subkey21" FROM ft_nested_json_test;
+    __OUTPUT__
+     key2.subkey21 
+    ---------------
+     hello-sub1
+    ```