Merge pull request #17 from DataRecce/feature/drc-1302-model-to-column-document-update

CLL: Add impact radius of a column

Author: DaveFlynn

docs/assets/images/features/cll-1.png

@@ -13,14 +13,18 @@ Common use-cases for column-level lineage are
 
 ## Usage
 
-1. Select a node in the lineage DAG, then click the **eye** icon next to the column you want to view.
+1. Select a node in the lineage DAG, then click the the column you want to view.
 
     ![alt text](../assets/images/features/cll-1.png){: .shadow}
 
 1. The column-level lineage for the selected column will be displayed.
 
     ![alt text](../assets/images/features/cll-2.png){: .shadow}
 
+1. To exit column-level lineage view, click the close button in the upper-left corner.
+    
+    ![alt text](../assets/images/features/cll-3.png){: .shadow}
+
 ## Transformation Types
 
 The transformation type is also displayed for each column, which will help you understand how the column was generated or modified.
@@ -34,6 +38,69 @@ The transformation type is also displayed for each column, which will help you u
 | Unknown | We have no information about the transformation type. This could be due to a parse error, or other unknown reason. | 
 
 
-## Limitation
+## Impact Radius of a Column
+
+The **right side of the Column-Level Lineage (CLL)** graph represents the **impact radius** of a selected column.
+This view helps you quickly understand what will be affected if that column changes.
+
+### What does the impact radius include?
+
+- **Downstream columns** that directly reference the selected column
+- **Downstream models** that directly depend on the selected column
+- **All indirect downstream columns and models** that transitively depend on it
+
+This helps you evaluate both the direct and downstream effects of a column change, making it easier to understand its overall impact.
+
+
+### Example: Simplified Model Chain
+
+Given the following models, here's how changes to `stg_orders.status` would impact downstream models:
+
+```sql
+-- stg_orders.sql
+select
+  order_id,
+  customer_id,
+  status,
+  ...
+from {{ ref("raw_orders") }}
+
+
+-- orders.sql
+select
+  order_id,
+  customer_id,
+  status,
+  ...
+from {{ ref("stg_orders") }}
+
+
+-- customers.sql
+select
+  c.customer_id,
+  ...
+from {{ ref("stg_customers") }} as c
+join {{ ref("stg_orders") }} as o
+  on c.customer_id = o.customer_id
+where o.status = 'completed'
+group by c.customer_id
+
+
+-- customer_segments.sql
+select
+  customer_id,
+  ...
+from {{ ref("customers") }}
+```
+
+![alt text](../assets/images/features/cll-example.png){: .shadow}
+
+The following impact is detected:
+
+- **orders**: This model is partially impacted, as it selects the `status` column directly from `stg_orders` but does not apply any transformation or filtering logic. The change is limited to the `status` column only.
+
+- **customers**: This model is fully impacted, because it uses `status` in a WHERE clause (`where o.status = 'completed'`). Any change to the logic in `stg_orders.status` can affect the entire output of the model.
+
+- **customer_segments**: This model is indirectly impacted, as it depends on the `customers` model, which itself is fully impacted. Even though `customer_segments` does not directly reference `status`, changes can still propagate downstream via its upstream dependency.
+
 
-Column-level lineage only displays column selection operations. It does not indicate if a column has been used in filters (WHERE clauses), with grouping (GROUP BY), joins, or other transformations.