Add an example of multiple primary keys

Author: igorlukanin

docs/pages/product/data-modeling/concepts.mdx

@@ -232,6 +232,10 @@ cubes:
 
 </CodeTabs>
 
+Note that the `id` dimension is defined as a [primary key][ref-ref-primary-key].
+It is also possible to have more than one primary key dimension in a cube if
+you'd like them all to be parts of a composite key.
+
 [The `line_items` table](#top) also has a couple of dimensions which can be
 represented as follows:
 
@@ -729,5 +733,6 @@ Pre-Aggregations][ref-caching-preaggs-intro].
 [ref-pmc]: /product/deployment/cloud/deployment-types#production-multi-cluster
 [ref-ref-time-dimensions]: /reference/data-model/types-and-formats#time-1
 [ref-ref-dimension-granularities]: /reference/data-model/dimensions#granularities
+[ref-ref-primary-key]: /reference/data-model/dimensions#primary_key
 [ref-custom-granularity-recipe]: /guides/recipes/data-modeling/custom-granularity
 [ref-proxy-granularity]: /product/data-modeling/concepts/calculated-members#time-dimension-granularity

docs/pages/reference/data-model/dimensions.mdx

@@ -264,17 +264,18 @@ cubes:
 
 ### `primary_key`
 
-Specify which dimension is a primary key for a cube. The default value is
+Specify if a dimension is a primary key for a cube. The default value is
 `false`.
 
 A primary key is used to make [joins][ref-schema-ref-joins] work properly.
 
-<WarningBox>
+<InfoBox>
 
-Setting `primary_key` to `true` will change the default value of `public`
-parameter to `false`. If you still want `public` to be `true`, set it manually.
+Setting `primary_key` to `true` will change the default value of the [`public`
+parameter](#public) to `false`. If you still want `public` to be `true`, set it
+explicitly.
 
-</WarningBox>
+</InfoBox>
 
 <CodeTabs>
 
@@ -306,6 +307,85 @@ cubes:
 
 </CodeTabs>
 
+It is possible to have more than one primary key dimension in a cube if you'd
+like them all to be parts of a composite key:
+
+<CodeTabs>
+
+```javascript
+cube(`products`, {
+  sql: `
+    SELECT 1 AS column_a, 1 AS column_b UNION ALL
+    SELECT 2 AS column_a, 1 AS column_b UNION ALL
+    SELECT 1 AS column_a, 2 AS column_b UNION ALL
+    SELECT 2 AS column_a, 2 AS column_b
+  `,
+
+  dimensions: {
+    composite_key_a: {
+      sql: `column_a`,
+      type: `number`,
+      primary_key: true
+    },
+
+    composite_key_b: {
+      sql: `column_b`,
+      type: `number`,
+      primary_key: true
+    }
+  },
+
+  measures: {
+    count: {
+      type: `count`
+    }
+  }
+})
+```
+
+```yaml
+cubes:
+  - name: products
+    sql: >
+      SELECT 1 AS column_a, 1 AS column_b UNION ALL
+      SELECT 2 AS column_a, 1 AS column_b UNION ALL
+      SELECT 1 AS column_a, 2 AS column_b UNION ALL
+      SELECT 2 AS column_a, 2 AS column_b
+
+    dimensions:
+      - name: composite_key_a
+        sql: column_a
+        type: number
+        primary_key: true
+
+      - name: composite_key_b
+        sql: column_b
+        type: number
+        primary_key: true
+
+    measures:
+      - name: count
+        type: count
+```
+
+</CodeTabs>
+
+Querying the `count` measure of the cube shown above will generate the following
+SQL to the upstream data source:
+
+```sql
+SELECT
+  count(
+    CAST("product".column_a as TEXT) || CAST("product".column_b as TEXT)
+  ) "product__count"
+FROM (
+  SELECT 1 AS column_a, 1 AS column_b UNION ALL
+  SELECT 2 AS column_a, 1 AS column_b UNION ALL
+  SELECT 1 AS column_a, 2 AS column_b UNION ALL
+  SELECT 2 AS column_a, 2 AS column_b
+) AS "product"
+```
+
 ### `propagate_filters_to_sub_query`
 
 When this statement is set to `true`, the filters applied to the query will be