Add deepest epic relation level to usage data

Send epics deepest relationship level of instance
to usage ping.

ee/app/models/ee/epic.rb

@@ -156,6 +156,46 @@ module EE
           )
         end
       end
+
+      # Return the deepest relation level for an epic.
+      # Example 1:
+      # epic1 - parent: nil
+      # epic2 - parent: epic1
+      # epic3 - parent: epic 2
+      # Returns: 3
+      # ------------
+      # Example 2:
+      # epic1 - parent: nil
+      # epic2 - parent: epic1
+      # Returns: 2
+      def deepest_relationship_level
+        return nil unless ::Gitlab::Database.postgresql?
+
+        result =
+          ActiveRecord::Base.connection.execute(
+            <<-SQL
+              WITH RECURSIVE descendants AS (
+                  SELECT id, 1 depth
+                  FROM epics
+                  WHERE parent_id IS NOT NULL
+              UNION
+                  SELECT e.id, d.depth+ 1
+                  FROM epics e
+                  INNER JOIN descendants d
+                  ON e.parent_id = d.id
+              )
+              SELECT MAX(depth) as deepest_level
+              FROM descendants
+            SQL
+          )
+
+        deepest_level = result.first["deepest_level"] || 0
+
+        # For performance reason epics without a parent_id are being
+        # ignored in the query.
+        # So we sum 1 to the result to take into account first parent.
+        deepest_level + 1
+      end
     end
 
     def assignees

ee/lib/ee/gitlab/usage_data.rb

@@ -105,8 +105,17 @@ module EE
           projects_with_tracing_enabled: count(ProjectTracingSetting)
         }).merge(service_desk_counts).merge(security_products_usage)
 
+        # MySql does not support recursive queries so we can't retrieve epics relationship depth
+        if ::Gitlab::Database.postgresql?
+          usage_data[:counts] = usage_data[:counts].merge(epics_deepest_relationship_level)
+        end
+
         usage_data
       end
+
+      def epics_deepest_relationship_level
+        { epics_deepest_relationship_level: ::Epic.deepest_relationship_level }
+      end
     end
   end
 end

ee/spec/lib/gitlab/usage_data_spec.rb

@@ -54,6 +54,7 @@ describe Gitlab::UsageData do
       expect(count_data.keys).to include(*%i(
         projects_mirrored_with_pipelines_enabled
         epics
+        epics_deepest_relationship_level
         geo_nodes
         ldap_group_links
         ldap_keys
@@ -73,6 +74,12 @@ describe Gitlab::UsageData do
       expect(count_data[:projects_with_packages]).to eq(2)
     end
 
+    it 'gathers deepest epic relationship level', :postgresql do
+      count_data = subject[:counts]
+
+      expect(count_data.keys).to include(:epics_deepest_relationship_level)
+    end
+
     it 'gathers security products usage data' do
       count_data = subject[:counts]
 

ee/spec/models/epic_spec.rb

@@ -443,6 +443,27 @@ describe Epic do
     end
   end
 
+  describe '.deepest_relationship_level' do
+    it 'returns the deepest relationship level between epics' do
+      group_1 = create(:group)
+      group_2 = create(:group)
+
+      # No relationship
+      create(:epic, group: group_1)
+
+      # Two levels relationship
+      group_1_epic_1 = create(:epic, group: group_1)
+      create(:epic, group: group_1, parent: group_1_epic_1)
+
+      # Three levels relationship
+      group_2_epic_1 = create(:epic, group: group_2)
+      group_2_epic_2 = create(:epic, group: group_2, parent: group_2_epic_1)
+      create(:epic, group: group_2, parent: group_2_epic_2)
+
+      expect(described_class.deepest_relationship_level).to eq(3)
+    end
+  end
+
   describe '#issues_readable_by' do
     let(:user) { create(:user) }
     let(:group) { create(:group, :private) }