Remove cross joins for ProjectMonthlyUsage.for_namespace_monthly_usage

config/feature_flags/development/ci_decompose_for_namespace_monthly_usage_query.yml

+---
+name: ci_decompose_for_namespace_monthly_usage_query
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/77952
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/350146
+milestone: '14.7'
+type: development
+group: group::pipeline execution
+default_enabled: false

ee/app/models/ci/minutes/project_monthly_usage.rb

@@ -12,10 +12,17 @@ module Ci
       scope :current_month, -> { where(date: beginning_of_month) }
 
       scope :for_namespace_monthly_usage, -> (namespace_monthly_usage) do
-        where(
-          date: namespace_monthly_usage.date,
-          project: namespace_monthly_usage.namespace.projects
-        ).allow_cross_joins_across_databases(url: 'https://gitlab.com/gitlab-org/gitlab/-/issues/343301')
+        if Feature.enabled?(:ci_decompose_for_namespace_monthly_usage_query, namespace_monthly_usage.namespace, default_enabled: :yaml)
+          where(
+            date: namespace_monthly_usage.date,
+            project: Ci::ProjectMirror.by_namespace_id(namespace_monthly_usage.namespace_id).select(:project_id)
+          )
+        else
+          where(
+            date: namespace_monthly_usage.date,
+            project: namespace_monthly_usage.namespace.projects
+          ).allow_cross_joins_across_databases(url: 'https://gitlab.com/gitlab-org/gitlab/-/issues/343301')
+        end
       end
 
       def self.beginning_of_month(time = Time.current)

ee/spec/models/ci/minutes/project_monthly_usage_spec.rb

@@ -77,17 +77,38 @@ RSpec.describe Ci::Minutes::ProjectMonthlyUsage do
   end
 
   describe '.for_namespace_monthly_usage' do
-    it "fetches project monthly usages matching the namespace monthly usage's date and namespace" do
-      date_for_usage = Date.new(2021, 5, 1)
-      date_not_for_usage = date_for_usage + 1.month
-      namespace_usage = create(:ci_namespace_monthly_usage, namespace: project.namespace, amount_used: 50, date: date_for_usage)
-      matching_project_usage = create(:ci_project_monthly_usage, project: project, amount_used: 50, date: date_for_usage)
-      create(:ci_project_monthly_usage, project: project, amount_used: 50, date: date_not_for_usage)
-      create(:ci_project_monthly_usage, project: create(:project), amount_used: 50, date: date_for_usage)
+    shared_examples 'namespace monthly usage' do
+      let(:date_for_usage) { Date.new(2021, 5, 1) }
+      let(:namespace_usage) { create(:ci_namespace_monthly_usage, namespace: project.namespace, amount_used: 50, date: date_for_usage) }
 
-      project_usages = described_class.for_namespace_monthly_usage(namespace_usage)
+      it "fetches project monthly usages matching the namespace monthly usage's date and namespace" do
+        date_not_for_usage = date_for_usage + 1.month
+        matching_project_usage = create(:ci_project_monthly_usage, project: project, amount_used: 50, date: date_for_usage)
+        create(:ci_project_monthly_usage, project: project, amount_used: 50, date: date_not_for_usage)
+        create(:ci_project_monthly_usage, project: create(:project), amount_used: 50, date: date_for_usage)
 
-      expect(project_usages).to contain_exactly(matching_project_usage)
+        project_usages = described_class.for_namespace_monthly_usage(namespace_usage)
+
+        expect(project_usages).to contain_exactly(matching_project_usage)
+      end
+
+      it 'does not join across databases' do
+        with_cross_joins_prevented do
+          described_class.for_namespace_monthly_usage(namespace_usage)
+        end
+      end
+    end
+
+    context 'when ci_decompose_for_namespace_monthly_usage_query is enabled' do
+      it_behaves_like 'namespace monthly usage'
+    end
+
+    context 'when ci_decompose_for_namespace_monthly_usage_query is disabled' do
+      before do
+        stub_feature_flags(ci_decompose_for_namespace_monthly_usage_query: false)
+      end
+
+      it_behaves_like 'namespace monthly usage'
     end
   end
 end