Improve the performance of MR analytics page

This change adds a feature flag which alters the COUNT query to only
access the merge_request_metrics table. This happens when loading the
data in the project level merge request analytics page.

app/finders/merge_request/metrics_finder.rb

+# frozen_string_literal: true
+
+class MergeRequest::MetricsFinder
+  include Gitlab::Allowable
+
+  def initialize(current_user, params = {})
+    @current_user = current_user
+    @params = params
+  end
+
+  def execute
+    items = init_collection
+    return items.none if target_project_missing? || user_not_authorized?
+
+    items = by_target_project(items)
+    items = by_merged_after(items)
+    items = by_merged_before(items)
+
+    items
+  end
+
+  private
+
+  attr_reader :current_user, :params
+
+  def by_target_project(items)
+    items.by_target_project(target_project)
+  end
+
+  def by_merged_after(items)
+    items = items.merged_after(params[:merged_after]) if params[:merged_after]
+
+    items
+  end
+
+  def by_merged_before(items)
+    items = items.merged_before(params[:merged_before]) if params[:merged_before]
+
+    items
+  end
+
+  def target_project_missing?
+    params[:target_project].blank?
+  end
+
+  def user_not_authorized?
+    !can?(current_user, :read_merge_request, target_project)
+  end
+
+  def init_collection
+    klass.all
+  end
+
+  def klass
+    MergeRequest::Metrics
+  end
+
+  def target_project
+    params[:target_project]
+  end
+end

app/graphql/resolvers/project_merge_requests_resolver.rb

@@ -6,5 +6,36 @@ module Resolvers
     accept_assignee
     accept_author
     accept_reviewer
+
+    def resolve(**args)
+      scope = super
+
+      if only_count_is_selected_with_merged_at_filter?(args) && Feature.enabled?(:optimized_merge_request_count_with_merged_at_filter)
+        MergeRequest::MetricsFinder
+          .new(current_user, args.merge(target_project: project))
+          .execute
+      else
+        scope
+      end
+    end
+
+    def only_count_is_selected_with_merged_at_filter?(args)
+      return unless lookahead
+
+      argument_names = args.keys
+      argument_names.delete(:lookahead)
+      argument_names.delete(:sort)
+      argument_names.delete(:merged_before)
+      argument_names.delete(:merged_after)
+
+      # Detecting a specific query pattern:
+      # mergeRequests(mergedAfter: "X", mergedBefore: "Y") {
+      #   count
+      # }
+      lookahead.selects?(:count) &&
+        lookahead.selections.size == 1 && # no other nodes are selected
+        (args[:merged_after] || args[:merged_before]) &&
+        argument_names.empty? # no extra filtering arguments are provided
+    end
   end
 end

app/models/merge_request/metrics.rb

@@ -5,12 +5,14 @@ class MergeRequest::Metrics < ApplicationRecord
   belongs_to :pipeline, class_name: 'Ci::Pipeline', foreign_key: :pipeline_id
   belongs_to :latest_closed_by, class_name: 'User'
   belongs_to :merged_by, class_name: 'User'
+  belongs_to :target_project, class_name: 'Project'
 
   before_save :ensure_target_project_id
 
   scope :merged_after, ->(date) { where(arel_table[:merged_at].gteq(date)) }
   scope :merged_before, ->(date) { where(arel_table[:merged_at].lteq(date)) }
   scope :with_valid_time_to_merge, -> { where(arel_table[:merged_at].gt(arel_table[:created_at])) }
+  scope :by_target_project, ->(project) { where(target_project_id: project) }
 
   def self.time_to_merge_expression
     Arel.sql('EXTRACT(epoch FROM SUM(AGE(merge_request_metrics.merged_at, merge_request_metrics.created_at)))')

config/feature_flags/development/optimized_merge_request_count_with_merged_at_filter.yml

+---
+name: optimized_merge_request_count_with_merged_at_filter
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/52113
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/299347
+milestone: '13.8'
+type: development
+group: group::optimize
+default_enabled: false

spec/finders/merge_request/metrics_finder_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe MergeRequest::MetricsFinder do
+  let_it_be(:current_user) { create(:user) }
+  let_it_be(:project) { create(:project, :repository) }
+  let_it_be(:merge_request_not_merged) { create(:merge_request, :unique_branches, source_project: project) }
+  let_it_be(:merge_request_merged) do
+    create(:merge_request, :unique_branches, :merged, source_project: project).tap do |mr|
+      mr.metrics.update!(merged_at: Time.new(2020, 5, 1))
+    end
+  end
+
+  let(:merged_at) { merge_request_merged.metrics.merged_at }
+  let(:params) do
+    {
+      target_project: project,
+      merged_after: merged_at - 10.days,
+      merged_before: merged_at + 10.days
+    }
+  end
+
+  subject { described_class.new(current_user, params).execute.to_a }
+
+  context 'when target project is missing' do
+    before do
+      params.delete(:target_project)
+    end
+
+    it { expect(subject).to be_empty }
+  end
+
+  context 'when the user is not part of the project' do
+    it { expect(subject).to be_empty }
+  end
+
+  context 'when user is part of the project' do
+    before do
+      project.add_developer(current_user)
+    end
+
+    it 'returns merge request records' do
+      expect(subject).to eq([merge_request_merged.metrics])
+    end
+
+    it 'excludes not merged records' do
+      expect(subject).not_to eq([merge_request_not_merged.metrics])
+    end
+
+    context 'when only merged_before is given' do
+      before do
+        params.delete(:merged_after)
+      end
+
+      it { expect(subject).to eq([merge_request_merged.metrics]) }
+    end
+
+    context 'when only merged_after is given' do
+      before do
+        params.delete(:merged_before)
+      end
+
+      it { expect(subject).to eq([merge_request_merged.metrics]) }
+    end
+
+    context 'when no records matching the date range' do
+      before do
+        params[:merged_before] = merged_at - 1.year
+        params[:merged_after] = merged_at - 2.years
+      end
+
+      it { expect(subject).to be_empty }
+    end
+  end
+end

spec/requests/api/graphql/project/merge_requests_spec.rb

@@ -396,4 +396,66 @@ RSpec.describe 'getting merge request listings nested in a project' do
       end
     end
   end
+
+  context 'when only the count is requested' do
+    context 'when merged at filter is present' do
+      let_it_be(:merge_request) do
+        create(:merge_request, :unique_branches, source_project: project).tap do |mr|
+          mr.metrics.update!(merged_at: Time.new(2020, 1, 3))
+        end
+      end
+
+      let(:query) do
+        graphql_query_for(:project, { full_path: project.full_path },
+        <<~QUERY
+        mergeRequests(mergedAfter: "2020-01-01", mergedBefore: "2020-01-05", first: 0) {
+          count
+        }
+        QUERY
+        )
+      end
+
+      context 'when "optimized_merge_request_count_with_merged_at_filter" feature flag is enabled' do
+        before do
+          stub_feature_flags(optimized_merge_request_count_with_merged_at_filter: true)
+        end
+
+        it 'does not query the merge requests table for the count' do
+          query_recorder = ActiveRecord::QueryRecorder.new { post_graphql(query, current_user: current_user) }
+
+          queries = query_recorder.data.each_value.first[:occurrences]
+          expect(queries).not_to include(match(/SELECT COUNT\(\*\) FROM "merge_requests"/))
+          expect(queries).to include(match(/SELECT COUNT\(\*\) FROM "merge_request_metrics"/))
+        end
+
+        it 'returns the correct count' do
+          post_graphql(query, current_user: current_user)
+
+          count = graphql_data.dig('project', 'mergeRequests', 'count')
+          expect(count).to eq(1)
+        end
+
+        context 'when "optimized_merge_request_count_with_merged_at_filter" feature flag is disabled' do
+          before do
+            stub_feature_flags(optimized_merge_request_count_with_merged_at_filter: false)
+          end
+
+          it 'does not query the merge requests table for the count' do
+            query_recorder = ActiveRecord::QueryRecorder.new { post_graphql(query, current_user: current_user) }
+
+            queries = query_recorder.data.each_value.first[:occurrences]
+            expect(queries).to include(match(/SELECT COUNT\(\*\) FROM "merge_requests"/))
+            expect(queries).not_to include(match(/SELECT COUNT\(\*\) FROM "merge_request_metrics"/))
+          end
+
+          it 'returns the correct count' do
+            post_graphql(query, current_user: current_user)
+
+            count = graphql_data.dig('project', 'mergeRequests', 'count')
+            expect(count).to eq(1)
+          end
+        end
+      end
+    end
+  end
 end