Optimize IssuesFinder redundent confidential check when assigned

This query is sub-optimal and is frequently reported in the top 15 in
total time taken due to the fact that this is used to count the number
of assigned open issues on header bar which is loaded on every page
load.

Since a common use case is filtering by assigned to self (ie. the count
of assigned issues) we can optimize this query by not bothering to check
for confidentiality as it doesn't affect the outcome as you can see
assigned issues regardless of confidentiality.

app/finders/issues_finder.rb

@@ -47,6 +47,15 @@ class IssuesFinder < IssuableFinder
   # rubocop: disable CodeReuse/ActiveRecord
   def with_confidentiality_access_check
     return Issue.all if params.user_can_see_all_confidential_issues?
+
+    if Feature.enabled?(:optimize_issue_filter_assigned_to_self, default_enabled: :yaml)
+      # If already filtering by assignee we can skip confidentiality since a user
+      # can always see confidential issues assigned to them. This is just an
+      # optimization since a very common usecase of this Finder is to load the
+      # count of issues assigned to the user for the header bar.
+      return Issue.all if current_user && params.assignees.include?(current_user)
+    end
+
     return Issue.where('issues.confidential IS NOT TRUE') if params.user_cannot_see_confidential_issues?
 
     Issue.where('

changelogs/unreleased/325470-optimize-confidentiality-filter-on-issue-finder.yml

+---
+title: Optimize database performance of loading assigned issue count on header bar
+merge_request: 57073
+author:
+type: performance

config/feature_flags/development/optimize_issue_filter_assigned_to_self.yml

+---
+name: optimize_issue_filter_assigned_to_self
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/57073
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/325470
+milestone: '13.11'
+type: development
+group: group::global search
+default_enabled: false

spec/support/shared_examples/finders/assignees_filter_shared_examples.rb

 # frozen_string_literal: true
 
 RSpec.shared_examples 'assignee ID filter' do
+  context 'when optimize_issue_filter_assigned_to_self is disabled' do
+    before do
+      stub_feature_flags(optimize_issue_filter_assigned_to_self: false)
+    end
+
+    it 'returns issuables assigned to that user' do
+      expect(issuables).to contain_exactly(*expected_issuables)
+    end
+  end
+
   it 'returns issuables assigned to that user' do
     expect(issuables).to contain_exactly(*expected_issuables)
   end
@@ -13,6 +23,16 @@ RSpec.shared_examples 'assignee NOT ID filter' do
 end
 
 RSpec.shared_examples 'assignee username filter' do
+  context 'when optimize_issue_filter_assigned_to_self is disabled' do
+    before do
+      stub_feature_flags(optimize_issue_filter_assigned_to_self: false)
+    end
+
+    it 'returns issuables assigned to those users' do
+      expect(issuables).to contain_exactly(*expected_issuables)
+    end
+  end
+
   it 'returns issuables assigned to those users' do
     expect(issuables).to contain_exactly(*expected_issuables)
   end