Skip projects filter on issues search

When searching for issues, an additional subquery
is added which filters only issues in a project. If global context is
used (no project is specified) this query filters all projects user has
access to.

In that case we can skip this filter because filtering only projects
user has access to is added anyway.

The filter is used only if a custom project context is specified

Related to #40540

changelogs/unreleased/16117-improve-search-for-issues.yml

+---
+title: Improve search query for issues.
+merge_request:
+author:
+type: performance

lib/gitlab/search_results.rb

@@ -82,7 +82,10 @@ module Gitlab
     end
 
     def issues
-      issues = IssuesFinder.new(current_user).execute.where(project_id: project_ids_relation)
+      issues = IssuesFinder.new(current_user).execute
+      unless default_project_filter
+        issues = issues.where(project_id: project_ids_relation)
+      end
 
       issues =
         if query =~ /#(\d+)\z/

spec/lib/gitlab/search_results_spec.rb

@@ -52,18 +52,39 @@ describe Gitlab::SearchResults do
       expect(results.objects('merge_requests')).to include merge_request_2
     end
 
+    describe '#merge_requests' do
       it 'includes project filter by default' do
         expect(results).to receive(:project_ids_relation).and_call_original
+
         results.objects('merge_requests')
       end
 
-    it 'it skips project filter if default is used' do
+      it 'it skips project filter if default project context is used' do
         allow(results).to receive(:default_project_filter).and_return(true)
+
         expect(results).not_to receive(:project_ids_relation)
+
         results.objects('merge_requests')
       end
     end
 
+    describe '#issues' do
+      it 'includes project filter by default' do
+        expect(results).to receive(:project_ids_relation).and_call_original
+
+        results.objects('issues')
+      end
+
+      it 'it skips project filter if default project context is used' do
+        allow(results).to receive(:default_project_filter).and_return(true)
+
+        expect(results).not_to receive(:project_ids_relation)
+
+        results.objects('issues')
+      end
+    end
+  end
+
   it 'does not list issues on private projects' do
     private_project = create(:project, :private)
     issue = create(:issue, project: private_project, title: 'foo')