Improve issue load time performance by avoiding ORDER BY in find_by call

The Sortable concern has a default scope that adds ORDER BY to all
queries. EXPLAIN ANALYZE shows that this additional ORDER BY statement
causes the SQL optimizer to use the wrong index, which leads to a load
time of 2.9 s vs 0.073 ms just for the SELECT call. The minimal
change here is to re-implement find_by using where and reorder to
remove the ORDER BY clause in IssuesController#index.

Closes #23075

CHANGELOG

@@ -3,6 +3,7 @@ Please view this file on the master branch, on stable branches it's out of date.
 v 8.13.0 (unreleased)
   - Update runner version only when updating contacted_at
   - Add link from system note to compare with previous version
+  - Improve issue load time performance by avoiding ORDER BY in find_by call
   - Use gitlab-shell v3.6.2 (GIT TRACE logging)
   - Fix centering of custom header logos (Ashley Dumaine)
   - AbstractReferenceFilter caches project_refs on RequestStore when active

app/controllers/projects/issues_controller.rb

@@ -159,7 +159,8 @@ class Projects::IssuesController < Projects::ApplicationController
   protected
 
   def issue
-    @noteable = @issue ||= @project.issues.find_by(iid: params[:id]) || redirect_old
+    # The Sortable default scope causes performance issues when used with find_by
+    @noteable = @issue ||= @project.issues.where(iid: params[:id]).reorder(nil).take || redirect_old
   end
   alias_method :subscribable_resource, :issue
   alias_method :issuable, :issue