Optimise SQL query

app/services/ci/register_build_service.rb

@@ -7,10 +7,14 @@ module Ci
 
       builds =
         if current_runner.shared?
-          # this returns builds that are ordered by number of running builds
-          # we prefer projects that don't use shared runners at all
-          builds.joins("JOIN (#{projects_with_builds_for_shared_runners.to_sql}) AS projects ON ci_builds.gl_project_id=projects.gl_project_id").
-            order('projects.running_builds ASC', 'ci_builds.id ASC')
+          builds.
+            # don't run projects which have not enables shared runners
+            joins(:project).where(projects: { builds_enabled: true, shared_runners_enabled: true }).
+
+            # this returns builds that are ordered by number of running builds
+            # we prefer projects that don't use shared runners at all
+            joins("LEFT JOIN (#{running_builds_for_shared_runners.to_sql}) AS project_builds ON ci_builds.gl_project_id=project_builds.gl_project_id").
+            order('COALESCE(project_builds.running_builds, 0) ASC', 'ci_builds.id ASC')
         else
           # do run projects which are only assigned to this runner (FIFO)
           builds.where(project: current_runner.projects.where(builds_enabled: true)).order('created_at ASC')
@@ -38,15 +42,9 @@ module Ci
 
     private
 
-    def projects_with_builds_for_shared_runners
-      Ci::Build.running_or_pending.
-        joins(:project).where(projects: { builds_enabled: true, shared_runners_enabled: true }).
-        group(:gl_project_id).
-        select(:gl_project_id, "count(case when status = 'running' AND runner_id = (#{shared_runners.to_sql}) then 1 end) as running_builds")
-    end
-
-    def shared_runners
-      Ci::Runner.shared.select(:id)
+    def running_builds_for_shared_runners
+      Ci::Build.running.where(runner: Ci::Runner.shared).
+        group(:gl_project_id).select(:gl_project_id, 'count(*) AS running_builds')
     end
   end
 end