Use subqueries instead of joins to migrate subscriptions

db/migrate/20161031174110_migrate_subscriptions_project_id.rb

@@ -5,51 +5,36 @@ class MigrateSubscriptionsProjectId < ActiveRecord::Migration
   DOWNTIME_REASON = 'Subscriptions will not work as expected until this migration is complete.'
 
   def up
-    if Gitlab::Database.mysql?
     execute <<-EOF.strip_heredoc
       UPDATE subscriptions
-          INNER JOIN issues ON issues.id = subscriptions.subscribable_id AND subscriptions.subscribable_type = 'Issue'
-        SET subscriptions.project_id = issues.project_id;
-      EOF
-
-      execute <<-EOF.strip_heredoc
-        UPDATE subscriptions
-          INNER JOIN merge_requests ON merge_requests.id = subscriptions.subscribable_id AND subscriptions.subscribable_type = 'MergeRequest'
-        SET subscriptions.project_id = merge_requests.target_project_id;
-      EOF
-
-      execute <<-EOF.strip_heredoc
-        UPDATE subscriptions
-          INNER JOIN labels ON labels.id = subscriptions.subscribable_id AND subscriptions.subscribable_type = 'Label'
-          INNER JOIN projects ON projects.id = labels.project_id
-        SET subscriptions.project_id = projects.id;
-      EOF
-    else
-      execute <<-EOF.strip_heredoc
-        UPDATE subscriptions
-        SET project_id = issues.project_id
+      SET project_id = (
+        SELECT issues.project_id
         FROM issues
-        WHERE issues.id = subscriptions.subscribable_id
-          AND subscriptions.subscribable_type = 'Issue';
+        WHERE issues.id = subscriptions.subscribable_id AND subscriptions.subscribable_type = 'Issue'
+      )
+      WHERE subscriptions.subscribable_type = 'Issue';
     EOF
 
     execute <<-EOF.strip_heredoc
       UPDATE subscriptions
-        SET project_id = merge_requests.target_project_id
+      SET project_id = (
+        SELECT merge_requests.target_project_id
         FROM merge_requests
-        WHERE merge_requests.id = subscriptions.subscribable_id
-          AND subscriptions.subscribable_type = 'MergeRequest';
+        WHERE merge_requests.id = subscriptions.subscribable_id AND subscriptions.subscribable_type = 'MergeRequest'
+      )
+      WHERE subscriptions.subscribable_type = 'MergeRequest';
     EOF
 
     execute <<-EOF.strip_heredoc
       UPDATE subscriptions
-        SET project_id = projects.id
+      SET project_id = (
+        SELECT projects.id
         FROM labels INNER JOIN projects ON projects.id = labels.project_id
-        WHERE labels.id = subscriptions.subscribable_id
-          AND subscriptions.subscribable_type = 'Label';
+        WHERE labels.id = subscriptions.subscribable_id AND subscriptions.subscribable_type = 'Label'
+      )
+      WHERE subscriptions.subscribable_type = 'Label';
     EOF
   end
-  end
 
   def down
     execute <<-EOF.strip_heredoc