Improve stages index migration code readability

lib/gitlab/background_migration/migrate_stage_index.rb

@@ -4,18 +4,20 @@
 module Gitlab
   module BackgroundMigration
     class MigrateStageIndex
-      module Migratable
-        class Stage < ActiveRecord::Base
-          self.table_name = 'ci_stages'
+      def perform(start_id, stop_id)
+        migrate_stage_index_sql(start_id.to_i, stop_id.to_i).tap do |sql|
+          ActiveRecord::Base.connection.execute(sql)
         end
       end
 
-      def perform(start_id, stop_id)
+      private
+
+      def migrate_stage_index_sql(start_id, stop_id)
         if Gitlab::Database.postgresql?
-          sql = <<~SQL
+          <<~SQL
             WITH freqs AS (
               SELECT stage_id, stage_idx, COUNT(*) AS freq FROM ci_builds
-                WHERE stage_id BETWEEN #{start_id.to_i} AND #{stop_id.to_i}
+                WHERE stage_id BETWEEN #{start_id} AND #{stop_id}
                   AND stage_idx IS NOT NULL
                 GROUP BY stage_id, stage_idx
             ), indexes AS (
@@ -29,18 +31,16 @@ module Gitlab
                 AND ci_stages.index IS NULL;
           SQL
         else
-          sql = <<~SQL
+          <<~SQL
             UPDATE ci_stages
               SET index =
                 (SELECT stage_idx FROM ci_builds
                   WHERE ci_builds.stage_id = ci_stages.id
                   GROUP BY ci_builds.stage_idx ORDER BY COUNT(*) DESC LIMIT 1)
-            WHERE ci_stages.id BETWEEN #{start_id.to_i} AND #{stop_id.to_i}
+            WHERE ci_stages.id BETWEEN #{start_id} AND #{stop_id}
               AND ci_stages.index IS NULL
           SQL
         end
-
-        ActiveRecord::Base.connection.execute(sql)
       end
     end
   end