Add the guard clause in #down method too

db/migrate/20180428200509_add_index_to_ci_builds_artifacts_file.rb

@@ -15,6 +15,8 @@ class AddIndexToCiBuildsArtifactsFile < ActiveRecord::Migration
   end
   def down
+    return unless Gitlab::Database.postgresql?
+
     remove_concurrent_index :ci_builds, :artifacts_file
   end
 end