DB schema generated by a migration may look different in rails 4 and 5 (because rails 5 may use different default values). For this reason it's important to explicitly set for which rails version a migration was written for. See https://stackoverflow.com/questions/35929869/activerecordmigration-deprecation-warning-asks-for-rails-version-but-im-no/35930912#35930912
By adding various composite indexes we can reduce the time spent retrieving issue lists. Because of the way these indexes are built column wise we can also remove some standalone indexes, keeping the total number of indexes in check.