You shouldn't need to schedule that activity to run. If you decide you need to rebuild the indexes, then go ahead and run it manually. Although, I'm not 100% clear, are you talking about a rule-declare-index column or an exposed column. Have you confirmed that running the activity solves your problem? If so that's a good data point for someone trying to investigate what's going wrong on your system. Generally speaking, that should all just work and if it's regularly not working, there is probably some sort of problem that should be investigated.