Reindexing : When do you use 'Update on schedule' or 'update on save'?

Update on Save :The index is set to update whenever a change is saved.

Update on Schedule :The index is set to update on schedule according to a cron job.

• On a small store,Update on Save is fine and will add very little load.
• If you have a larger store with multiple admins, working in the backend then your indexes could be getting triggered constantly. Update on Save ties up MySQL and for large stores can take hours to complete. To avoid this, it is recommended setting your indexers to Update on Schedule. Then only run the indexers as needed (via command) or via a scheduled cron task.

Update on save (realtime): Indexed data is updated as soon as a change is made in the Admin. (For example, the category products index is reindex after products are added to a category in the Admin.) This is the default.

Update by schedule (schedule): Data is indexed according to the schedule set by your Magento cron job.

Update by schedule (schedule): Using a cron job (preferred because indexing runs every minute)

Update on save : Using the magento indexer:reindex [indexer] command, which reindexes selected indexers, or all indexers, one time only