What can be the use of an external ID field, not marked as unique? Helps in indexing?
I checked the documentation , and found that to upsert records with an external ID, there is no requirement that the external ID be unique. So, if I upsert records using a non-unique external ID, will I not get error saying "Duplicate External IDs" found? If not, how will the upsert pick records that have duplicate values in this external ID field?