Database upgrade; may fail with an error "Key does not exist in collection."

The database upgrade process may fail with an error "Failed to Upgrade database. Failed to drop Extended Indexes. Failed to retrieve Database schema. Key does not exist in collection.".

This may be caused by indexes or statistics being created by the Database Engine Tuning Advisor (DTA). The table will have index names beginning with "_dta_".

The upgrade process was not handling these indexes correctly.

Note, in most cases this wizard would not have been run and therefore there is no issue.