Some indexes exist only created in ABAP dictionary. But in some situations, if specifically indicated by SAP, these can be also activated on the database level. For example this would be one scenario, described in the note 2489084.
What are the necessary steps to activate an index at DB level?
Go to SE11 enter the name of the table of which the index belongs -> Display
Then on Indexes button -> pick the corresponding index
And you will see that the index is marked as a “no database index” and under it states “Index does not exist in database system ORACLE”
Go to change and select instead “Index on all database systems”
A transport request will have to be created for this change that will record the modifications made.
After this option is saved, the index will become inactive, go to the same SE11 transaction and activate it 
Then go to SE14, enter the name of the corresponding table -> Edit -> Indexes -> Select the wanted index
You will also see here the message “Does not exist in the database”. At the processing type, select Background, as this will be a time consuming action and then the button Create database index.
You can monitor the progress of this action in SM37:
After the job is completed you can see that the index exists now also created on the DB side:
In order to pass the modification into the next systems go to SE09, release the transport and then import the TR in the target systems. No other action will be needed on the target system.