I just loaded some NHD line feature classes and an EROM table into a Enterprise Geodatabase based on 10.3.1, SQL Server 2012, and was going to make a join, but got the warning that there was no index built for the join attribute. Went to my data owner connection in ArcCatalog 10.3.1, right click > properties>Indexes to add the index to the feature class, but found that add index is greyed out. I saw that there is already an index for OBJECTID and an auto grid spatial index, but can't figure out how I can add a new index. Add Index is also grayed out with my sa and my sde connection.
The heck of it is that I did this a couple months ago at a different 10.3.1, SQL Server, ArcServer installation. I did a Remote Desktop via VPN to this other location, inspected the properties of the same feature class with ArcCatalog, and saw that I had already built this index for this installation, and Add Index is not grayed out. The Join is still working at this remote installation.
What did I forget to build on this current installation that is preventing me from adding my Index.