Just wondering an Insight in the difference between these two indexes IdKey / PrimaryKey =================  Property Identifier As %Integer Index Index1 on Identifier [Idkey] Index Index2 on Identifier [PrimaryKey] **What's the difference?** 1. If I don't have Index1 and only have Index2,  then cache does still make its own id. So how and why  do I ever use the PrimaryKey.  In Joins ?? Table1.Identifier = Table2.Identifier instead of Table1.Id = Table2.id ?? But I can still use Table1.Id = Table2.Id as cache still made one ID field So where is PrimaryKey useful in cache? 2. If I declare Index1 , I am not able to have any Bitmap indexes [Cache throws an error on compilation saying I have an Idkey index]