To summarize:  ID Apps uses Compound Indexes alot.  One interesting side effect is that any time there are multiple columns and it has a sort icon, then there is an underlying compound index.

With Indexes there are a couple of types. The standard index MIGHT already exist as a System index.

When a attribute exists, with more than 25 values, or any value is longer than 2048 characters eDir automatically makes an index of type System.  The problem is, as a normal Admin you cannot change it from System to the normal index type, since it does something different with it at the FLAIM level (The underlying Database under eDirectory) and you need to contact support to help convert it away from the FLAIM Container so you can then make a manual index.  After which eDir won't try to make it a FLAIM container again.

Once an attribute is 'containerized' via FLAIM, it cannot be used in a Compound index.  I ran into this with CN, where some object was created with a very long CN that caused the attribute to containerizedm and in fact forced eDir to delete the compound indexes that contained CN, so we lost functionality when it happened