This is obviously neither Oracle nor DB2 feature, but I like it so much that I have put it in my wishlist. For traditional row-based RDBMS, OLAP kind of activities can really hurt performance. On the flip side, newer column-based databases excel in OLAP, but suffer in OLTP type of activities. Forget about throwing in SSD and terrabytes of RAM to 'solve' the issues ( eg: Oracle's Exadata, IBM's pureSparta ), this new kind of index offers partial relief to traditional RBDMS in OLAP system. 'Best' of both worlds at a fairly low cost.
Ellison, are you listening?