Two of the many performance enhancements in DB2 10.1 are called PED (partial early distinct) and PEA (partial early aggregation). Work is done as early as possible, though only partially, to reduce data volume already in an early stage and speed up the total query. Having said that, I am done. :)
P.S.: This may be only partial information on PED and PEA, but it helps you speed up query performance and is light in reading...