Today's topic is a general primer on something that is called "local-global aggregation".  Effectively, this is a technique to allow you to take a query containing joins and group bys and perform the group by partially "before" the joins.  This can dramatically reduce the number of rows that a query has to process to return a result, and therefore it is a very powerful operation.  This technique is also quite useful to split a grouping operation into something that can be done in parallel.  That obviously can also speed your query as well.

I hope to put something more formal together soon, work permitting, but here is an introduction that you can read in the meantime:

http://citeseer.ist.psu.edu/jaedicke97framework.html

Thanks,

Conor