Which database to choose (Cassandra, MongoDB,?) for storing and querying event / log / metrics data?

Group by" and "stupidly fast" do not go together. That's just the nature of that beast... Hence the limitations on Mongo's group operation; Cassandra doesn't even support it natively (although it does for Hive or Pig queries via Hadoop... but those are not intended to be stupidly fast).

To clarify, this is 10,000 rows returned. In your example, this will work for up to 10,000 combinations of dimension1/dimension2. If that's too large, then you can also use the slower Map / Reduce.

Note that if you're running a query with more than 10k results, it may best to use Map / Reduce and save this data. 10k is a large query result to otherwise just "throw away".

I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.