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

Question

In sql terms we\'re storing data like this:

table events (
  id
  timestamp
  dimension1
  dimension2
  dimension3
  etc.
)

All dimension value

Answer 1

"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).

Systems like Twitter's Rainbird (which uses Cassandra) doing realtime analytics do it by denormalizing/pre-computing the counts: http://www.slideshare.net/kevinweil/rainbird-realtime-analytics-at-twitter-strata-2011