First, the background. I used to have a collection logs and used map/reduce to generate various reports. Most of these reports were based on data from within a single day, so I always had a condition d: SOME_DATE. When the logs collection grew extremely big, inserting became extremely slow (slower than the app we were monitoring was generating logs), even after dropping lots of indexes. So we decided to have each day's data in a separate collection - logs_YYYY-mm-dd - that way indexes are smaller, and we don't even need an index on date. This is cool since most reports (thus map/reduce) are on daily data. However, we have a report where we need to cover multiple days.
And now the question. Is there a way to run a map/reduce (or more precisely, the map) over multiple collections as if it were only one?