Comment by hunterpayne

An aggregation is less work than a join. You are segmenting the data in basically the same way in ideal conditions for a join as you are in an aggregation. Think of an aggregation as an inner join against a table of buckets (plus updating a single value instead of keeping a number of copies around). In practice this holds with aggregation being a linear amount faster than a join over the same data. That delta is the extra work the join needs to do to keep around a list of rows rather than a single value being updated (and in cache) repeatedly. Depending on the data this delta might be quite small. But without a very obtuse aggregation function (maybe ketosis perhaps), the aggregation will be faster. Its updating a single value vs appending to a list with the extra memory overhead this introduces.