fabianmenges commented on issue #3920: [Bugfix] Single Druid dimension spec error (part 2) URL: https://github.com/apache/incubator-superset/pull/3920#issuecomment-346372377 > Curious, why is there a branch to specifically handle a single dimension (which can't be a spec), with no having_filters and order_desc = True? I can bring some light into this. [TopN](http://druid.io/docs/latest/querying/topnquery.html) queries in Druid are much faster than [groupby](http://druid.io/docs/latest/querying/groupbyquery.html) queries, which is why if you could use either one, your definitely want to use `TopN` (look it gets mentioned in the `groupBy` spec). `TopN` queries are much more limited than `groupBy` queries and also don't guarantee correctness. Their main limitations is that they can only handle a single dimension and you cannot specify the sort order. The much better performance and their broad use case of `TopN` queries justify their use case. We run a very large Druid cluster (>300 machines) with a lot data and it makes a huge difference.
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