Apache Druid is a distributed, high-performance columnar store for real-time analytics on a large dataset. Druid core design combines the OLAP analytics, time series database and search system to create a single operational analysis. Druid is most suitable for data with high cardinality column or queries having higher aggregation or group by.
Druid has very specific use cases. If you don’t fit one of the use cases, it’s not a good solution at all; but if you do fit one of the use cases, it’s excellent.