Apache Pinot™ -- "The real-time analytics open source platform for lightning-fast insights, effortless scaling, and cost-effective data-driven decisions."
What is Apache Pinot? | Apache Pinot 101 - YouTube by Tim Berglund. latency, concurrency, freshness.
15 Years of Realtime OLAP (Part 1) - by Chris Riccomini
Pinot used columnar storage and in-memory buffering. But they also included a new feature: star-tree indexes. Star-trees give operators a knob to tune, trading off space for query latency. Though data was stored in columnar files as with Druid, for very fast (single-digit millisecond) reads, data could be partially pre-materialized. Pinot became widely adopted within LinkedIn and Uber for internal business intelligence, user-facing analytics features, log analytics, metrics, and more.