What is rollup in a nutshell?

Rollup is a form of summarization or pre-aggregation. Or, as stated by one of the PMC members:

At ingest time rollup simply means apply a transformation to my data that is logically a simple group by (by all dimensions, all metrics are aggregates).

Rollup helps Druid be a high performance real-time analytics database.

Here’s some context, and here’s a great video demonstration:

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This is such a critical technique in Druid – nice to see it highlighted!