Queries are made using an HTTP REST style request to queryable nodes (Broker, Historical, or Realtime). The query is expressed in JSON and each of these node types expose the same REST query interface. For normal Druid operations, queries should be issued to the broker nodes.
Druid's native query language is JSON over HTTP, although many members of the community have contributed different client libraries in other languages to query Druid.
Druid's native query is relatively low level, mapping closely to how computations are performed internally. Druid queries are designed to be lightweight and complete very quickly. This means that for more complex analysis, or to build more complex visualizations, multiple Druid queries may be required.
Druid has numerous query types for various use cases. Queries are composed of various JSON properties and Druid has different types of queries for different use cases. The documentation for the various query types describe all the JSON properties that can be set.
Where possible, we recommend using Timeseries and TopN queries instead of GroupBy. GroupBy is the most flexible Druid query, but also has the poorest performance. Timeseries are significantly faster than groupBy queries for aggregations that don't require grouping over dimensions. For grouping and sorting over a single dimension, topN queries are much more optimized than groupBys.
Queries can be cancelled explicitly using their unique identifier. If the query identifier is set at the time of query, or is otherwise known, the following endpoint can be used on the broker or router to cancel the query.
For example, if the query ID is
abc123, the query can be cancelled as follows:
curl -X DELETE "http://host:port/druid/v2/abc123"