In Druid adapter, deduce columns by running a "segmentMetadata" query. Currently a Druid model must contain "dimensions" and "metrics" fields. If either of these are absent, Calcite should run a segment metadata query and take the resulting "columns" and "aggregators" fields and make them into columns.
The effect will be that the Druid adapter will be easier to configure. You will be able to connect without a model, per
By the way, those URLs are the same as the defaults, and if you don't care about the schema name the following will suffice:
It will also adapt to schema changes. If there are multiple segments, and the schema evolves over time, I suppose that the segments might have different columns and aggregators. Calcite should use "merge": true, "lenientAggregatorMerge": false to combine them.