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I don’t think there is an explicit maximum. But too many column merges at once during the segment building phase could get you in trouble with memory. You can throttle that step with maxColumnsToMerge parameter.

I would say generally tens of columns is 100% totally fine, hundreds is potentially fine depending on how big they are, and thousands of columns is pushing it If you find you need a lot of columns because you have a lot of complexity in your input data, you might prefer a design involving nested columns instead: https://druid.apache.org/docs/latest/querying/nested-columns.html