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I believe this is expected as per the guide, Could you please try setting

useGroupingSetForExactDistinct

When useApproximateCountDistinct is set to "false", the computation will be exact. In this case, expr must be string or numeric, since exact counts are not possible using prebuilt sketches. In exact mode, only one distinct count per query is permitted unless useGroupingSetForExactDistinct is enabled.

https://druid.apache.org/docs/latest/querying/sql-aggregations.html

Thank you , it’s solved my problem, I think I know the reason, thank you again!!