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Calcite throws this error when it cannot figure out how to plan an SQL query. The extension you are writing, are you skipping some conversion in some case. You may want to set a planning error when you do skip conversion. E.g. - https://github.com/apache/druid/blob/b0e52c99bbc248d54b43e5f06880a4c13be00383/sql/[…]main/java/org/apache/druid/sql/calcite/rule/DruidUnionRule.java