Addressed

Author: xuzifu666

core/src/main/java/org/apache/calcite/adapter/enumerable/RexImpTable.java

@@ -1253,6 +1253,8 @@ void populate2() {
                   NullPolicy.NONE, false)));
       // Generates conditional expressions for aggregate FILTER clause:
       // e.g. SUM(salary) FILTER (WHERE dept='Sales') → condition ? sum : NULL
+      // FilterImplementor is used for all FILTER operations, but the restriction only affects
+      // OVER clause, because normal aggregates never pass through SqlOverOperator.
       define(FILTER, new FilterImplementor());
     }