public boolean hasSingleGlobalAggregation() {
    return hasEmptyGroupingSet() && getGroupingSetCount() == 1;
  }
@return true if the aggregation collapses all rows into a single global group (e.g., as a
    result of a GROUP BY () query). Otherwise, false.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/planner/node/AggregationNode.java