@Override
    public List<Symbol> visitAggregation(AggregationNode node, GroupContext context) {
      return ImmutableSet.copyOf(node.getGroupingKeys()).equals(context.groupingKeys)
          ? node.getGroupingKeys()
          : ImmutableList.of();
    }
This visitor returns the list of preGroupedSymbols of current AggregationNode. Attention: The
preGroupedSymbols of child-AggregationNode should have been calculated. GroupContext: The
GroupingKeys of current AggregationNode.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/planner/optimizations/TransformAggregationToStreamable.java