public Map<List<String>, List<CrossSeriesAggregationDescriptor>>
      getTagValuesToAggregationDescriptors() {
    return tagValuesToAggregationDescriptors;
  }
The CrossSeriesAggregationDescriptor may be null if there exists a key containing no
timeseries.

<p>e.g. we have following timeseries:

<ul>
  <li>root.sg.d1.s1(k1=v1)
  <li>root.sg.d1.s2(k1=v1)
  <li>root.sg.d2.s1(k1=v2)
  <li>root.sg.d3.s1(k1=v2)
</ul>

Then the query <code>
SELECT avg(s1), avg(s2) FROM root.sg.** GROUP BY TAGS(k1)
</code>will generate a {@link GroupByTagNode} with the <code>TagValuesToAggregationDescriptors
</code> as below: <code>
  {
    ["v1"]: [["avg(root.sg.d1.s1)"], ["avg(root.sg.d1.s2)"]],
    ["v2"]: [["avg(root.sg.d2.s1)","avg(root.sg.d3.s1)"], null],
  }
</code>

<p>So we should use it carefully with null values.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/planner/plan/node/process/GroupByTagNode.java