public InputLocation getSourceLocation(int index) {
    checkArgument(index < sourceLocations.size(), "index is not valid");
    return sourceLocations.get(index);
  }
used for case that this OutputColumn only has one input column
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/planner/plan/parameter/OutputColumn.java