@Override
  public <R, C> R accept(PlanVisitor<R, C> visitor, C context) {
    return visitor.visitFill(this, context);
  }
FillNode is used to fill the empty field in one row.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/planner/plan/node/process/FillNode.java