public Symbol getSymbol(int fieldIndex) {
    checkArgument(
        fieldIndex >= 0 && fieldIndex < fieldMappings.size(),
        "No field->symbol mapping for field %s",
        fieldIndex);
    return fieldMappings.get(fieldIndex);
  }
The purpose of this class is to hold the current plan built so far for a relation (query, table,
values, etc.), and the mapping to indicate how the fields (by position) in the relation map to
the outputs of the plan.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/relational/planner/RelationPlan.java