src/main/java/com/conveyal/gtfs/loader/Table.java
public String getIndexFields() {
        String orderFieldName = getOrderFieldName();
        if (orderFieldName == null) return getKeyFieldName();
        else return String.join(",", getKeyFieldName(), orderFieldName);
    }
public String getIndexFields() {
        String orderFieldName = getOrderFieldName();
        if (orderFieldName == null) return getKeyFieldName();
        else return String.join(",", getKeyFieldName(), orderFieldName);
    }
Gets index fields for the spec table. WARNING: this MUST be called on a spec table (i.e., one of the constant
tables defined in this class). Otherwise, it could return fields that should not be indexed.
@return