@Override
  public String vectorExpressionParameters() {
    return getColumnParamString(0, inputColumnNum[0]) + ", start " + startIdx + ", length " + length;
  }
Populates the substring start and end offsets based on the substring start and length params.

@param utf8String byte array that holds the utf8 string
@param start start offset of the byte array the string starts at
@param len length of the bytes the string holds in the byte array
@param substrStart the Start index for the substring operation
@param substrLen the length of the substring
@param offsetArray the array that indexes are populated to. Assume its length >= 2.
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/StringSubstrColStartLen.java