@Override
  public String vectorExpressionParameters() {
    return getColumnParamString(0, inputColumnNum[0]) + ", start " + startIdx;
  }
Given the substring start index param it finds the starting offset of the passed in utf8
string byte array that matches the index.
@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
ql/src/java/org/apache/hadoop/hive/ql/exec/vector/expressions/StringSubstrColStart.java