private void reAlignPositionMap(int startPosition, int offset) {
    for (Map.Entry<String, Integer> entry : fieldPositionMap.entrySet()) {
      // Re-align the columns appearing on or after startPostion(say, column 1) such that
      // column 2 becomes column (2+offset), column 3 becomes column (3+offset) and so on.
      Integer entryVal = entry.getValue();
      if (entryVal >= startPosition) {
        entry.setValue(entryVal+offset);
      }
    }
  }
Note : The position will be re-numbered when one of the preceding columns are removed.
Hence, the client should not cache this value and expect it to be always valid.
@param fieldName
@return the index of field named fieldName in Schema. If field is not
present, returns null.
hcatalog/core/src/main/java/org/apache/hive/hcatalog/data/schema/HCatSchema.java