How to get some statistic number in custom script?

For example, I wanna normalize one field. So I need the max and min value of this field.

In the document, I only found this three statistic

_index['FIELD'].docCount()
Number of documents containing the field FIELD. Does not take deleted documents into account.
_index['FIELD'].sumttf()
Sum of ttf over all terms that appear in field FIELD in all documents.
_index['FIELD'].sumdf()
The sum of df s over all terms that appear in field FIELD in all documents.