String stats aggregation

String stats aggregation #

A multi-value metrics aggregation computing length and character statistics over keyword/string values: count, min/max/avg length and Shannon entropy of the character distribution.

How it works #

Every document that has a string value for the field contributes one string. Over those strings the engine reports:

• count — how many values were seen (documents without the field are skipped, not counted);
• min_length, max_length, avg_length — length statistics in UTF-8 bytes, so a non-ASCII character counts as its encoded width;
• entropy — Shannon entropy of the pooled character frequencies, in bits: -Σ p(c) · log2 p(c) over the share p(c) of each character across all strings. Constant text gives 0; text where every character is equally likely gives log2(alphabet size).

The character frequencies are pooled across all contributing values before the entropy is taken, so repetition across documents counts: the teaching type field repeats the same few words 120 times and still scores ≈3.22 bits.