minDistinct

Distinct values a path needs to be worth an inverted index.

Two, because an index over a path with one value returns every document and has cost nothing but space. There is deliberately no upper bound: a path with a distinct value per document is a perfectly good equality index — it is the best one — and excluding it would be confusing a bitmap's storage shape with an index's usefulness.

It does not apply to IndexKind.SHREDDED_COLUMN, which is not answering an equality question: a column exists so that a scan of one field never touches the documents, and it pays in proportion to the bytes it avoids reading whatever the cardinality is.