pub const EXACT_NDV_CAP: usize = 1_000_000;Expand description
Approximate NDV cap above which we drop to a HyperLogLog-style estimate.
The exact-count implementation uses a HashSet<Box<dyn Any>> which is
O(unique-values) memory. Above this cap we use HyperLogLog (HllSketch)
instead, which is bounded. The threshold is deliberately generous so
typical small/medium tables stay exact; lakehouse-scale tables switch
to the sketch.