analyze {mode: "ndv"} currently rejects sharded collections (uassert 13175803). system.stats.field_stats is shard-local and not visible through mongos, and JOO does not support sharded collections yet, so no NDV statistics are collected for them.

Once JOO supports sharded collections, NDV should work similarly: each shard builds HLL sketches for its local data, and the cluster merges them using register-wise maxima. The registers are already persisted for this purpose; the accumulator’s merge guards still need to be implemented.