polars.Expr.approx_n_unique

Expr.approx_n_unique() → Self

Approximate count of unique values.

This is done using the HyperLogLog++ algorithm for cardinality estimation.

Examples

>>> df = pl.DataFrame({"a": [1, 1, 2]})
>>> df.select(pl.col("a").approx_n_unique())
shape: (1, 1)
┌─────┐
│ a   │
│ --- │
│ u32 │
╞═════╡
│ 2   │
└─────┘