polars.approx_n_unique

polars.approx_n_unique(column: str | Expr) → Expr

Approximate count of unique values.

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

Parameters:

column

Column name or Series.

Examples

>>> df = pl.DataFrame({"a": [1, 8, 1], "b": [4, 5, 2], "c": ["foo", "bar", "foo"]})
>>> df.select(pl.approx_n_unique("a"))
shape: (1, 1)
┌─────┐
│ a   │
│ --- │
│ u32 │
╞═════╡
│ 2   │
└─────┘