feat(polars): implement approx_nunique

Author: jcrist

ibis/backends/polars/compiler.py

@@ -708,6 +708,7 @@ def struct_column(op, **kw):
     ops.All: "all",
     ops.Any: "any",
     ops.ApproxMedian: "median",
+    ops.ApproxCountDistinct: "approx_n_unique",
     ops.Count: "count",
     ops.CountDistinct: "n_unique",
     ops.Max: "max",

ibis/backends/tests/test_aggregation.py

@@ -478,9 +478,6 @@ def mean_and_std(v):
             lambda t, where: t.string_col.approx_nunique(where=where),
             lambda t, where: t.string_col[where].nunique(),
             id="approx_nunique",
-            marks=[
-                pytest.mark.notimpl(["polars"], raises=com.OperationNotDefinedError),
-            ],
         ),
         param(
             lambda t, where: t.bigint_col.bit_and(where=where),