fix: recursively check allowed streaming dtypes (#11879)

crates/polars-lazy/src/physical_plan/streaming/convert_alp.rs

@@ -357,6 +357,10 @@ pub(crate) fn insert_streaming_nodes(
                         DataType::Object(_) => false,
                         #[cfg(feature = "dtype-categorical")]
                         DataType::Categorical(_) => string_cache,
+                        DataType::List(inner) => allowed_dtype(inner, string_cache),
+                        DataType::Struct(fields) => fields
+                            .iter()
+                            .all(|fld| allowed_dtype(fld.data_type(), string_cache)),
                         _ => true,
                     }
                 }

py-polars/tests/unit/streaming/test_streaming_categoricals.py

@@ -0,0 +1,14 @@
+import polars as pl
+
+
+def test_streaming_nested_categorical() -> None:
+    assert (
+        pl.LazyFrame({"numbers": [1, 1, 2], "cat": [["str"], ["foo"], ["bar"]]})
+        .with_columns(pl.col("cat").cast(pl.List(pl.Categorical)))
+        .group_by("numbers")
+        .agg(pl.col("cat").first())
+        .sort("numbers")
+    ).collect(streaming=True).to_dict(False) == {
+        "numbers": [1, 2],
+        "cat": [["str"], ["bar"]],
+    }