fix: sum_horizontal should not always cast to int (#12031)

crates/polars-ops/src/series/ops/horizontal.rs

@@ -1,34 +1,34 @@
+use std::borrow::Cow;
 use std::ops::{BitAnd, BitOr};
 
 use polars_core::prelude::*;
 use polars_core::POOL;
 use rayon::prelude::*;
 
 pub fn sum_horizontal(s: &[Series]) -> PolarsResult<Series> {
-    let out = POOL
-        .install(|| {
-            s.par_iter()
-                .try_fold(
-                    || UInt32Chunked::new("", &[0u32]).into_series(),
-                    |acc, b| {
-                        PolarsResult::Ok(
-                            acc.fill_null(FillNullStrategy::Zero)?
-                                + b.fill_null(FillNullStrategy::Zero)?,
-                        )
-                    },
-                )
-                .try_reduce(
-                    || UInt32Chunked::new("", &[0u32]).into_series(),
-                    |a, b| {
-                        PolarsResult::Ok(
-                            a.fill_null(FillNullStrategy::Zero)?
-                                + b.fill_null(FillNullStrategy::Zero)?,
-                        )
-                    },
-                )
-        })?
-        .with_name("sum");
-    Ok(out)
+    let sum_fn = |acc: &Series, s: &Series| {
+        PolarsResult::Ok(
+            acc.fill_null(FillNullStrategy::Zero)? + s.fill_null(FillNullStrategy::Zero)?,
+        )
+    };
+    let out = match s.len() {
+        0 => Ok(UInt32Chunked::new("", &[0u32]).into_series()),
+        1 => Ok(s[0].clone()),
+        2 => sum_fn(&s[0], &s[1]),
+        _ => {
+            // the try_reduce_with is a bit slower in parallelism,
+            // but I don't think it matters here as we parallelize over series, not over elements
+            POOL.install(|| {
+                s.par_iter()
+                    .map(|s| Ok(Cow::Borrowed(s)))
+                    .try_reduce_with(|l, r| sum_fn(&l, &r).map(Cow::Owned))
+                    // we can unwrap the option, because we are certain there is a series
+                    .unwrap()
+                    .map(|cow| cow.into_owned())
+            })
+        },
+    };
+    out.map(|ok| ok.with_name("sum"))
 }
 
 pub fn any_horizontal(s: &[Series]) -> PolarsResult<Series> {

py-polars/tests/unit/functions/aggregation/test_horizontal.py

@@ -1,5 +1,6 @@
 from __future__ import annotations
 
+import datetime
 from typing import Any
 
 import pytest
@@ -227,3 +228,19 @@ def test_cumsum_fold() -> None:
     )
     result = df.select(pl.cumsum_horizontal("a", "c"))
     assert result.to_dict(False) == {"cumsum": [{"a": 1, "c": 6}, {"a": 2, "c": 8}]}
+
+
+def test_sum_dtype_12028() -> None:
+    result = pl.select(
+        pl.sum_horizontal([pl.duration(seconds=10)]).alias("sum_duration")
+    )
+    expected = pl.DataFrame(
+        [
+            pl.Series(
+                "sum_duration",
+                [datetime.timedelta(seconds=10)],
+                dtype=pl.Duration(time_unit="us"),
+            ),
+        ]
+    )
+    assert_frame_equal(expected, result)