feat: Sort decimal fields (pola-rs#14649)

crates/polars-core/src/chunked_array/ops/sort/mod.rs

@@ -629,6 +629,9 @@ pub(crate) fn convert_sort_column_multi_sort(s: &Series) -> PolarsResult<Series>
                 .collect::<PolarsResult<Vec<_>>>()?;
             return StructChunked::new(ca.name(), &new_fields).map(|ca| ca.into_series());
         },
+        // we could fallback to default branch, but decimal is not numeric dtype for now, so explicit here
+        #[cfg(feature = "dtype-decimal")]
+        Decimal(_, _) => s.clone(),
         _ => {
             let phys = s.to_physical_repr().into_owned();
             polars_ensure!(

crates/polars-core/src/series/implementations/decimal.rs

@@ -66,6 +66,12 @@ impl private::PrivateSeries for SeriesWrap<DecimalChunked> {
             .into_decimal_unchecked(self.0.precision(), self.0.scale())
             .into_series())
     }
+    fn into_total_eq_inner<'a>(&'a self) -> Box<dyn TotalEqInner + 'a> {
+        (&self.0).into_total_eq_inner()
+    }
+    fn into_total_ord_inner<'a>(&'a self) -> Box<dyn TotalOrdInner + 'a> {
+        (&self.0).into_total_ord_inner()
+    }
 
     #[cfg(feature = "algorithm_group_by")]
     unsafe fn agg_sum(&self, groups: &GroupsProxy) -> Series {
@@ -211,6 +217,17 @@ impl SeriesTrait for SeriesWrap<DecimalChunked> {
         self.0.get_any_value_unchecked(index)
     }
 
+    fn sort_with(&self, options: SortOptions) -> Series {
+        self.0
+            .sort_with(options)
+            .into_decimal_unchecked(self.0.precision(), self.0.scale())
+            .into_series()
+    }
+
+    fn arg_sort(&self, options: SortOptions) -> IdxCa {
+        self.0.arg_sort(options)
+    }
+
     fn null_count(&self) -> usize {
         self.0.null_count()
     }

crates/polars-row/src/encode.rs

@@ -153,6 +153,7 @@ pub fn encoded_size(data_type: &ArrowDataType) -> usize {
         Int16 => i16::ENCODED_LEN,
         Int32 => i32::ENCODED_LEN,
         Int64 => i64::ENCODED_LEN,
+        Decimal(_, _) => i128::ENCODED_LEN,
         Float32 => f32::ENCODED_LEN,
         Float64 => f64::ENCODED_LEN,
         Boolean => bool::ENCODED_LEN,

crates/polars-row/src/fixed.rs

@@ -105,6 +105,7 @@ encode_signed!(1, i8);
 encode_signed!(2, i16);
 encode_signed!(4, i32);
 encode_signed!(8, i64);
+encode_signed!(16, i128);
 
 impl FixedLengthEncoding for f32 {
     type Encoded = [u8; 4];

crates/polars-row/src/utils.rs

@@ -9,6 +9,7 @@ macro_rules! with_match_arrow_primitive_type {(
         Int16 => __with_ty__! { i16 },
         Int32 => __with_ty__! { i32 },
         Int64 => __with_ty__! { i64 },
+        Decimal(_, _) => __with_ty__! { i128 },
         UInt8 => __with_ty__! { u8 },
         UInt16 => __with_ty__! { u16 },
         UInt32 => __with_ty__! { u32 },

py-polars/tests/unit/datatypes/test_decimal.py

@@ -336,6 +336,33 @@ def test_decimal_in_filter() -> None:
     }
 
 
+def test_decimal_sort() -> None:
+    df = pl.DataFrame(
+        {
+            "foo": [1, 2, 3],
+            "bar": [D("3.4"), D("2.1"), D("4.5")],
+            "baz": [1, 1, 2],
+        }
+    )
+    assert df.sort("bar").to_dict(as_series=False) == {
+        "foo": [2, 1, 3],
+        "bar": [D("2.1"), D("3.4"), D("4.5")],
+        "baz": [1, 1, 2],
+    }
+    assert df.sort(["foo", "bar"]).to_dict(as_series=False) == {
+        "foo": [1, 2, 3],
+        "bar": [D("3.4"), D("2.1"), D("4.5")],
+        "baz": [1, 1, 2],
+    }
+
+    assert df.select([pl.col("foo").sort_by("bar", descending=True).alias("s1")])[
+        "s1"
+    ].to_list() == [3, 1, 2]
+    assert df.select([pl.col("foo").sort_by(["baz", "bar"]).alias("s2")])[
+        "s2"
+    ].to_list() == [2, 1, 3]
+
+
 def test_decimal_write_parquet_12375() -> None:
     f = io.BytesIO()
     df = pl.DataFrame(