perf: Ensure metadata flags are maintained on vertical parallelization (

#17804)

crates/polars-core/src/series/mod.rs

@@ -195,13 +195,30 @@ impl Series {
     // TODO! this probably can now be removed, now we don't have special case for structs.
     pub fn select_chunk(&self, i: usize) -> Self {
         let mut new = self.clear();
+        let flags = self.get_flags();
+
+        let mut new_flags = MetadataFlags::empty();
+        new_flags.set(
+            MetadataFlags::SORTED_ASC,
+            flags.contains(MetadataFlags::SORTED_ASC),
+        );
+        new_flags.set(
+            MetadataFlags::SORTED_DSC,
+            flags.contains(MetadataFlags::SORTED_DSC),
+        );
+        new_flags.set(
+            MetadataFlags::FAST_EXPLODE_LIST,
+            flags.contains(MetadataFlags::FAST_EXPLODE_LIST),
+        );
+
         // Assign mut so we go through arc only once.
         let mut_new = new._get_inner_mut();
         let chunks = unsafe { mut_new.chunks_mut() };
         let chunk = self.chunks()[i].clone();
         chunks.clear();
         chunks.push(chunk);
         mut_new.compute_len();
+        mut_new._set_flags(new_flags);
         new
     }
 

py-polars/tests/unit/operations/test_is_sorted.py

@@ -407,3 +407,13 @@ def test_sorted_flag_group_by_dynamic() -> None:
 
 def test_is_sorted_rle_id() -> None:
     assert pl.Series([12, 3345, 12, 3, 4, 4, 1, 12]).rle_id().flags["SORTED_ASC"]
+
+
+def test_is_sorted_chunked_select() -> None:
+    df = pl.DataFrame({"a": np.ones(14)})
+
+    assert (
+        pl.concat([df, df, df], rechunk=False)
+        .set_sorted("a")
+        .select(pl.col("a").alias("b"))
+    )["b"].flags["SORTED_ASC"]