feat: register 'set_sorted' as batch/elementwise (#13896)

crates/polars-plan/src/dsl/mod.rs

@@ -1608,7 +1608,8 @@ impl Expr {
     /// This can lead to incorrect results if this `Series` is not sorted!!
     /// Use with care!
     pub fn set_sorted_flag(self, sorted: IsSorted) -> Expr {
-        self.apply_private(FunctionExpr::SetSortedFlag(sorted))
+        // This is `map`. If a column is sorted. Chunks of that column are also sorted.
+        self.map_private(FunctionExpr::SetSortedFlag(sorted))
     }
 
     #[cfg(feature = "row_hash")]

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

@@ -160,3 +160,11 @@ def test_slice_nullcount(ref: list[int | None]) -> None:
     assert s.null_count() == sum(x is None for x in ref)
     assert s.slice(64).null_count() == sum(x is None for x in ref[64:])
     assert s.slice(50, 60).slice(25).null_count() == sum(x is None for x in ref[75:110])
+
+
+def test_slice_pushdown_set_sorted() -> None:
+    ldf = pl.LazyFrame({"foo": [1, 2, 3]})
+    ldf = ldf.set_sorted("foo").head(5)
+    plan = ldf.explain()
+    # check the set sorted is above slice
+    assert plan.index("set_sorted") < plan.index("SLICE")