fix: Toggle 'fast_unique' on new_from_index (#19956)

crates/polars-core/src/chunked_array/logical/categorical/builder.rs

@@ -162,8 +162,8 @@ impl CategoricalChunkedBuilder {
                 false,
                 self.ordering,
             )
+            .with_fast_unique(true)
         }
-        .with_fast_unique(true)
     }
 
     pub fn drain_iter_and_finish<'a, I>(mut self, i: I) -> CategoricalChunked
@@ -187,8 +187,8 @@ impl CategoricalChunkedBuilder {
                 &self.categories.into(),
                 self.ordering,
             )
+            .with_fast_unique(true)
         }
-        .with_fast_unique(true)
     }
 }
 
@@ -369,7 +369,8 @@ impl CategoricalChunked {
                 Arc::new(rev_map),
                 true,
                 ordering,
-            ))
+            )
+            .with_fast_unique(false))
         }
     }
 }

crates/polars-core/src/chunked_array/logical/categorical/mod.rs

@@ -297,12 +297,18 @@ impl CategoricalChunked {
         }
     }
 
-    pub(crate) fn with_fast_unique(mut self, toggle: bool) -> Self {
+    /// Set `FAST_UNIQUE` metadata
+    /// # Safety
+    /// This invariant must hold `unique(categories) == unique(self)`
+    pub(crate) unsafe fn with_fast_unique(mut self, toggle: bool) -> Self {
         self.set_fast_unique(toggle);
         self
     }
 
-    pub fn _with_fast_unique(self, toggle: bool) -> Self {
+    /// Set `FAST_UNIQUE` metadata
+    /// # Safety
+    /// This invariant must hold `unique(categories) == unique(self)`
+    pub unsafe fn _with_fast_unique(self, toggle: bool) -> Self {
         self.with_fast_unique(toggle)
     }
 

crates/polars-core/src/chunked_array/logical/enum_/mod.rs

@@ -94,7 +94,7 @@ impl EnumChunkedBuilder {
         // SAFETY: keys and values are in bounds
         unsafe {
             CategoricalChunked::from_cats_and_rev_map_unchecked(ca, self.rev, true, self.ordering)
+                .with_fast_unique(true)
         }
-        .with_fast_unique(true)
     }
 }

crates/polars-core/src/frame/group_by/mod.rs

@@ -258,7 +258,6 @@ impl<'df> GroupBy<'df> {
         } else {
             &self.groups
         };
-
         POOL.install(|| {
             self.selected_keys
                 .par_iter()

crates/polars-core/src/frame/group_by/perfect.rs

@@ -170,6 +170,7 @@ impl CategoricalChunked {
         let mut out = match &**rev_map {
             RevMapping::Local(cached, _) => {
                 if self._can_fast_unique() {
+                    assert!(cached.len() <= self.len(), "invalid invariant");
                     if verbose() {
                         eprintln!("grouping categoricals, run perfect hash function");
                     }

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

@@ -215,7 +215,7 @@ impl SeriesTrait for SeriesWrap<CategoricalChunked> {
     }
 
     fn new_from_index(&self, index: usize, length: usize) -> Series {
-        self.with_state(true, |cats| cats.new_from_index(index, length))
+        self.with_state(false, |cats| cats.new_from_index(index, length))
             .into_series()
     }
 

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

@@ -57,24 +57,24 @@ fn map_cats(
                 },
             });
 
-        let outvals = [
-            brk_vals.finish().into_series(),
+        let outvals = [brk_vals.finish().into_series(), unsafe {
             bld.finish()
                 ._with_fast_unique(label_has_value.iter().all(bool::clone))
-                .into_series(),
-        ];
+                .into_series()
+        }];
         Ok(StructChunked::from_series(out_name, outvals[0].len(), outvals.iter())?.into_series())
     } else {
-        Ok(bld
-            .drain_iter_and_finish(s_iter.map(|opt| {
+        Ok(unsafe {
+            bld.drain_iter_and_finish(s_iter.map(|opt| {
                 opt.filter(|x| !x.is_nan()).map(|x| {
                     let pt = sorted_breaks.partition_point(|v| op(&x, v));
-                    unsafe { *label_has_value.get_unchecked_mut(pt) = true };
-                    unsafe { labels.get_unchecked(pt).as_str() }
+                    *label_has_value.get_unchecked_mut(pt) = true;
+                    labels.get_unchecked(pt).as_str()
                 })
             }))
             ._with_fast_unique(label_has_value.iter().all(bool::clone))
-            .into_series())
+        }
+        .into_series())
     }
 }
 

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

@@ -876,3 +876,14 @@ def test_perfect_group_by_19452() -> None:
     )
 
     assert df2.with_columns(a=(pl.col("b")).over(pl.col("a")))["a"].is_sorted()
+
+
+def test_perfect_group_by_19950() -> None:
+    dtype = pl.Enum(categories=["a", "b", "c"])
+
+    left = pl.DataFrame({"x": "a"}).cast(dtype)
+    right = pl.DataFrame({"x": "a", "y": "b"}).cast(dtype)
+    assert left.join(right, on="x").group_by("y").first().to_dict(as_series=False) == {
+        "y": ["b"],
+        "x": ["a"],
+    }