Fix order-preservation in cudf-polars groupby

When we are requested to maintain order in groupby aggregations we
must post-process the result by computing a permutation between the
wanted order (of the input keys) and the order returned by the groupby
aggregation. To do this, we can perform a join between the two unique
key tables. Previously, we assumed that the gather map returned in
this join for the left (wanted order) table was the identity. However,
this is not guaranteed, in addition to computing the match between the
wanted key order and the key order we have, we must also apply the
permutation between the left gather map order and the identity.

- Closes #16893

python/cudf_polars/cudf_polars/dsl/ir.py

@@ -603,24 +603,39 @@ def evaluate(self, *, cache: MutableMapping[int, DataFrame]) -> DataFrame:
             req.evaluate(result_subs, mapping=mapping) for req in self.agg_requests
         ]
         broadcasted = broadcast(*result_keys, *results)
-        result_keys = broadcasted[: len(result_keys)]
-        results = broadcasted[len(result_keys) :]
         # Handle order preservation of groups
-        # like cudf classic does
-        # https://github.com/rapidsai/cudf/blob/5780c4d8fb5afac2e04988a2ff5531f94c22d3a3/python/cudf/cudf/core/groupby/groupby.py#L723-L743
         if self.maintain_order and not sorted:
-            left = plc.stream_compaction.stable_distinct(
+            # The order we want
+            want = plc.stream_compaction.stable_distinct(
                 plc.Table([k.obj for k in keys]),
                 list(range(group_keys.num_columns())),
                 plc.stream_compaction.DuplicateKeepOption.KEEP_FIRST,
                 plc.types.NullEquality.EQUAL,
                 plc.types.NanEquality.ALL_EQUAL,
             )
-            right = plc.Table([key.obj for key in result_keys])
-            _, indices = plc.join.left_join(left, right, plc.types.NullEquality.EQUAL)
+            # The order we have
+            have = plc.Table([key.obj for key in broadcasted[: len(keys)]])
+
+            # We know an inner join is OK because by construction
+            # want and have are permutations of each other.
+            left_order, right_order = plc.join.inner_join(
+                want, have, plc.types.NullEquality.EQUAL
+            )
+            # Now left_order is an arbitrary permutation of the ordering we
+            # want, and right_order is a matching permutation of the ordering
+            # we have. To get to the original ordering, we need
+            # left_order == iota(nrows), with right_order permuted
+            # appropriately. This can be obtained by sorting
+            # right_order by left_order.
+            (right_order,) = plc.sorting.sort_by_key(
+                plc.Table([right_order]),
+                plc.Table([left_order]),
+                [plc.types.Order.ASCENDING],
+                [plc.types.NullOrder.AFTER],
+            ).columns()
             ordered_table = plc.copying.gather(
                 plc.Table([col.obj for col in broadcasted]),
-                indices,
+                right_order,
                 plc.copying.OutOfBoundsPolicy.DONT_CHECK,
             )
             broadcasted = [

python/cudf_polars/tests/test_groupby.py

@@ -4,6 +4,7 @@
 
 import itertools
 
+import numpy as np
 import pytest
 
 import polars as pl
@@ -191,3 +192,24 @@ def test_groupby_literal_in_agg(df, key, expr):
 def test_groupby_unary_non_pointwise_raises(df, expr):
     q = df.group_by("key1").agg(expr)
     assert_ir_translation_raises(q, NotImplementedError)
+
+
+@pytest.mark.parametrize("nrows", [30, 300, 300_000])
+@pytest.mark.parametrize("nkeys", [1, 2, 4])
+def test_groupby_maintain_order_random(nrows, nkeys, with_nulls):
+    key_names = [f"key{key}" for key in range(nkeys)]
+    key_values = [np.random.randint(100, size=nrows) for _ in key_names]
+    value = np.random.randint(-100, 100, size=nrows)
+    df = pl.DataFrame(dict(zip(key_names, key_values, strict=True), value=value))
+    if with_nulls:
+        df = df.with_columns(
+            *(
+                pl.when(pl.col(name) == 1)
+                .then(None)
+                .otherwise(pl.col(name))
+                .alias(name)
+                for name in key_names
+            )
+        )
+    q = df.lazy().group_by(key_names, maintain_order=True).agg(pl.col("value").sum())
+    assert_gpu_result_equal(q)