Avoid stacking when grouping by chunked array

doc/whats-new.rst

@@ -62,6 +62,8 @@ Documentation
 
 Internal Changes
 ~~~~~~~~~~~~~~~~
+- Avoid stacking when grouping by a chunked array. This can be a large performance improvement.
+  By `Deepak Cherian <https://github.com/dcherian>`_.
 
 .. _whats-new.2025.03.1:
 

xarray/core/groupby.py

@@ -661,18 +661,26 @@ def __init__(
         # specification for the groupby operation
         # TODO: handle obj having variables that are not present on any of the groupers
         #       simple broadcasting fails for ExtensionArrays.
-        # FIXME: Skip this stacking when grouping by a dask array, it's useless in that case.
-        (self.group1d, self._obj, self._stacked_dim, self._inserted_dims) = _ensure_1d(
-            group=self.encoded.codes, obj=obj
-        )
-        (self._group_dim,) = self.group1d.dims
+        codes = self.encoded.codes
+        self._by_chunked = is_chunked_array(codes._variable._data)
+        if not self._by_chunked:
+            (self.group1d, self._obj, self._stacked_dim, self._inserted_dims) = (
+                _ensure_1d(group=codes, obj=obj)
+            )
+            (self._group_dim,) = self.group1d.dims
+        else:
+            self.group1d = None
+            # This transpose preserves dim order behaviour
+            self._obj = obj.transpose(..., *codes.dims)
+            self._stacked_dim = None
+            self._inserted_dims = []
+            self._group_dim = None
 
         # cached attributes
         self._groups = None
         self._dims = None
         self._sizes = None
         self._len = len(self.encoded.full_index)
-        self._by_chunked = is_chunked_array(self.encoded.codes.data)
 
     @property
     def sizes(self) -> Mapping[Hashable, int]:
@@ -817,6 +825,7 @@ def __getitem__(self, key: GroupKey) -> T_Xarray:
         """
         Get DataArray or Dataset corresponding to a particular group label.
         """
+        self._raise_if_by_is_chunked()
         return self._obj.isel({self._group_dim: self.groups[key]})
 
     def __len__(self) -> int:
@@ -1331,9 +1340,6 @@ def quantile(
            "Sample quantiles in statistical packages,"
            The American Statistician, 50(4), pp. 361-365, 1996
         """
-        if dim is None:
-            dim = (self._group_dim,)
-
         # Dataset.quantile does this, do it for flox to ensure same output.
         q = np.asarray(q, dtype=np.float64)
 
@@ -1348,6 +1354,8 @@ def quantile(
             )
             return result
         else:
+            if dim is None:
+                dim = (self._group_dim,)
             return self.map(
                 self._obj.__class__.quantile,
                 shortcut=False,
@@ -1491,6 +1499,7 @@ class DataArrayGroupByBase(GroupBy["DataArray"], DataArrayGroupbyArithmetic):
 
     @property
     def dims(self) -> tuple[Hashable, ...]:
+        self._raise_if_by_is_chunked()
         if self._dims is None:
             index = self.encoded.group_indices[0]
             self._dims = self._obj.isel({self._group_dim: index}).dims
@@ -1702,6 +1711,7 @@ class DatasetGroupByBase(GroupBy["Dataset"], DatasetGroupbyArithmetic):
 
     @property
     def dims(self) -> Frozen[Hashable, int]:
+        self._raise_if_by_is_chunked()
         if self._dims is None:
             index = self.encoded.group_indices[0]
             self._dims = self._obj.isel({self._group_dim: index}).dims