Allow setting (or skipping) new indexes in open_dataset

xarray/backends/api.py

@@ -403,6 +403,7 @@ def open_dataset(
     concat_characters: bool | None = None,
     decode_coords: Literal["coordinates", "all"] | bool | None = None,
     drop_variables: str | Iterable[str] | None = None,
+    set_indexes: bool = True,
     inline_array: bool = False,
     chunked_array_type: str | None = None,
     from_array_kwargs: dict[str, Any] | None = None,
@@ -494,6 +495,12 @@ def open_dataset(
         A variable or list of variables to exclude from being parsed from the
         dataset. This may be useful to drop variables with problems or
         inconsistent values.
+    set_indexes : bool, optional
+        If True (default), create new indexes from coordinates. Both the number and
+        the type(s) of those indexes depend on the backend used to open the dataset.
+        For most common backends this creates a pandas index for each
+        :term:`Dimension coordinate`, which loads the coordinate data fully in memory.
+        Set it to False if you want to avoid loading data into memory.
     inline_array: bool, default: False
         How to include the array in the dask task graph.
         By default(``inline_array=False``) the array is included in a task by
@@ -572,6 +579,7 @@ def open_dataset(
     backend_ds = backend.open_dataset(
         filename_or_obj,
         drop_variables=drop_variables,
+        set_indexes=set_indexes,
         **decoders,
         **kwargs,
     )
@@ -606,6 +614,7 @@ def open_dataarray(
     concat_characters: bool | None = None,
     decode_coords: Literal["coordinates", "all"] | bool | None = None,
     drop_variables: str | Iterable[str] | None = None,
+    set_indexes: bool = True,
     inline_array: bool = False,
     chunked_array_type: str | None = None,
     from_array_kwargs: dict[str, Any] | None = None,
@@ -699,6 +708,12 @@ def open_dataarray(
         A variable or list of variables to exclude from being parsed from the
         dataset. This may be useful to drop variables with problems or
         inconsistent values.
+    set_indexes : bool, optional
+        If True (default), create new indexes from coordinates. Both the number and
+        the type(s) of those indexes depend on the backend used to open the dataset.
+        For most common backends this creates a pandas index for each
+        :term:`Dimension coordinate`, which loads the coordinate data fully in memory.
+        Set it to False if you want to avoid loading data into memory.
     inline_array: bool, default: False
         How to include the array in the dask task graph.
         By default(``inline_array=False``) the array is included in a task by
@@ -756,6 +771,7 @@ def open_dataarray(
         chunks=chunks,
         cache=cache,
         drop_variables=drop_variables,
+        set_indexes=set_indexes,
         inline_array=inline_array,
         chunked_array_type=chunked_array_type,
         from_array_kwargs=from_array_kwargs,

xarray/backends/common.py

@@ -490,6 +490,7 @@ def open_dataset(
         filename_or_obj: str | os.PathLike[Any] | BufferedIOBase | AbstractDataStore,
         *,
         drop_variables: str | Iterable[str] | None = None,
+        set_indexes: bool = True,
         **kwargs: Any,
     ) -> Dataset:
         """

xarray/backends/store.py

@@ -9,6 +9,7 @@
     AbstractDataStore,
     BackendEntrypoint,
 )
+from xarray.core.coordinates import Coordinates
 from xarray.core.dataset import Dataset
 
 if TYPE_CHECKING:
@@ -35,6 +36,7 @@ def open_dataset(  # type: ignore[override]  # allow LSP violation, not supporti
         concat_characters=True,
         decode_coords=True,
         drop_variables: str | Iterable[str] | None = None,
+        set_indexes: bool = True,
         use_cftime=None,
         decode_timedelta=None,
     ) -> Dataset:
@@ -55,8 +57,22 @@ def open_dataset(  # type: ignore[override]  # allow LSP violation, not supporti
             decode_timedelta=decode_timedelta,
         )
 
-        ds = Dataset(vars, attrs=attrs)
-        ds = ds.set_coords(coord_names.intersection(vars))
+        # split data and coordinate variables (promote dimension coordinates)
+        data_vars = {}
+        coord_vars = {}
+        for name, var in vars.items():
+            if name in coord_names or var.dims == (name,):
+                coord_vars[name] = var
+            else:
+                data_vars[name] = var
+
+        if set_indexes:
+            coords = coord_vars
+        else:
+            # explict Coordinates object with no index passed
+            coords = Coordinates(coord_vars)
+
+        ds = Dataset(data_vars, coords=coords, attrs=attrs)
         ds.set_close(filename_or_obj.close)
         ds.encoding = encoding
 

xarray/backends/zarr.py

@@ -803,6 +803,7 @@ def open_zarr(
     zarr_version=None,
     chunked_array_type: str | None = None,
     from_array_kwargs: dict[str, Any] | None = None,
+    set_indexes=True,
     **kwargs,
 ):
     """Load and decode a dataset from a Zarr store.
@@ -896,6 +897,10 @@ def open_zarr(
         chunked arrays, via whichever chunk manager is specified through the `chunked_array_type` kwarg.
         Defaults to {'manager': 'dask'}, meaning additional kwargs will be passed eventually to
         :py:func:`dask.array.from_array`. Experimental API that should not be relied upon.
+    set_indexes : bool, optional
+        If True (default), create a default (pandas) index for each
+        :term:`Dimension coordinate`. Set it to False if the dataset contains
+        dimension coordinate arrays that are too large to load fully in memory.
 
     Returns
     -------
@@ -952,6 +957,7 @@ def open_zarr(
         engine="zarr",
         chunks=chunks,
         drop_variables=drop_variables,
+        set_indexes=set_indexes,
         chunked_array_type=chunked_array_type,
         from_array_kwargs=from_array_kwargs,
         backend_kwargs=backend_kwargs,
@@ -996,6 +1002,7 @@ def open_dataset(  # type: ignore[override]  # allow LSP violation, not supporti
         concat_characters=True,
         decode_coords=True,
         drop_variables: str | Iterable[str] | None = None,
+        set_indexes: bool = True,
         use_cftime=None,
         decode_timedelta=None,
         group=None,
@@ -1032,6 +1039,7 @@ def open_dataset(  # type: ignore[override]  # allow LSP violation, not supporti
                 drop_variables=drop_variables,
                 use_cftime=use_cftime,
                 decode_timedelta=decode_timedelta,
+                set_indexes=set_indexes,
             )
         return ds