Reduce graph size through writing indexes directly into graph for map_blocks (#9658)

* Reduce graph size through writing indexes directly into graph for map_blocks

* Reduce graph size through writing indexes directly into graph for map_blocks

* Update xarray/core/parallel.py

---------

Co-authored-by: Deepak Cherian <dcherian@users.noreply.github.com>

Author: phofl

xarray/core/parallel.py

@@ -25,7 +25,6 @@ class ExpectedDict(TypedDict):
     shapes: dict[Hashable, int]
     coords: set[Hashable]
     data_vars: set[Hashable]
-    indexes: dict[Hashable, Index]
 
 
 def unzip(iterable):
@@ -337,6 +336,7 @@ def _wrapper(
         kwargs: dict,
         arg_is_array: Iterable[bool],
         expected: ExpectedDict,
+        expected_indexes: dict[Hashable, Index],
     ):
         """
         Wrapper function that receives datasets in args; converts to dataarrays when necessary;
@@ -372,7 +372,7 @@ def _wrapper(
 
             # ChainMap wants MutableMapping, but xindexes is Mapping
             merged_indexes = collections.ChainMap(
-                expected["indexes"],
+                expected_indexes,
                 merged_coordinates.xindexes,  # type: ignore[arg-type]
             )
             expected_index = merged_indexes.get(name, None)
@@ -412,6 +412,7 @@ def _wrapper(
     try:
         import dask
         import dask.array
+        from dask.base import tokenize
         from dask.highlevelgraph import HighLevelGraph
 
     except ImportError:
@@ -551,6 +552,20 @@ def _wrapper(
             for isxr, arg in zip(is_xarray, npargs, strict=True)
         ]
 
+        # only include new or modified indexes to minimize duplication of data
+        indexes = {
+            dim: coordinates.xindexes[dim][
+                _get_chunk_slicer(dim, chunk_index, output_chunk_bounds)
+            ]
+            for dim in (new_indexes | modified_indexes)
+        }
+
+        tokenized_indexes: dict[Hashable, str] = {}
+        for k, v in indexes.items():
+            tokenized_v = tokenize(v)
+            graph[f"{k}-coordinate-{tokenized_v}"] = v
+            tokenized_indexes[k] = f"{k}-coordinate-{tokenized_v}"
+
         # raise nice error messages in _wrapper
         expected: ExpectedDict = {
             # input chunk 0 along a dimension maps to output chunk 0 along the same dimension
@@ -562,17 +577,18 @@ def _wrapper(
             },
             "data_vars": set(template.data_vars.keys()),
             "coords": set(template.coords.keys()),
-            # only include new or modified indexes to minimize duplication of data, and graph size.
-            "indexes": {
-                dim: coordinates.xindexes[dim][
-                    _get_chunk_slicer(dim, chunk_index, output_chunk_bounds)
-                ]
-                for dim in (new_indexes | modified_indexes)
-            },
         }
 
         from_wrapper = (gname,) + chunk_tuple
-        graph[from_wrapper] = (_wrapper, func, blocked_args, kwargs, is_array, expected)
+        graph[from_wrapper] = (
+            _wrapper,
+            func,
+            blocked_args,
+            kwargs,
+            is_array,
+            expected,
+            (dict, [[k, v] for k, v in tokenized_indexes.items()]),
+        )
 
         # mapping from variable name to dask graph key
         var_key_map: dict[Hashable, str] = {}

xarray/tests/test_dask.py

@@ -14,6 +14,7 @@
 from xarray import DataArray, Dataset, Variable
 from xarray.core import duck_array_ops
 from xarray.core.duck_array_ops import lazy_array_equiv
+from xarray.core.indexes import PandasIndex
 from xarray.testing import assert_chunks_equal
 from xarray.tests import (
     assert_allclose,
@@ -1375,6 +1376,13 @@ def test_map_blocks_da_ds_with_template(obj):
         actual = xr.map_blocks(func, obj, template=template)
     assert_identical(actual, template)
 
+    # Check that indexes are written into the graph directly
+    dsk = dict(actual.__dask_graph__())
+    assert len({k for k in dsk if "x-coordinate" in k})
+    assert all(
+        isinstance(v, PandasIndex) for k, v in dsk.items() if "x-coordinate" in k
+    )
+
     with raise_if_dask_computes():
         actual = obj.map_blocks(func, template=template)
     assert_identical(actual, template)