Micro optimize dataset.isel for speed on large datasets

doc/whats-new.rst

@@ -33,6 +33,13 @@ New Features
 - Added zarr backends for :py:func:`open_groups` (:issue:`9430`, :pull:`9469`).
   By `Eni Awowale <https://github.com/eni-awowale>`_.
 
+Performance
+~~~~~~~~~~~
+
+- Small optimizations to help reduce indexing speed of datasets (:pull:`9003`).
+  By `Mark Harfouche <https://github.com/hmaarrfk>`_.
+
+
 Breaking changes
 ~~~~~~~~~~~~~~~~
 

xarray/core/dataset.py

@@ -3039,20 +3039,40 @@ def isel(
         coord_names = self._coord_names.copy()
 
         indexes, index_variables = isel_indexes(self.xindexes, indexers)
+        all_keys = set(indexers.keys())
 
         for name, var in self._variables.items():
             # preserve variable order
             if name in index_variables:
                 var = index_variables[name]
-            else:
-                var_indexers = {k: v for k, v in indexers.items() if k in var.dims}
-                if var_indexers:
+                dims.update(zip(var.dims, var.shape))
+            # Fastpath, skip all this metadata analysis for variables
+            # with no dimensions
+            # Keep the result of var.dims cached for future accesss to it
+            #
+            # Optimization Note from hmaarrfk - 2024/06
+            #     https://github.com/pydata/xarray/pull/9003#discussion_r1592767493
+            # It was found that accessing var.dims is faster than
+            # using var.shape or var.ndim since resolving both is typically
+            # left to the underlying array that each Xarray structure wraps.
+            # By using var.dims, we can avoid the cost of resolving the
+            # underlying array's shape and ndim since the dims are already
+            # cached by the Variable
+            elif len(var_dims := var.dims):
+                # Large datasets with alot of metadata may have many scalars
+                # without any relevant dimensions for slicing.
+                # Pick those out quickly and avoid paying the cost below
+                # of resolving the var_indexers variables
+                if var_indexer_keys := all_keys.intersection(var_dims):
+                    var_indexers = {k: indexers[k] for k in var_indexer_keys}
                     var = var.isel(var_indexers)
                     if drop and var.ndim == 0 and name in coord_names:
                         coord_names.remove(name)
                         continue
+                    # Update our reference to `var_dims` after the call to isel
+                    var_dims = var.dims
+                dims.update(zip(var_dims, var.shape))
             variables[name] = var
-            dims.update(zip(var.dims, var.shape, strict=True))
 
         return self._construct_direct(
             variables=variables,