Repository: UXARRAY/uxarray
Issue category: Performance Issue

Issue:
`edge_node_connectivity` unnecessarily constructed when slicing
When doing `Grid.isel()` with `n_face` or `n_node`, the `edge_node_connectivity` is constructed when the grid does not currently have any edges defined, which is not necessary and impacts the execution time. See also #1138 


Hints:

Patch:
diff --git a/uxarray/grid/slice.py b/uxarray/grid/slice.py
index 94e8e0eb8..8cce19d15 100644
--- a/uxarray/grid/slice.py
+++ b/uxarray/grid/slice.py
@@ -111,18 +111,23 @@ def _slice_face_indices(
     node_indices = np.unique(grid.face_node_connectivity.values[face_indices].ravel())
     node_indices = node_indices[node_indices != INT_FILL_VALUE]
 
-    # edges of each face (inclusive)
-    edge_indices = np.unique(grid.face_edge_connectivity.values[face_indices].ravel())
-    edge_indices = edge_indices[edge_indices != INT_FILL_VALUE]
-
     # index original dataset to obtain a 'subgrid'
     ds = ds.isel(n_node=node_indices)
     ds = ds.isel(n_face=face_indices)
-    ds = ds.isel(n_edge=edge_indices)
+
+    # Only slice edge dimension if we already have the connectivity
+    if "face_edge_connectivity" in grid._ds:
+        edge_indices = np.unique(
+            grid.face_edge_connectivity.values[face_indices].ravel()
+        )
+        edge_indices = edge_indices[edge_indices != INT_FILL_VALUE]
+        ds = ds.isel(n_edge=edge_indices)
+        ds["subgrid_edge_indices"] = xr.DataArray(edge_indices, dims=["n_edge"])
+    else:
+        edge_indices = None
 
     ds["subgrid_node_indices"] = xr.DataArray(node_indices, dims=["n_node"])
     ds["subgrid_face_indices"] = xr.DataArray(face_indices, dims=["n_face"])
-    ds["subgrid_edge_indices"] = xr.DataArray(edge_indices, dims=["n_edge"])
 
     # mapping to update existing connectivity
     node_indices_dict = {
@@ -152,9 +157,12 @@ def _slice_face_indices(
 
         index_types = {
             "face": face_indices,
-            "edge": edge_indices,
             "node": node_indices,
         }
+
+        if edge_indices is not None:
+            index_types["edge"] = edge_indices
+
         if isinstance(inverse_indices, bool):
             inverse_indices_ds["face"] = face_indices
         else:
diff --git a/uxarray/subset/dataarray_accessor.py b/uxarray/subset/dataarray_accessor.py
index 1775baa68..f5f64d9f0 100644
--- a/uxarray/subset/dataarray_accessor.py
+++ b/uxarray/subset/dataarray_accessor.py
@@ -67,7 +67,7 @@ def bounding_circle(
         self,
         center_coord: Union[Tuple, List, np.ndarray],
         r: Union[float, int],
-        element: Optional[str] = "nodes",
+        element: Optional[str] = "face centers",
         inverse_indices: Union[List[str], Set[str], bool] = False,
         **kwargs,
     ):
@@ -97,7 +97,7 @@ def nearest_neighbor(
         self,
         center_coord: Union[Tuple, List, np.ndarray],
         k: int,
-        element: Optional[str] = "nodes",
+        element: Optional[str] = "face centers",
         inverse_indices: Union[List[str], Set[str], bool] = False,
         **kwargs,
     ):
diff --git a/uxarray/subset/grid_accessor.py b/uxarray/subset/grid_accessor.py
index e42720db7..971ab477f 100644
--- a/uxarray/subset/grid_accessor.py
+++ b/uxarray/subset/grid_accessor.py
@@ -69,7 +69,7 @@ def bounding_circle(
         self,
         center_coord: Union[Tuple, List, np.ndarray],
         r: Union[float, int],
-        element: Optional[str] = "nodes",
+        element: Optional[str] = "face centers",
         inverse_indices: Union[List[str], Set[str], bool] = False,
         **kwargs,
     ):
@@ -108,7 +108,7 @@ def nearest_neighbor(
         self,
         center_coord: Union[Tuple, List, np.ndarray],
         k: int,
-        element: Optional[str] = "nodes",
+        element: Optional[str] = "face centers",
         inverse_indices: Union[List[str], Set[str], bool] = False,
         **kwargs,
     ):


Test patch:
diff --git a/test/test_subset.py b/test/test_subset.py
index 252719500..71be6dff5 100644
--- a/test/test_subset.py
+++ b/test/test_subset.py
@@ -24,14 +24,20 @@ def test_grid_face_isel():
     for grid_path in GRID_PATHS:
         grid = ux.open_grid(grid_path)
 
+        grid_contains_edge_node_conn = "edge_node_connectivity" in grid._ds
+
         face_indices = [0, 1, 2, 3, 4]
         for n_max_faces in range(1, len(face_indices)):
             grid_subset = grid.isel(n_face=face_indices[:n_max_faces])
             assert grid_subset.n_face == n_max_faces
+            if not grid_contains_edge_node_conn:
+                assert "edge_node_connectivity" not in grid_subset._ds
 
         face_indices = [0, 1, 2, grid.n_face]
         with pytest.raises(IndexError):
             grid_subset = grid.isel(n_face=face_indices)
+            if not grid_contains_edge_node_conn:
+                assert "edge_node_connectivity" not in grid_subset._ds
 
 
 def test_grid_node_isel():