id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
526754376,MDU6SXNzdWU1MjY3NTQzNzY=,3558,optimizing xarray operations for lazy array equality test,2448579,closed,0,,,0,2019-11-21T18:01:51Z,2020-02-24T18:26:30Z,2020-02-24T18:26:30Z,MEMBER,,,,"TLDR: I think we want `A.sel(x=A.x).equals(A)` to pass lazily. It doesn't do so currently.

Currently if I do `A.sel(x=A.x)`, this sticks in a `getitem` call in the dask graph which breaks our lazy array equality optimization. Here's an example

``` python
>>> A = xr.DataArray(np.arange(100), dims=""x"", coords={""x"": np.arange(100)}).chunk({""x"": 1})
>>> A.sel(x=A.x).variable.equals(A, equiv=xr.core.duck_array_ops.lazy_array_equiv)
None
```

**Questions:**

1. Where is the best place to do this? In `sel` or `isel`? Both? 
    Sticking the following in `sel` makes the above check return `True` which is what we want:
    ``` python
        if self._indexes:
            equals = []
            for index in indexers:
                equals.append(indexers[index].to_index().equals(self._indexes[index]))

            if all(equals):
                return self
    ```

    This doesn't handle slice objects though so that makes me think we'd want to add something similar to `isel` too.
2. What is the behaviour we want? `A.sel(x=A.x).equals(A)` or `A.sel(x=A.x) is A`? 
3. Doing the latter will mean changing `_to_temp_dataset` and `_from_temp_dataset` which suggests the constraint `A._from_temp_dataset(A._to_temp_dataset()) is A`? But this seems too strong to me. Do we only want to lazily satisfy an `equals` constraint rather than an `identical` constraint?
4. It seems like we'll want to add such short-circuits in many places (I have not checked all of these): `sortby`, `broadcast`, `align`, `reindex` (`transpose` does this now).","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3558/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue