html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/pull/4979#issuecomment-789599056,https://api.github.com/repos/pydata/xarray/issues/4979,789599056,MDEyOklzc3VlQ29tbWVudDc4OTU5OTA1Ng==,10194086,2021-03-03T10:11:43Z,2021-03-03T10:11:43Z,MEMBER,"One thing I did not see discussed is alignment (or did I miss this?). Currently alignment is based on the ""indexes"" or well I guess `pd.Index(dimension-coords)`. I assume not all potential indexes are useful for this? Alignment will still be based on 1D dimension-coords?

One (potential) edge case are `MultiIndex` which allow to align several non-dimension coordinates.


<details>

```python
import xarray as xr
da1 = xr.DataArray([1, 2, 3], dims=""x"", coords=dict(time=(""x"", [1, 2, 3]), exp=(""x"", [""a"", ""a"", ""b""])))
da2 = xr.DataArray([2, 3, 4], dims=""x"", coords=dict(time=(""x"", [1, 2, 3]), exp=(""x"", [""a"", ""a"", ""a""])))
da1 = da1.set_index(x=(""time"", ""exp""))
da2 = da2.set_index(x=(""time"", ""exp""))

da1 + da2
```

```python
<xarray.DataArray (x: 2)>
array([3, 5])
Coordinates:
  * x        (x) MultiIndex
  - time     (x) int64 1 2
  - exp      (x) object 'a' 'a'
```

</details>
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