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
494095795,MDU6SXNzdWU0OTQwOTU3OTU=,3311,optimize compatibility checks in merge.unique_variable,2448579,closed,0,,,0,2019-09-16T14:43:39Z,2019-11-05T15:41:15Z,2019-11-05T15:41:15Z,MEMBER,,,,"Currently `merge.unique_variable` does this

```
    if equals is None:
        out = out.compute()
        for var in variables[1:]:
            equals = getattr(out, compat)(var)
            if not equals:
                break
```

`out` (`=variables[0]`) is always computed though it may not be necessary. 

One solution would be to loop through once checking attrs, shapes and `_data`. If these checks were satisfied, then we execute the above code","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3311/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,completed,13221727,issue
494079482,MDExOlB1bGxSZXF1ZXN0MzE3OTAzNDYz,3310,Raise error if cmap is list of colors,2448579,closed,0,,,0,2019-09-16T14:16:48Z,2019-09-17T14:49:35Z,2019-09-17T14:49:32Z,MEMBER,,0,pydata/xarray/pulls/3310,,"{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3310/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull