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/2621#issuecomment-448817147,https://api.github.com/repos/pydata/xarray/issues/2621,448817147,MDEyOklzc3VlQ29tbWVudDQ0ODgxNzE0Nw==,1217238,2018-12-20T01:07:27Z,2018-12-20T01:07:27Z,MEMBER,"> This issue does not matter only in unstack but also in sel (reindex also?).

Can you explain why matters for `.sel()`? I guess it slows down repeated indexing?

My inclination was just to copy what pandas does (which is only removing unused levels in `unstack`)","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,392535505
https://github.com/pydata/xarray/pull/2621#issuecomment-448677802,https://api.github.com/repos/pydata/xarray/issues/2621,448677802,MDEyOklzc3VlQ29tbWVudDQ0ODY3NzgwMg==,1217238,2018-12-19T17:26:36Z,2018-12-19T17:26:36Z,MEMBER,"Should we update levels after indexing or when stacking? Pandas does the later:
```
In [31]: import pandas as pd

In [32]: mindex = pd.MultiIndex.from_product([[1, 2, 3], ['a', 'b']])

In [33]: s = pd.Series(range(6), mindex)

In [34]: s.loc[:1].index
Out[34]:
MultiIndex(levels=[[1, 2, 3], ['a', 'b']],
           labels=[[0, 0], [0, 1]])

In [35]: s.loc[:1].unstack()
Out[35]:
   a  b
1  0  1
```

The advantage of waiting until unstacking is that removing unused levels could be expensive compared to indexing: it runs in time O(L+N), where L is the size of the original level and N is the length of the new multi-index.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,392535505