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/issues/7376#issuecomment-1352989233,https://api.github.com/repos/pydata/xarray/issues/7376,1352989233,IC_kwDOAMm_X85QpPox,4160723,2022-12-15T12:27:37Z,2022-12-15T12:27:37Z,MEMBER,"> Thanks @benbovy! Are you also aware of the issue with plain assign being slower on MultiIndex (comment above: https://github.com/pydata/xarray/issues/7376#issuecomment-1350446546)? Do you know what could be the issue there by any chance?

I see that in `ds.assign(foo=~ds[""d3""])`, the coordinates of `~ds[""d3""]` are dropped (#2087), which triggers re-indexing of the multi-index when aligning `ds` with `~ds[""d3""]`. This is a quite expensive operation.

It is not clear to me what would be a clean fix (see, e.g., #2180), but we could probably optimize the alignment logic so that when all unindexed dimension sizes match with indexed dimension sizes (like your example) no re-indexing is performed. 

","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,1495605827
https://github.com/pydata/xarray/issues/7376#issuecomment-1352318926,https://api.github.com/repos/pydata/xarray/issues/7376,1352318926,IC_kwDOAMm_X85Qmr_O,4160723,2022-12-14T22:43:11Z,2022-12-14T22:47:37Z,MEMBER,"> Are you aware of any workarounds for this issue with the current code (assuming I would like to preserve MultiIndex).

Unfortunately I don't know about any workaround that would preserve the MultiIndex. Depending on how you use the multi-index, you could instead set two single indexes for ""i1"" and ""i2"" respectively (it is supported now, use `set_xindex()`). I think that groupby will work well in that case. If you really need a multi-index, you could still build it afterwards from the groupby result.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,1495605827
https://github.com/pydata/xarray/issues/7376#issuecomment-1350738301,https://api.github.com/repos/pydata/xarray/issues/7376,1350738301,IC_kwDOAMm_X85QgqF9,4160723,2022-12-14T09:40:57Z,2022-12-14T09:40:57Z,MEMBER,"Thanks for the report @ravwojdyla.

Since #5692, multi-indexes level have each their own coordinate variable so copying takes a bit more time as we need to create more variables. Not sure what's happening with `_maybe_cast_to_cftimeindex`, though.

The real issue here, however, is the same than in #6836. In your example, `.groupby(""i1"")` creates 400 000 groups whereas it should create only 4 groups.","{""total_count"": 1, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 1}",,1495605827