issue_comments: 1352989233

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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. 

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