issues: 864249974

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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
864249974 MDU6SXNzdWU4NjQyNDk5NzQ= 5202 Make creating a MultiIndex in stack optional 1217238 closed 0     7 2021-04-21T20:21:03Z 2022-03-17T17:11:42Z 2022-03-17T17:11:42Z MEMBER      

As @Hoeze notes in https://github.com/pydata/xarray/issues/5179, calling stack() can be "incredibly slow and memory-demanding, since it creates a MultiIndex of every possible coordinate in the array."

This is true with how stack() works currently, but I'm not sure this is necessary. I suspect it's a vestigial design choice from copying pandas, back from before Xarray had optional indexes. One benefit is that it's convenient for making unstack() the inverse of stack(), but isn't always required.

Regardless of how we define the semantics for boolean indexing (https://github.com/pydata/xarray/issues/1887), it seems like it could be a good idea to allow stack to skip creating a MultiIndex for the new dimension, via a new keyword argument such as ds.stack(index=False). This would be equivalent to calling reset_index() after stack() but would be cheaper because the MultiIndex is never created in the first place.

 
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