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/4804#issuecomment-849813462,https://api.github.com/repos/pydata/xarray/issues/4804,849813462,MDEyOklzc3VlQ29tbWVudDg0OTgxMzQ2Mg==,2448579,2021-05-27T17:33:45Z,2021-05-27T17:33:45Z,MEMBER,"Reopening for the suggestions in https://github.com/pydata/xarray/issues/4804#issuecomment-760114285

cc @AndrewWilliams3142 if you're looking for a small followup PR with potentially large impact :)","{""total_count"": 1, ""+1"": 1, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,785329941
https://github.com/pydata/xarray/issues/4804#issuecomment-767138669,https://api.github.com/repos/pydata/xarray/issues/4804,767138669,MDEyOklzc3VlQ29tbWVudDc2NzEzODY2OQ==,2448579,2021-01-25T21:57:03Z,2021-01-25T21:57:03Z,MEMBER,@kathoef we'd be happy to merge a PR with some of the suggestions proposed here.,"{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,785329941
https://github.com/pydata/xarray/issues/4804#issuecomment-759780514,https://api.github.com/repos/pydata/xarray/issues/4804,759780514,MDEyOklzc3VlQ29tbWVudDc1OTc4MDUxNA==,10194086,2021-01-13T22:32:47Z,2021-01-14T01:15:02Z,MEMBER,"@aaronspring I had a quick look at your version - do you have an idea why it is is faster? Does yours also work for dask arrays?

* In `a, b = xr.broadcast(a, b, exclude=dim)` why can you exclude `dim`?
* I think you could also use `a, b = xr.align(a, b, exclude=dim)` (`broadcast` has `join=""outer""` which fills it with `NA` which then get ignored; `align` uses  `join=""inner""`)
* Does your version work if the weights contain `NA`?","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,785329941
https://github.com/pydata/xarray/issues/4804#issuecomment-759795213,https://api.github.com/repos/pydata/xarray/issues/4804,759795213,MDEyOklzc3VlQ29tbWVudDc1OTc5NTIxMw==,10194086,2021-01-13T22:52:19Z,2021-01-13T22:52:19Z,MEMBER,"Another possibility is to replace

https://github.com/pydata/xarray/blob/cc53a77ff0c8aaf8686f0b0bd7f75985b74e2054/xarray/core/computation.py#L1327

with `xr.dot`. However, to do so, you need to replace `NA` with `0` (and I am not sure if that's worth it). Also the `min_count` needs to be addressed (but that should not be too difficult).","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,785329941
https://github.com/pydata/xarray/issues/4804#issuecomment-759745055,https://api.github.com/repos/pydata/xarray/issues/4804,759745055,MDEyOklzc3VlQ29tbWVudDc1OTc0NTA1NQ==,10194086,2021-01-13T21:17:34Z,2021-01-13T21:17:34Z,MEMBER,"Yes `if not valid_values.all()` is not lazy. That's the same problem as #4541 and therefore  #4559 can be an inspiration how to tackle this. It would be good to test if the check also makes this slower for numpy arrays? Then it could also be removed entirely. That would be counter-intuitive for me, but it seems to be faster for dask arrays...

Other improvements
* I am not sure if `/=` avoids a copy but if so, that's also a possibility to make it faster.
* We could add a short-cut for `skipna=False` (would require adding this option) or dtypes that cannot have `NA` values as follows:

```python
if skipna:
    # 2. Ignore the nans
    valid_values = da_a.notnull() & da_b.notnull()

    if not valid_values.all():
        da_a = da_a.where(valid_values)
        da_b = da_b.where(valid_values)

    valid_count = valid_values.sum(dim) - ddof
else:
    # shortcut for skipna=False
    # da_a and da_b are aligned, so the have the same dims and shape
    axis = da_a.get_axis_num(dim)
    valid_count = np.take(da_a.shape, axis).prod() - ddof

```


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