issue_comments: 454351420

@malmans2 thanks for this reproducible test case!

From xarray's perspective, the difference is the order in which the arrays are concatenated/processed. This is determined by sorting the (globbed) file names: ``` In [16]: sorted(glob.glob('rep/.nc')) Out[16]: ['rep0/dsA0.nc', 'rep0/dsB0.nc', 'rep1/dsA1.nc', 'rep1/dsB1.nc']

In [17]: sorted(glob.glob('*.nc')) Out[17]: ['dsA0.nc', 'dsA1.nc', 'dsB0.nc', 'dsB1.nc'] ```

It appears that the slow case [A0, B0, A1, B1] now requires computing data with dask, whereas [A0, A1, B0, B1] does not.

I suspect the issue is that we're now using some different combination of merge/concat. In particular it looks like the compute is being triggered from within merge. This sort of makes sense: if we're using merge instead of concat for joining along the dimension T, that is super slow because that goes through a path that checks arrays for conflicting values by loading data into memory (even though in this case that isn't possible, because the original coordinates were not overlapping).

We could (and should) optimize this path in merge to avoid eagerly loading data, but the immediate fix here is probably to make sure we're using concat instead of merge.