issue_comments: 326750703

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https://github.com/pydata/xarray/issues/1521#issuecomment-326750703 https://api.github.com/repos/pydata/xarray/issues/1521 326750703 MDEyOklzc3VlQ29tbWVudDMyNjc1MDcwMw== 6213168 2017-09-02T15:24:20Z 2017-09-02T15:27:12Z MEMBER

As suspected, the problem is caused specifically by non-index coords:

``` import xarray import numpy

data = numpy.random.randint(1<<63, size=1000000)

for r in range(50): ds = xarray.Dataset( coords={'r': [r], 'c': data, 'otherindex': data}, data_vars={'data': (('r', 'c'), data.reshape(1, data.size))}) ds.to_netcdf('fast.%02d.nc' % r) del ds['otherindex'] ds.coords['nonindex'] = ('c', data) ds.to_netcdf('slow.%02d.nc' % r)

def load_coords(ds): for coord in ds.coords.values(): coord.load() return ds %time xarray.open_mfdataset('fast..nc') %time xarray.open_mfdataset('fast..nc', preprocess=load_coords) %time xarray.open_mfdataset('slow..nc') %time xarray.open_mfdataset('slow..nc', preprocess=load_coords) output: CPU times: user 332 ms, sys: 88 ms, total: 420 ms Wall time: 420 ms CPU times: user 348 ms, sys: 84 ms, total: 432 ms Wall time: 430 ms CPU times: user 1.13 s, sys: 200 ms, total: 1.33 s Wall time: 1.07 s CPU times: user 596 ms, sys: 104 ms, total: 700 ms Wall time: 697 ms ```

 
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