issues: 513916063

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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
513916063 MDU6SXNzdWU1MTM5MTYwNjM= 3454 Large coordinate arrays trigger computation 14314623 closed 0     2 2019-10-29T13:27:00Z 2019-10-29T15:07:43Z 2019-10-29T15:07:43Z CONTRIBUTOR      

I want to bring up an issue that has tripped up my workflow with large climate models many times. I am dealing with large data arrays of vertical cell thickness. These are 4d arrays (x, y, z, time) but I would define them as coordinates, not data_variables in the xarrays data model (e.g. they should not be multiplied by a value if a dataset is multiplied). These sort of coordinates might become more prevalent with newer ocean models like MOM6

Whenever I assign these arrays as coordinates operations on the arrays seem to trigger computation, whereas they don't if I set them up as data_variables. The example below shows this behavior. Is this a bug or done on purpose? Is there a workaround to keep these vertical thicknesses as coordinates?

``` import xarray as xr import numpy as np import dask.array as dsa

create dataset with with vertical thickness dz as data variable

data = xr.DataArray(dsa.random.random([30, 50, 200, 1000]), dims=['x','y', 'z', 't']) dz = xr.DataArray(dsa.random.random([30, 50, 200, 1000]), dims=['x','y', 'z', 't']) ds = xr.Dataset({'data':data, 'dz':dz})

another dataset with dz as coordinate

ds_new = xr.Dataset({'data':data}) ds_new.coords['dz'] = dz ```

%%time test = ds['data'] * ds['dz'] CPU times: user 1.94 ms, sys: 19.1 ms, total: 21 ms Wall time: 21.6 ms

%%time test = ds_new['data'] * ds_new['dz'] CPU times: user 17.4 s, sys: 1.98 s, total: 19.4 s Wall time: 12.5 s

Output of xr.show_versions()

INSTALLED VERSIONS ------------------ commit: None python: 3.7.4 (default, Aug 13 2019, 15:17:50) [Clang 4.0.1 (tags/RELEASE_401/final)] python-bits: 64 OS: Darwin OS-release: 18.6.0 machine: x86_64 processor: i386 byteorder: little LC_ALL: None LANG: en_US.UTF-8 LOCALE: en_US.UTF-8 libhdf5: None libnetcdf: None xarray: 0.13.0+24.g4254b4af pandas: 0.25.1 numpy: 1.17.2 scipy: None netCDF4: None pydap: None h5netcdf: None h5py: None Nio: None zarr: None cftime: None nc_time_axis: None PseudoNetCDF: None rasterio: None cfgrib: None iris: None bottleneck: None dask: 2.5.0 distributed: 2.5.1 matplotlib: None cartopy: None seaborn: None numbagg: None setuptools: 41.2.0 pip: 19.2.3 conda: None pytest: 5.2.0 IPython: 7.8.0 sphinx: None 
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