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
174390114,MDU6SXNzdWUxNzQzOTAxMTQ=,995,"xarray slicing is very slow, and reading time differs a lot between variables.",7747527,closed,0,,,2,2016-08-31T22:02:42Z,2016-09-01T03:24:49Z,2016-09-01T03:17:52Z,NONE,,,,"Hi everyone,

I've been working on calculating ocean heat transport across a prescribed section. Therefore, I need to calculate the V*T (velocity times T) on the grid points along that section.

Xarray has been the my favorite tool, ever since the day it was still xray. In my code, there is a loop (looping through grid points along the line) that determine if I need to use U (zonal velocity) or V (meridional velocity). Then times it by the temperature on that location, which is full depth. 

VVEL, UVEL and TEMP have structure like this (time: 1, z_t: 42, lat: 1800, lon: 3600)

```
ds = xarray.open_dataset(filename, decode_times=False)
vvel0=ds.VVEL.sel(lat=slice(-60,-20),lon=slice(0,40))/100    
uvel0=ds.UVEL.sel(lat=slice(-60,-20),lon=slice(0,40))/100  
temp0=ds.TEMP.sel(lat=slice(-60,-20),lon=slice(0,40)) 
```

The weird thing is, for VVEL and UVEL, it takes about 5sec with slicing, about 2 sec without slicing, but for TEMP, it only needs 6ms. 

This then leads to another issue, within the loop, I need to extract a vertical column of temperature, 

```
tt=np.squeeze(temp[:,yidx,xidx].values)
```

This line drags everything down, it takes about 4sec... but this is repeated for each loop (rouhgly 300 times).
I found out that, removing `.values` reduces the time to 2ms, but I need to extract the values to calculate V*T.

What's more interesting is that, in the loop, I also have this line

```
vv=np.squeeze(vvel[:,yidx,xidx].values)
```

Which has the exact same structure as the line I used to call the column values of temperature. But this line only needs about 1ms, with and without calling `.values`

---

Could someone please explain how is this possible? All these variables are from the same netcdf file. And I called them in the exact same way. Also, why is slicing takes even longer then loading in the full field? Any suggestion I can circumvent this bottleneck? Thank you.
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