issue_comments: 364625429

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https://github.com/pydata/xarray/pull/1899#issuecomment-364625429 https://api.github.com/repos/pydata/xarray/issues/1899 364625429 MDEyOklzc3VlQ29tbWVudDM2NDYyNTQyOQ== 1217238 2018-02-10T04:33:44Z 2018-02-10T04:33:44Z MEMBER

in case we want to get three diagonal elements (1, 1), (2, 2), (3, 3) from a 1000x1000 array. What we want is array[[1, 2, 3], [1, 2, 3]]. It can be decomposed to array[1: 4, 1:4][[0, 1, 2], [0, 1, 2]]. We only need to load 3 x 3 part of the 1000 x 1000 array, then take its diagonal elements.

OK, this is pretty clever.

There are some obvious fail cases, e.g., if they want to pull out indices array[[1, -1], [1, -1]], in which case the entire array needs to be sliced. I wonder if we should try to detect these with some heuristics, e.g., if the size of the result is much (maybe 10x or 100x) smaller than the size of sliced arrays.

Also, we would want to avoid separating basic/vectorized for backends that support efficient vectorized indexing (scipy and zarr).

 
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