issue_comments: 398240724

With groupby in xarray, we have two main cases:

1. groupby with reduction -- (e.g. ds.groupby('baz').mean(dim='x')). There is currently no problem here. The new dimension becomes baz and the array is chunked as {'baz': 1}.
2. groupby with no reduction -- (e.g. ds.groubpy('baz').apply(lambda x: x - x.mean())). In this case, the point of the out-of-order indexing is actually to put the array back together in its original order. In my last example above, according to the dot graph, it looks like there are four chunks right up until the end. They just have to be re-ordered. I imagine this should be cheap and simple, but I am probably overlooking something.

Case 2 seems similar to @shoyer's example: x[np.arange(4)[::-1]. Here we would just want to reorder the existing chunks.

If the chunk size before reindexing is not 1, then yes, one needs to do something more sophisticated. But I would argue that, if the array is being re-indexed along a dimension in which the chunk size is 1, a sensible default behavior would be to avoid aggregating into a big chunk and instead just pass the original chunks though in a new order.