html_url,issue_url,id,node_id,user,created_at,updated_at,author_association,body,reactions,performed_via_github_app,issue
https://github.com/pydata/xarray/issues/186#issuecomment-73464476,https://api.github.com/repos/pydata/xarray/issues/186,73464476,MDEyOklzc3VlQ29tbWVudDczNDY0NDc2,1217238,2015-02-09T06:48:28Z,2015-02-09T06:48:28Z,MEMBER,"I have most of a working implementation for this that will be up for a PR shortly. Here's some of my thinking on expected behavior.

Based on the principle that combining arrays into a new dataset should not remove information, it makes sense to use outer joins for `Dataset.__init__` and `Dataset.merge`. This is the same behavior pandas uses

When adding an item to an existing dataset, it would be surprising if indexes or dimension sizes changed. So I think we should be using left joins for `__setitem__` and `update`. This is also what pandas does.

Right now, we use `Dataset.merge` to handle _all_ operations that add new items to a dataset. Adding automatic alignment is turning that into even more of a kludgy mess than it already was. I think some simplification of scope for `update`/`__setitem__`/`merge` would help, though I'm not sure it's worth breaking existing code.
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