Should Xarray stop doing automatic index-based alignment?

What is your issue?

I am increasingly thinking that automatic index-based alignment in Xarray (copied from pandas) may have been a design mistake. Almost every time I work with datasets with different indexes, I find myself writing code to explicitly align them:

1. Automatic alignment is hard to predict. The implementation is complicated, and the exact mode of automatic alignment (outer vs inner vs left join) depends on the specific operation. It's also no longer possible to predict the shape (or even the dtype) resulting from most Xarray operations purely from input shape/dtype.
2. Automatic alignment brings unexpected performance penalty. In some domains (analytics) this is OK, but in others (e.g,. numerical modeling or deep learning) this is a complete deal-breaker.
3. Automatic alignment is not useful for float indexes, because exact matches are rare. In practice, this makes it less useful in Xarray's usual domains than it for pandas.

Would it be insane to consider changing Xarray's behavior to stop doing automatic alignment? I imagine we could roll this out slowly, first with warnings and then with an option for disabling it.

If you think this is a good or bad idea, consider responding to this issue with a 👍 or 👎 reaction.

Start by reading the issue body and its 13-comment discussion to understand the competing concerns around automatic index-based alignment. The issue names no files, tests, or entry points. Done would require an agreed design for changing or deprecating automatic alignment, including the warning and opt-out behavior described here.