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/2064#issuecomment-531818131,https://api.github.com/repos/pydata/xarray/issues/2064,531818131,MDEyOklzc3VlQ29tbWVudDUzMTgxODEzMQ==,2448579,2019-09-16T15:03:12Z,2019-09-16T15:03:12Z,MEMBER," #3239 has been merged. Now `minimal` is more useful since you can specify `compat=""override""` to skip compatibility checking.

What's left is to change defaults to implement @shoyer's comment

> So I'm thinking that we probably want to combine ""all"" and ""minimal"" into a single mode to use as the default, and remove the other behavior, which is either useless or broken. Maybe it would make sense to come up with a new name for this mode, and to make both ""all"" and ""minimal"" deprecated aliases for it? In the long term, this leaves only two ""automatic"" modes for xarray.concat, which should make things simpler for users trying to figure this out.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,314764258
https://github.com/pydata/xarray/issues/2064#issuecomment-524021001,https://api.github.com/repos/pydata/xarray/issues/2064,524021001,MDEyOklzc3VlQ29tbWVudDUyNDAyMTAwMQ==,2448579,2019-08-22T18:22:37Z,2019-08-22T18:22:37Z,MEMBER,"Thanks for your input @bonnland. 

> The pandas concat() function uses the option join = {'inner', 'outer', 'left', 'right'} in order to mimic logical database join operations. If there is a reason that xarray cannot do the same, it is not obvious to me. I think the pandas options have the advantage of logical simplicity and traditional usage within database systems.

We do have a `join` argument that takes these arguments + 'override' which was added recently to skip expensive comparisons. This works for ""indexes"" or ""dimension coordinates"". An example: if you have 2 dataarrays, one on a coordinate `x=[1, 2, 3]` and the other on `x=[2,3,4]`, `join` lets you control the `x` coordinate of the output. This is done by `xr.align`.

What's under discussion here is what to do about variables duplicated across datasets or indeed, how do we know that these variables are duplicated across datasets when concatenating other variables.
","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,314764258
https://github.com/pydata/xarray/issues/2064#issuecomment-523960862,https://api.github.com/repos/pydata/xarray/issues/2064,523960862,MDEyOklzc3VlQ29tbWVudDUyMzk2MDg2Mg==,2448579,2019-08-22T15:42:10Z,2019-08-22T15:42:10Z,MEMBER,"I have a draft solution in #3239. It adds a new mode called ""sensible"" that acts like ""all"" when the concat dimension doesn't exist in the dataset and acts like ""minimal"" when the dimension is present. We can decide whether this is the right way i.e. add a new mode but the more fundamental problem is below.

The issue is dealing with variables that should not be concatentated in ""minimal"" mode (e.g. time-invariant non dim coords when concatenating in time). In this case, we want to skip the equality checks in `_calc_concat_over`. This is a common reason for poor `open_mfdataset` performance.

I thought the clean way to do this would be to add the `compat` kwarg to `concat` and then add `compat='override'` since the current behaviour is effectively `compat='equals'`. 

However, `merge` takes `compat` too and `concat` and `merge` support different `compat` arguments at present. This makes it complicated to easily thread `compat` down from `combine` or `open_mfdataset` without adding `concat_compat` and `merge_compat` which is silly.

So do we want to support all the other `compat` modes in `concat`? Things like `broadcast_equals` or `no_conflicts` are funny because they're basically `merge` operations and it means `concat` acts like both `stack`, `concat` and `merge`. OTOH if you have a set of variables with the same name from different datasets and you want to pick one of those (i.e. no concatenation), then you're basically doing `merge` anyway. This would require some refactoring since `concat` assumes the first dataset is a template for the rest.

@shoyer What do you think? ","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,314764258
https://github.com/pydata/xarray/issues/2064#issuecomment-519149757,https://api.github.com/repos/pydata/xarray/issues/2064,519149757,MDEyOklzc3VlQ29tbWVudDUxOTE0OTc1Nw==,2448579,2019-08-07T15:32:16Z,2019-08-07T15:32:16Z,MEMBER,"> Maybe it would make sense to come up with a new name for this mode, and to make both ""all"" and ""minimal"" deprecated aliases for it?

I'm in favour of this. What should we name this mode?

One comment on ""existing dimensions"" mode:
>    -  ""minimal"" does the right thing, concatenating only variables with the dimension.

For variables without the dimension, this will still raise a `ValueError` because `compat` can only be `'equals'` or `'identical'`. It seems to me like we need `compat='override'` and/or `compat='tolerance', tolerance=...` that would use numpy's approximate equality testing. This checking of non-dimensional coordinates is a common source of `mfdataset` issues. What do you think?
","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,314764258
https://github.com/pydata/xarray/issues/2064#issuecomment-511468454,https://api.github.com/repos/pydata/xarray/issues/2064,511468454,MDEyOklzc3VlQ29tbWVudDUxMTQ2ODQ1NA==,2448579,2019-07-15T16:15:51Z,2019-07-15T16:15:51Z,MEMBER,"@bonnland I don't think you want to change the default `data_vars` but instead update the heuristics as in this comment
> we shouldn't implicitly add a new dimensions to variables in the case where the dimension already exists in the dataset. We only need the heuristics/comparisons when an entirely new dimension is being added.","{""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,314764258