id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
1976752481,PR_kwDOAMm_X85ekPdj,8412,Minimize duplication in `map_blocks` task graph,2448579,closed,0,,,7,2023-11-03T18:30:02Z,2024-01-03T04:10:17Z,2024-01-03T04:10:15Z,MEMBER,,0,pydata/xarray/pulls/8412,"Builds on #8560 

- [x] Closes #8409
- [x] Tests added
- [ ] User visible changes (including notable bug fixes) are documented in `whats-new.rst`

cc @max-sixty 

```
print(len(cloudpickle.dumps(da.chunk(lat=1, lon=1).map_blocks(lambda x: x))))
# 779354739 -> 47699827
print(len(cloudpickle.dumps(da.chunk(lat=1, lon=1).drop_vars(da.indexes).map_blocks(lambda x: x))))
# 15981508
```

This is a quick attempt. I think we can generalize this to minimize duplication.

The downside is that the graphs are not totally embarrassingly parallel any more.
This PR:
![image](https://github.com/pydata/xarray/assets/2448579/6e10d00a-53d5-42b9-8564-2008c6b65fbb)

vs main:
![image](https://github.com/pydata/xarray/assets/2448579/cb0c8c56-e636-45c5-9c0e-b37c64ed0c04)

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1975400777,PR_kwDOAMm_X85efqSl,8408,Generalize explicit_indexing_adapter,2448579,open,0,,,0,2023-11-03T03:29:40Z,2023-11-03T03:53:25Z,,MEMBER,,1,pydata/xarray/pulls/8408,Use `as_indexable` instead of `NumpyIndexingAdapter`,"{""url"": ""https://api.github.com/repos/pydata/xarray/issues/8408/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull