Disallow picking output columns from nested columns.

[devavret]

Only top level columns can be selected by name

Fixes #7229

[vuule]

As we discussed offline - you can use the file included in the issue for testing.

[jlowe]

Only top level columns can be selected by name

To implement efficient queries on nested types we'll need the ability to specify a non-top-level column to load (e.g.: only loading one of the three fields in a top-level struct column). Will we be able to specify a fully-qualified path to that column from the root?

[devavret]

Will we be able to specify a fully-qualified path to that column from the root?

That's exactly what we discussed offline. That we should probably enable that after this fix and the method to do that should be to specify not just the name but the path in schema e.g. "name.first"

[codecov]

Codecov Report

Merging #7248 (12120c0) into branch-0.18 (8860baf) will increase coverage by 0.10%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           branch-0.18    #7248      +/-   ##
===============================================
+ Coverage        82.09%   82.19%   +0.10%     
===============================================
  Files               97       99       +2     
  Lines            16474    16841     +367     
===============================================
+ Hits             13524    13843     +319     
- Misses            2950     2998      +48     

Impacted Files	Coverage Δ
python/cudf/cudf/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/_fuzz_testing/parquet.py	0.00% <ø> (ø)
python/cudf/cudf/_lib/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/_typing.py	92.30% <ø> (ø)
python/cudf/cudf/core/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/core/abc.py	87.23% <ø> (ø)
python/cudf/cudf/core/buffer.py	80.00% <ø> (+0.95%)	⬆️
python/cudf/cudf/core/column/__init__.py	100.00% <ø> (ø)
python/cudf/cudf/core/column/categorical.py	92.73% <ø> (-0.62%)	⬇️
python/cudf/cudf/core/column/column.py	87.75% <ø> (-0.39%)	⬇️
... and 68 more

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[vuule]

rerun tests

[vuule]

Perhaps the title should have 'disallow', rather than 'disallowing'.

[devavret]

@gpucibot merge