Reorganize the GroupColumn implementations into more coherent modules

[alamb]

Is your feature request related to a problem or challenge?

While reviewing #12996 I noticed that all the implementations of GroupColumn for different types were in datafusion/physical-plan/src/aggregates/group_values/group_column.rs which is:

1. a single large file
2. Not obvious that it is like an implementation detail of GroupValuesColumn: https://github.com/apache/datafusion/blob/main/datafusion/physical-plan/src/aggregates/group_values/column.rs#L94-L93

Describe the solution you'd like

I would like to propose rearrainging the code:

1. Rename https://github.com/apache/datafusion/blob/main/datafusion/physical-plan/src/aggregates/group_values/column.rs to https://github.com/apache/datafusion/blob/main/datafusion/physical-plan/src/aggregates/group_values/multi_column/mod.rs
2. Move the implementations in datafusion/physical-plan/src/aggregates/group_values/group_column.rs to different modules within https://github.com/apache/datafusion/blob/main/datafusion/physical-plan/src/aggregates/group_values/multi_column (like primitive.rs, bytes.rs, etc)

Describe alternatives you've considered

No response

Additional context

No response

[alamb]

Once #12996 is merged, this would be a good first issue I think as it is just code movement and somewhat mechanical It would be a good way to get introduced to the code

[Chen-Yuan-Lai]

take

[alamb]

#12996 has now merged so this issue should now be unblocked

[jayzhan211]

@Chen-Yuan-Lai Have you started on this?

[Chen-Yuan-Lai]

Yes, I'm working on this, but have some questions:

1. I trying to rename /column.rs to /multi_column/mod.rs, and move /group_column.rs to different module. simultaneously, I've also moved some shared trait and function to /multi_column/mod.rs. However, /multi_column/mod.rs itself also defines and implements some data structures (e.g.,GroupValuesColumn, GroupIndexView). It feels too complex as a mod.rs file. Should I split it into other files as well?

2. In the original /group_column.rs, there were some test cases. After splitting /group_column.rs into three modules (/multi_column/byte.rs, /multi_column/byte_view.rs, and /multi_column/primitive.rs), I move the corresponding tests to their respective files. I'm not sure if this is reasonable, or if it would be better to keep all tests together in multi_column/test.rs?"

[jayzhan211]

Or you can name it as multi_column.rs and add the submodule in this this file. Structure is similar to datafusion/expr-common/src/type_coercion.rs and you can place trait in multi_column.rs.

I think it is fine to place test in separated file.