[BUG] Legacy join APIs return extraneous columns

[jlowe]

Describe the bug
While updating the Java bindings to the new join gather map APIs added in #7454, I noticed that the old join APIs now return more columns than they did before. They seem to be returning a table that contains the key columns from both the left and right tables, whereas before they returned only the key columns as they appeared in the left table (updated for the join result).

For example take two tables, the left having columns A,B and the right having columns C,D and we want to perform a join on A == C. Previously the join APIs would return a table with 3 columns, A, B, and D, but now they return 4 columns.

Steps/Code to reproduce bug
Call the left_join, inner_join, and full_join APIs before and after #7454.

Expected behavior
Redundant key columns should not be manifested. Manifesting the right key columns on an inner or left join seems to serve no purpose, as the left key columns already contain the proper answer for the join.

[harrism]

@shwina did you see this?

[shwina]

This was by design, although I can concede a lazy one on my part. @jrhemstad wanted the "common columns" parameter from the legacy APIs gone and I didn't replace it with reasonable default behaviour

One solution is to get rid of these APIs entirely. Python (and I believe Spark) don't use them anymore. If we still want them for the convenience of libcudf users, we could make it such that only key columns from the left are preserved for inner and left joins.

[shwina]

Sorry for closing! Wrong button

[github-actions]

This issue has been labeled inactive-30d due to no recent activity in the past 30 days. Please close this issue if no further response or action is needed. Otherwise, please respond with a comment indicating any updates or changes to the original issue and/or confirm this issue still needs to be addressed. This issue will be labeled inactive-90d if there is no activity in the next 60 days.

[jlowe]

Still relevant. The API should either be fixed or removed.

[github-actions]

This issue has been labeled inactive-90d due to no recent activity in the past 90 days. Please close this issue if no further response or action is needed. Otherwise, please respond with a comment indicating any updates or changes to the original issue and/or confirm this issue still needs to be addressed.

[jlowe]

I think this is still relevant, but low priority.

One solution is to get rid of these APIs entirely.

I think this is the most appropriate solution, since I cannot see a valid reason callers would want or expect duplicated key columns in the join output.