Date, IDate, and POSIXct should be distinct classes for joins
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Assessment
- Difficulty
- 5/5
- Estimated time
- Over a week
- Newbie friendliness
- 35/100
Research direction
Start by examining the existing mergeType behavior and the join compatibility table linked from #6627. Read the context in #3506, #6602, and #6627 to determine how Date, IDate, and POSIXct should be treated as separate cases. Done means the join behavior follows the agreed compatibility rules and the referenced issues are addressed.
Written by the indexing model from the issue text.
Description
I think we should implement mergeType returning I?Date, POSIXct as separate cases. That would for example solve #3506 and could make the solution here clearer.
Originally posted by @MichaelChirico in https://github.com/Rdatatable/data.table/issues/6603#issuecomment-2504992807
#6602 highlights difficulty with our current approach of ignoring column class attributes for well-known classes. We already cater for two very common classes (factor and integer64); I see no reason to treat Date, POSIXct any differently.
Solving this would also solve #3506 with more generality. We should also keep #6627 (in particular the linked "join compatibility table") in mind.
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