fix: Disallows dropping duplicate keys when using full outer join #1320
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kosiew
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Dec 28, 2025
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kosiew
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Thanks for working on this.
Documentation gaps:
The doc string for join in python/datafusion/dataframe.py states:
drop_duplicate_keys: When True, the columns from the right DataFrame
that have identical names in the ``on`` fields to the left DataFrame
will be dropped.It does not mention the full join exception. Users reading this may assume the parameter works the same for all join types.
Similarly, docs/source/user-guide/common-operations/joins.rst should also document for the full join and drop_duplicate_keys behaviour.
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Which issue does this PR close?
Closes #1305
Rationale for this change
Datafusion-python should follow datafusion implementation and disallow dropping keys when doing a full outer join as both keys are not equivalent thus they can't just be dropped. Users can then decide on how to proceed based on their use case.
What changes are included in this PR?
fix + unit test
Are there any user-facing changes?
yes, disallowing dropping keys when doing a full outer join as that is not semantically correct.