Merging deletion vectors across a deletion-vectors.bitmap64 flip crashes with a class-type error

[LuciferYang]

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• I searched in the issues and found no similar issues.

Paimon version

master (1.5-SNAPSHOT)

Compute Engine

Any engine writing an append or bucketed-append table with deletion vectors.

Minimal reproduce step

1. Create an append table with deletion vectors and write some deletes, stored as bitmap32 (the default).
2. ALTER TABLE t SET ('deletion-vectors.bitmap64' = 'true').
3. Delete more rows on a data file that already has a stored bitmap32 deletion vector.

What doesn't meet your expectations?

The delete crashes with RuntimeException: Only instance with the same class type can be merged.. deletion-vectors.bitmap64 is not an immutable option, so it can be flipped on a table that already has deletion vectors. After the flip, a freshly created deletion vector (bitmap64) has to merge with the stored one (bitmap32), and Bitmap64DeletionVector.merge / BitmapDeletionVector.merge reject a different class. The reverse flip (bitmap64 back to false, stored bitmap64 + fresh bitmap32) crashes the same way. Both the unaware-append and bucketed-append maintainers are affected.

Anything else?

Deletion vectors are deserialized by magic number, so bitmap32 and bitmap64 vectors already coexist in one index file; merging across the two formats just needs to promote the narrower one first.

Are you willing to submit a PR?

• I'm willing to submit a PR!