`ComplexCopier` missing support for FixedSizeBinary columns

[nbauernfeind]

Complex copier does not support fixed size binary.

java.lang.UnsupportedOperationException: FIXEDSIZEBINARY
	at org.apache.arrow.vector.complex.impl.ComplexCopier.getListWriterForReader(ComplexCopier.java:824)
	at org.apache.arrow.vector.complex.impl.ComplexCopier.writeValue(ComplexCopier.java:93)
	at org.apache.arrow.vector.complex.impl.ComplexCopier.copy(ComplexCopier.java:76)
	at org.apache.arrow.vector.complex.ListVector.copyFrom(ListVector.java:409)

[Maria-Berta]

Hi @lidavidm @nbauernfeind @axreldable! I'd like to work on this issue. I'm a CS graduate with hands-on Java experience and I've recently contributed to Apache phpMyAdmin through CodePath's AI301 course, so I'm familiar with the Apache contribution process.
From reading the stack trace, it looks like getListWriterForReader in ComplexCopier.java is missing a case for FIXEDSIZEBINARY — the fix would involve adding that case similar to how other types like VARBINARY are already handled. Could you confirm this approach is on the right track, and point me to any existing tests for ComplexCopier I should model new tests after?
Thanks!

[sandeshkr419]

Hi @axreldable!

I see your commits from Nov 2025 and Mar 2026 in issue history, are you planning to open a PR? Happy to help review or help @Maria-Berta to resolve this if you've moved on.

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And @Maria-Berta: yes, your analysis is on the right track!

If you happen to continue on this, one small correction: the missing case isn't only in getListWriterForReader. Check getStructWriterForReader, getMapWriterForReader, and the copy() switch. Also, take inspiration from the FIXED_SIZE_LIST case rather than VARBINARY as both FixedSizeBinary and FixedSizeList are parameterized types (they carry a width/size), so the pattern should be the same.

For tests, TestComplexCopier.java should be a good anchor point.

[Maria-Berta]

Okay thank you for the pointers! I will keep on working on this hopefully send a PR soon!

[Maria-Berta]

Hi @sandeshkr419, thanks for the guidance — just opened #1253 with the fix across all 4 switches (getListWriterForReader, getStructWriterForReader, getMapWriterForReader, and copy()), plus test coverage for List and Struct. Flagged a pre-existing limitation I found with Map values in the PR description — happy to discuss there.

@axreldable — didn't want to duplicate effort, so flagging in case you were still planning to pick this up! Happy to close my PR if you'd prefer to continue with yours.