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Posted to github@datafusion.apache.org by "kumarUjjawal (via GitHub)" <gi...@apache.org> on 2026/01/14 14:46:30 UTC
[PR] fix: select array_union([[null]], []) failing [datafusion]
kumarUjjawal opened a new pull request, #19813:
URL: https://github.com/apache/datafusion/pull/19813
## Which issue does this PR close?
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- Closes #19798
## Rationale for this change
The array_union function fails with an error when one of the input arrays contains nested null types (e.g., [[null]]) and the other is empty.
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## What changes are included in this PR?
- Helper function to detect if a data type contains Null in a nested structure
- Specialized set operation for List(Null) types that avoids using RowConverter.convert_rows()
- Specialized distinct operation for List(Null) types that uses length-based comparison instead of RowConverter
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## Are these changes tested?
Yes, existing sqllogictests cover these cases
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## Are there any user-facing changes?
No
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Re: [PR] fix: select array_union([[null]], []) failing [datafusion]
Posted by "kumarUjjawal (via GitHub)" <gi...@apache.org>.
kumarUjjawal commented on PR #19813:
URL: https://github.com/apache/datafusion/pull/19813#issuecomment-3750140053
I looked at the arrow-rs handling and as per my understanding, Arrow's RowConverter uses a Stateless codec for Null type elements. However, when Null is nested inside a List, the list codec wraps each element in variable-length encoding bytes. During `convert_rows()`, the nested Stateless decoder doesn't consume these wrapper bytes, causing the "did not consume all bytes" error.
This is a gap in arrow-rs where the list codec's byte protocol doesn't account for the zero-width nature of Null child elements. While this could be fixed upstream, List(Null) is a rare edge case that only arises from SQL literals like [[null]], so adding a workaround in DataFusion seemed more practical than modifying arrow-rs's row format. Let me know what are your thoughts on this?
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Re: [PR] fix: select array_union([[null]], []) failing [datafusion]
Posted by "Jefffrey (via GitHub)" <gi...@apache.org>.
Jefffrey commented on PR #19813:
URL: https://github.com/apache/datafusion/pull/19813#issuecomment-3750208917
I would prefer this to be fixed upstream if the error originates from arrow-rs code
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Re: [PR] fix: select array_union([[null]], []) failing [datafusion]
Posted by "kumarUjjawal (via GitHub)" <gi...@apache.org>.
kumarUjjawal closed pull request #19813: fix: select array_union([[null]], []) failing
URL: https://github.com/apache/datafusion/pull/19813
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Re: [PR] fix: select array_union([[null]], []) failing [datafusion]
Posted by "Jefffrey (via GitHub)" <gi...@apache.org>.
Jefffrey commented on PR #19813:
URL: https://github.com/apache/datafusion/pull/19813#issuecomment-3749993325
This seems like a lot of handling code for this niche case; do we have an explanation for why rowconverter can't handle this case? I see lots of mention of this but no reasoning behind it 🤔
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Re: [PR] fix: select array_union([[null]], []) failing [datafusion]
Posted by "kumarUjjawal (via GitHub)" <gi...@apache.org>.
kumarUjjawal commented on PR #19813:
URL: https://github.com/apache/datafusion/pull/19813#issuecomment-3750330136
I'll close this, maybe you can open an issue in arrow-rs.
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