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https://issues.apache.org/jira/browse/DRILL-6810?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16661119#comment-16661119
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Paul Rogers commented on DRILL-6810:
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Disabling of {{NULL_IF_NULL}} handling may lead to unexpected behavior: the UDF
writer things Drill will do it, Drill thinks the UDF writer will do it.
Better would be to fail to load the function if it has a complex writer and
requests {{NULL_IF_NULL}} handling.
Even better is to extend handling to correctly handle complex types. The rules
are simple (if a bit hard to implement):
* Arrays are set to zero length. (This must be done explicitly to ensure that
the offset vector entry is created.)
* Recurse into map members.
* Each map member must be nullable. If not, we can't set it to null. Either
fail the query, or punt and set the non-nullable field to a default value (0 or
"").
* For each nullable member, no need to do anything: Drill's fill-empties logic
will handle the case.
> Disable NULL_IF_NULL NullHandling for functions with ComplexWriter
> ------------------------------------------------------------------
>
> Key: DRILL-6810
> URL: https://issues.apache.org/jira/browse/DRILL-6810
> Project: Apache Drill
> Issue Type: Bug
> Affects Versions: 1.14.0
> Reporter: Bohdan Kazydub
> Assignee: Bohdan Kazydub
> Priority: Major
> Fix For: 1.15.0
>
>
> Currently NullHandling.NULL_IF_NULL is allowed for UDFs with @Output of type
> org.apache.drill.exec.vector.complex.writer.BaseWriter.ComplexWriter but no
> null handling is performed for the kind of functions which leads to
> confusion. The problem is ComplexWriter holds list/map values and Drill does
> not yet support NULL values for the types (there is an issue to allow null
> maps/lists in [DRILL-4824|https://issues.apache.org/jira/browse/DRILL-4824]).
> For such functions support for NULL_IF_NULL will be disabled, as it is done
> for aggregate functions, and NullHandling.INTERNAL should be used instead.
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