Bohdan Kazydub created DRILL-6815:
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Summary: Improve code generation to handle functions with
NullHandling.NULL_IF_NULL better
Key: DRILL-6815
URL: https://issues.apache.org/jira/browse/DRILL-6815
Project: Apache Drill
Issue Type: Improvement
Reporter: Bohdan Kazydub
If a (simple) function is declared with NULL_IF_NULL null handling strategy
(`nulls = NullHandling.NULL_IF_NULL`) there is a additional code generated
which checks if any of the inputs is NULL (not set). In case if there is,
output is set to be null otherwise function's code is executed and at the end
output value is marked as set in case if ANY of the inputs is OPTIONAL (see
[https://github.com/apache/drill/blob/8edeb49873d1a1710cfe28e0b49364d07eb1aef4/exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/DrillSimpleFuncHolder.java#L143).]
The problem is, this behavior makes it impossible to make output value NULL
from within [function's evaluation
body|https://github.com/apache/drill/blob/7b0c9034753a8c5035fd1c0f1f84a37b376e6748/exec/java-exec/src/main/java/org/apache/drill/exec/expr/DrillSimpleFunc.java#L22].
Which may prove useful in certain situations, e.g. when input is an empty
string and output should be NULL in the case etc. Sometimes it may result in
two separate functions instead of one with NULL_IF_NULL. It does not follow a
[Principle of Least
Astonishment|https://en.wikipedia.org/wiki/Principle_of_least_astonishment] as
effectively it behaves more like "null if and only if null" and documentation
for NULL_IF_NULL is currently
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