[jira] [Created] (DRILL-6815) Improve code generation to handle functions with NullHandling.NULL_IF_NULL better

["Bohdan Kazydub (JIRA)" <ji...@apache.org>]

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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