Danny Chen commented on CALCITE-4094:

Well, it could, but i would prefer adding an interface to override instead of a 

Actually, there are many sql functions declared as SqlKind.OTHER_FUNCTION, not 
just the SqlUserDefinedFunction, should we consider those functions too ?

> Allow SqlUserDefinedFunction to define an optional Strong.Policy
> ----------------------------------------------------------------
>                 Key: CALCITE-4094
>                 URL: https://issues.apache.org/jira/browse/CALCITE-4094
>             Project: Calcite
>          Issue Type: Improvement
>          Components: core
>            Reporter: Ruben Q L
>            Assignee: Ruben Q L
>            Priority: Major
> {{FilterJoinRule}} performs some optimizations based on 
> {{RelOptUitl#simplifyJoin}}. Specifically, this part of the code:
> {code}
> if (joinType.generatesNullsOnRight()
>   && Strong.isNotTrue(filter, rightBitmap)) {
>     joinType = joinType.cancelNullsOnRight();
> }
> {code}
> allows e.g. a LEFT join with a condition on its RHS to be transformed into an 
> INNER join, with the condition pushed on its right input.
> In order to achieve this, the utility class {{Strong}} defines a certain 
> {{Policy}} map, depending on the {{SqlKind}} of a {{RexNode}} (i.e. depending 
> on the filter condition). In case of operators such as {{EQUALS}}, 
> {{NOT_EQUALS}}, {{LESS_THAN}}, etc, it defines {{Policy.ANY}} (i.e. 
> expression is null if and only if at least one of its arguments is null). In 
> case of a left join with conditions with this policy on the right-hand-side, 
> the join gets simplified by this module because of the nullability of the 
> right columns. However, in the case of {{SqlUserDefinedFunction}} the policy 
> is the default {{Policy.AS_IS}}, which prevents the simplification from 
> happening.
> It is proposed to enrich {{SqlUserDefinedFunction}} with an optional 
> {{Strong.Policy}}, and adapt {{Strong#isNull}} so that these functions can 
> also benefit from this simplification.

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