pepijnve opened a new pull request, #17991:
URL: https://github.com/apache/datafusion/pull/17991

   ## Which issue does this PR close?
   
   - Closes #17982
   
   ## Rationale for this change
   
   By making `NVLFunc` a wrapper for `CoalesceFunc` with a more restrictive 
signature the implementation automatically benefits from any optimisation work 
related to `coalesce`.
   
   ## What changes are included in this PR?
   
   - Make `NVLFunc` a thin wrapper of `CoalesceFunc`. This seemed like the 
simplest way to reuse the coalesce logic, but keep the stricter signature of 
`nvl`.
   - Add `ScalarUDF::conditional_arguments` as a more precise complement to 
`ScalarUDF::short_circuits`. By letting each function expose which arguments 
are eager and which are lazy, we provide more precise information to the 
optimizer which may enable better optimisation.
   
   ## Are these changes tested?
   
   Assumed to be covered by sql logic tests.
   Unit tests for the custom implementation were removed since those are no 
longer relevant.
   
   ## Are there any user-facing changes?
   
   The rewriting of `nvl` to `case when ... then ... else ... end is visible in 
the physical query plan.


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