viirya commented on pull request #32559:
URL: https://github.com/apache/spark/pull/32559#issuecomment-843648970


   > I did actually hit a bug today where the when value was being evaluated 
even though the condition was false. I wasn't able to find the exact root cause 
yet but turning off subexpression elimination fixed the issue. It was basically 
`when(col.rlike(...), udf(col))`, but more complex on both sides so somehow the 
UDF was getting subexpression eval'd early and failed because it didn't match 
the regular expression
   
   I see. Normally, for catalyst expressions, it is performance issue only. But 
for UDF, we cannot expect the logic put in UDF by users. It is possibly the UDF 
fails unexpectedly in such cases. So looks like it is better to backport it as 
a bug fix. 


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