adriangb commented on issue #19187:
URL: https://github.com/apache/datafusion/issues/19187#issuecomment-3627692913

   > > Always returns the same output regardless of the input.
   > 
   > Isn't constant the same as "has no column references and no volatile 
functons" ?
   
   Maybe? I'm not sure that covers things like `generate_series()` or if 
someone wants to write a `PhysicalExpr` that produces different data every time 
(i.e. is like a volatile function but is not a function). Maybe best to wait 
until someone complains that this definition doesn't cover their use case?
   
   > Maybe we can consolidate a function (with good examples, etc) that helps 
define this more
   
   That might be a good step.
   
   > > "stable" expression (like column > 123) or a "
   > 
   > What does "stable" mean in this context?
   
   Has references to columns and produces the same output for the same data 
input, but is not constant / cannot be evaluated at aplanning time or 
simplified.
   
   


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