Re: [PR] Minor: Move `Monotonicity` to `expr` crate [arrow-datafusion]

["alamb (via GitHub)" <gi...@apache.org>]

alamb commented on code in PR #7820:
URL: https://github.com/apache/arrow-datafusion/pull/7820#discussion_r1359317116


##########
datafusion/physical-expr/src/functions.rs:
##########
@@ -964,47 +957,6 @@ fn func_order_in_one_dimension(
     }
 }
 
-/// This function specifies monotonicity behaviors for built-in scalar functions.
-/// The list can be extended, only mathematical and datetime functions are
-/// considered for the initial implementation of this feature.
-pub fn get_func_monotonicity(fun: &BuiltinScalarFunction) -> Option<FuncMonotonicity> {

Review Comment:
   Can we please avoid a breaking api change by at least adding `pub use datafusion_expr::get_func_monotonicity` in this file? 
   
   However, since we are going to move this code anyways, what  would you think about putting this code as a method on `BuiltinScalarFunction` itself, so that it was easier to find
   
   Something like
   
   ```rust
   impl BuiltinScalarFunction {
     pub fn monotonicity(&self) -> Option<FuncMonotonicity> {
     ...
      }
   }
   ```
   
   And leaving the `get_func_monotonicity` as a deprecated function, perhaps like
   
   ```rust
   #[deprecated(message="use BuiltinScalarFunction::monotonicity")]
   pub fn get_func_monotonicity(fun: &BuiltinScalarFunction) -> Option<FuncMonotonicity> {
     fun.monotonicity()
   }
   ```
   
   



##########
datafusion/physical-expr/src/functions.rs:
##########
@@ -903,14 +904,6 @@ pub fn create_physical_fun(
     })
 }
 
-/// Monotonicity of the `ScalarFunctionExpr` with respect to its arguments.
-/// Each element of this vector corresponds to an argument and indicates whether
-/// the function's behavior is monotonic, or non-monotonic/unknown for that argument, namely:
-/// - `None` signifies unknown monotonicity or non-monotonicity.
-/// - `Some(true)` indicates that the function is monotonically increasing w.r.t. the argument in question.
-/// - Some(false) indicates that the function is monotonically decreasing w.r.t. the argument in question.
-pub type FuncMonotonicity = Vec<Option<bool>>;

Review Comment:
   Perhaps we can leave a `pub use datafusion_expr::FuncMonotonicty` in this module to ease backwards compatibility?



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