adriangb commented on code in PR #17351:
URL: https://github.com/apache/datafusion/pull/17351#discussion_r2324657452
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datafusion/physical-expr-common/src/physical_expr.rs:
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@@ -377,6 +377,19 @@ pub trait PhysicalExpr: Any + Send + Sync + Display +
Debug + DynEq + DynHash {
// static expressions will always return 0.
0
}
+
+ /// Returns true if the expression node is volatile, i.e. whether it can
return
+ /// different results when evaluated multiple times with the same input.
+ ///
+ /// Note: unlike [`is_volatile`], this function does not consider inputs:
+ /// - `random()` returns `true`,
+ /// - `a + random()` returns `false`
+ ///
+ /// By default, expressions are not volatile to avoid imposing API churn
on implementers.
+ /// It is highly recommended that volatile expressions implement this
method and return `true`.
+ fn is_volatile_node(&self) -> bool {
+ false
Review Comment:
I agree this is not a safe default long term. I can add a comment about it.
Do you think it's better to just eat the cost of the breaking change now? My
thought was that we've lived with a world where if you have a custom volatile
expression you can't teach DataFusion about it for a long time now. This just
means that you can fix it in your code instead of ours. But having no default
means breaking changes for a lot of people.
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