jayzhan211 commented on code in PR #10214:
URL: https://github.com/apache/datafusion/pull/10214#discussion_r1579366076
##########
datafusion/expr/src/udaf.rs:
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@@ -354,6 +360,24 @@ pub trait AggregateUDFImpl: Debug + Send + Sync {
fn aliases(&self) -> &[String] {
&[]
}
+
+ /// Construct an expression that calculates the aggregate in reverse.
+ /// Typically the "reverse" expression is itself (e.g. SUM, COUNT).
+ /// For aggregates that do not support calculation in reverse,
+ /// returns None (which is the default value).
+ fn reverse_expr(&self) -> ReversedExpr {
+ ReversedExpr::NotSupported
+ }
+}
+
+#[derive(Debug)]
+pub enum ReversedExpr {
+ /// The expression is the same as the original expression, like SUM, COUNT
+ Identical,
+ /// The expression does not support reverse calculation, like ArrayAgg
+ NotSupported,
+ /// The expression is different from the original expression
+ Reversed(AggregateFunction),
Review Comment:
You can see the reverse expr in OrderSensitiveArrayAgg
```rust
fn reverse_expr(&self) -> Option<Arc<dyn AggregateExpr>> {
Some(Arc::new(Self {
name: self.name.to_string(),
input_data_type: self.input_data_type.clone(),
expr: self.expr.clone(),
nullable: self.nullable,
order_by_data_types: self.order_by_data_types.clone(),
// Reverse requirement:
ordering_req: reverse_order_bys(&self.ordering_req),
reverse: !self.reverse,
}))
}
```
We need the ordering info, it is not contained in either `AggregateUDF` or
`AggregateUDFImpl` function. Therefore, I return `AggregateFunction` instead.
Then, we can create AggregateExec with `create_physical_expr` to get the
physical-expr from these logical exprs.
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