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Improve documentation for AggregateUDFImpl::value_from_stats #12689

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Improve documentation for AggregateUDFImpl::value_from_stats #12689

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20 changes: 17 additions & 3 deletions datafusion/expr/src/udaf.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -95,8 +95,11 @@ impl fmt::Display for AggregateUDF {
}
}

/// Arguments passed to [`AggregateUDFImpl::value_from_stats`]
pub struct StatisticsArgs<'a> {
/// The statistics of the aggregate input
pub statistics: &'a Statistics,
/// The resolved return type of the aggregate function
pub return_type: &'a DataType,
/// Whether the aggregate function is distinct.
///
Expand Down Expand Up @@ -251,13 +254,16 @@ impl AggregateUDF {
}

/// Returns true if the function is max, false if the function is min
/// None in all other cases, used in certain optimizations or
/// None in all other cases, used in certain optimizations for
/// or aggregate
///
pub fn is_descending(&self) -> Option<bool> {
self.inner.is_descending()
}

/// Return the value of this aggregate function if it can be determined
/// entirely from statistics and arguments.
///
/// See [`AggregateUDFImpl::value_from_stats`] for more details.
pub fn value_from_stats(
&self,
statistics_args: &StatisticsArgs,
Expand Down Expand Up @@ -577,7 +583,15 @@ pub trait AggregateUDFImpl: Debug + Send + Sync {
fn is_descending(&self) -> Option<bool> {
None
}
// Return the value of the current UDF from the statistics

/// Return the value of this aggregate function if it can be determined
/// entirely from statistics and arguments.
///
/// Using a scalar valie rather than a runtime computation can significantly
/// improving query performance.
///
/// For example, if the minimum value of column `x` is known to be `42` from
/// statistics, then the aggregate `MIN(x)` should return `Some(ScalarValue(42))`
fn value_from_stats(&self, _statistics_args: &StatisticsArgs) -> Option<ScalarValue> {
None
}
Expand Down