[GitHub] [arrow-datafusion] Jimexist commented on a change in pull request #375: add window expression stream, delegated window aggregation to aggregate functions, and implement `row_number`

[GitBox <gi...@apache.org>]

Jimexist commented on a change in pull request #375:
URL: https://github.com/apache/arrow-datafusion/pull/375#discussion_r638458423



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File path: datafusion/src/physical_plan/mod.rs
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@@ -509,6 +540,43 @@ pub trait Accumulator: Send + Sync + Debug {
     fn evaluate(&self) -> Result<ScalarValue>;
 }
 
+/// A window accumulator represents a stateful object that lives throughout the evaluation of multiple
+/// rows and generically accumulates values.
+///
+/// An accumulator knows how to:
+/// * update its state from inputs via `update`
+/// * convert its internal state to a vector of scalar values
+/// * update its state from multiple accumulators' states via `merge`
+/// * compute the final value from its internal state via `evaluate`
+pub trait WindowAccumulator: Send + Sync + Debug {
+    /// scans the accumulator's state from a vector of scalars, similar to Accumulator it also
+    /// optionally generates values.
+    fn scan(&mut self, values: &[ScalarValue]) -> Result<Option<ScalarValue>>;
+
+    /// scans the accumulator's state from a vector of arrays.
+    fn scan_batch(&mut self, values: &[ArrayRef]) -> Result<Option<Vec<ScalarValue>>> {

Review comment:
       @alamb I have updated to use `ArrayRef` instead of `Vec<ScalarValue>` whereas possible




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