Jimexist commented on a change in pull request #375:
URL: https://github.com/apache/arrow-datafusion/pull/375#discussion_r638360831
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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:
it is used down the road, in [`windows.rs` line 432][1]. I think if
possible i can change the return type of `scan_batch` directly to `ArrayRef` so
that upon scanning multiple batches, a single `concat` is needed.
[1]:
https://github.com/apache/arrow-datafusion/pull/413/files#diff-8b6b5ea3976c91229244e4e7a31a7026422b1374d1683e44b41af67a6bd43187R432
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