alamb commented on a change in pull request #8480:
URL: https://github.com/apache/arrow/pull/8480#discussion_r506512980
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File path: rust/datafusion/src/physical_plan/hash_aggregate.rs
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@@ -210,13 +260,60 @@ Example: average
* Once all N record batches arrive, `merge` is performed, which builds a
RecordBatch with N rows and 2 columns.
* Finally, `get_value` returns an array with one entry computed from the state
*/
-struct GroupedHashAggregateIterator {
- mode: AggregateMode,
- schema: SchemaRef,
+async fn grouped_aggregate(
+ input: DynFutureRecordBatchIterator,
group_expr: Vec<Arc<dyn PhysicalExpr>>,
aggr_expr: Vec<Arc<dyn AggregateExpr>>,
- input: SendableRecordBatchReader,
- finished: bool,
+ schema: SchemaRef,
+ mode: AggregateMode,
+) -> ArrowResult<RecordBatch> {
+ // the expressions to evaluate the batch, one vec of expressions per
aggregation
+ let aggregate_expressions = aggregate_expressions(&aggr_expr, &mode)
+ .map_err(|e| ExecutionError::into_arrow_external_error(e))?;
+
+ // mapping key -> (set of accumulators, indices of the key in the batch)
+ // * the indexes are updated at each row
+ // * the accumulators are updated at the end of each batch
+ // * the indexes are `clear`ed at the end of each batch
+ let accumulators: FnvHashMap<Vec<GroupByScalar>, (AccumulatorSet,
Box<Vec<u32>>)> =
+ FnvHashMap::default();
+
+ // this will be shared by multiple threads, which will update the
accumulator as required.
+ // todo: a mutex over all groups is _brutal_: this requires more care
+ let accumulators = Arc::new(Mutex::new(accumulators));
+ // place under an arc to avoid cloning vectors
+ let aggregate_expressions = Arc::new(aggregate_expressions);
+
+ let futures = input.map(|future_batch| {
+ // send each aggregation to its own thread
+ let accumulators = accumulators.clone();
+ let aggr_expr = aggr_expr.clone();
+ let aggregate_expressions = aggregate_expressions.clone();
+ let group_expr = group_expr.clone();
+ tokio::spawn(async move {
+ let batch = future_batch.await?;
+ let mut accumulators = accumulators.lock().unwrap();
+ group_aggregate_batch(
+ &mode,
+ &group_expr,
+ &aggr_expr,
+ &batch,
+ &mut accumulators,
+ &aggregate_expressions,
+ )
+ .map_err(ExecutionError::into_arrow_external_error)
+ })
+ });
+
+ // parallel computation of the aggregation
+ try_join_all(futures)
Review comment:
This pattern will have the downside of potentially requiring the entire
input to the hash_aggregate to be buffered if the aggregate consumes data more
slowly than it can be produced
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