avantgardnerio commented on code in PR #7192:
URL: https://github.com/apache/arrow-datafusion/pull/7192#discussion_r1286487885


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datafusion/core/src/physical_plan/aggregates/priority_queue.rs:
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@@ -0,0 +1,235 @@
+use uuid::Uuid;
+use std::collections::{BTreeMap, HashMap};
+use std::pin::Pin;
+use std::sync::Arc;
+use std::task::{Context, Poll};
+use arrow::row::{OwnedRow, RowConverter, SortField};
+use arrow::util::pretty::print_batches;
+use arrow_array::{RecordBatch};
+use arrow_schema::{DataType, SchemaRef, SortOptions};
+use futures::stream::{Stream, StreamExt};
+use hashbrown::HashSet;
+use datafusion_common::DataFusionError;
+use datafusion_execution::TaskContext;
+use datafusion_physical_expr::{PhysicalExpr};
+use crate::physical_plan::aggregates::{aggregate_expressions, AggregateExec, 
evaluate_group_by, evaluate_many, group_schema, PhysicalGroupBy};
+use crate::physical_plan::{RecordBatchStream, SendableRecordBatchStream};
+use datafusion_common::Result;
+use datafusion_physical_expr::expressions::{Max, Min};
+
+pub(crate) struct GroupedPriorityQueueAggregateStream {
+    schema: SchemaRef,
+    input: SendableRecordBatchStream,
+    aggregate_arguments: Vec<Vec<Arc<dyn PhysicalExpr>>>,
+    group_by: PhysicalGroupBy,
+    group_converter: RowConverter,
+    value_converter: RowConverter, // TODO: use accumulators
+    group_to_val: HashMap<OwnedRow, OwnedRow>, // TODO: BTreeMap->BinaryHeap, 
OwnedRow->Rows
+    val_to_group: BTreeMap<OwnedRow, HashSet<OwnedRow>>,

Review Comment:
   > BinaryHeap sounds good
   
   It does, but the standard library implementation doesn't have all the 
requisite [traversal and rewrite 
methods](https://github.com/avantgardnerio/arrow-datafusion/blob/c6ce44f3bc5b787c96c52e574727bfd1f820e05d/datafusion/core/src/physical_plan/aggregates/priority_queue.rs#L260),
 which was why I started with `TreeMap`.
   
   I could see using something like https://lib.rs/crates/btreemultimap, but 
most of the multimaps I can find are just wrappers around `BTreeMap` with heap 
allocation of a `Vec` or `Set` similar to what I had already.
   
   I can begin hunting for a suitable off-the-shelf implementation, or build my 
own, but advice from those here would be appreciated.



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