compute: form the min/max hierarchy's stage keys by byte slicing

The initial map rebuilt the values row datum by datum and packed (hash, key)
by iterating the key's datums. The values are the row's leading datums and
the key goes behind the hash unchanged, so copy both as bytes. Skip the row
hash when the hint leaves only the final stage, whose modulus is 1.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

Author: frankmcsherry

src/compute/src/render/reduce.rs

@@ -871,17 +871,25 @@ impl<'scope, T: RenderTimestamp> Context<'scope, T> {
                 let first_mod = buckets.get(0).copied().unwrap_or(1);
                 let aggregations = aggr_funcs.len();
 
-                // Gather the relevant keys with their hashes along with values ordered by aggregation_index.
+                // Gather the relevant keys with their hashes along with values ordered by
+                // aggregation_index. The values are the row's leading datums and the key goes
+                // behind the hash unchanged, so both are byte copies rather than datum round
+                // trips.
                 let mut stage = input.map(move |(key, row)| {
-                    let mut row_builder = SharedRow::get();
-                    let mut row_packer = row_builder.packer();
-                    row_packer.extend(row.iter().take(aggregations));
-                    let values = row_builder.clone();
-
-                    // Apply the initial mod here.
-                    let hash = values.hashed() % first_mod;
-                    let hash_key =
-                        row_builder.pack_using(std::iter::once(Datum::from(hash)).chain(&key));
+                    let (values, _) = row.split_at_datum(aggregations);
+                    let values = values.to_owned();
+
+                    // Apply the initial mod here; a lone final stage keys everything by zero
+                    // and need not hash at all.
+                    let hash = if first_mod == 1 {
+                        0
+                    } else {
+                        values.hashed() % first_mod
+                    };
+                    let mut hash_key = Row::default();
+                    let mut packer = hash_key.packer();
+                    packer.push(Datum::from(hash));
+                    packer.extend_by_row_ref(&key);
                     (hash_key, values)
                 });