Aggregate spill: List<Dictionary> state whose values repeat within a group still fails under a memory limit #27240

Describe the bug

An aggregate whose state holds a large dictionary can still fail with Failed to allocate additional ... after #25853 (which fixes #25851), when the dictionary values repeat within a group but not across groups. #25853 sizes a dictionary by the values its keys reference only when fewer than two keys reference each value on average; otherwise it counts the keys only and leaves the dictionary shared, as on main. A dictionary whose values repeat a few times within a group, but whose groups share no values, takes the shared path, and spill and replay batches still carry the whole dictionary.

To Reproduce

Measured with datafusion-cli (release-nonlto) at the head of #25853 (29c6661):

SET datafusion.execution.target_partitions = 1;
SELECT count(*), sum(n) FROM (
  SELECT (v / 10) % 32 AS k,
         cardinality(array_agg(arrow_cast(concat(CAST(v / 10 AS VARCHAR), repeat('x', 100)), 'Dictionary(Int32, Utf8)'))) AS n
  FROM generate_series(1, 5000000) AS t(v)
  GROUP BY (v / 10) % 32);

datafusion-cli -m 24M fails with Failed to allocate additional 24.7 MB for SingleHashAggregateStream[0] with 0.0 B already allocated for this reservation. It passes at -m 64M.

Changing only how often each value repeats in a group (v / 10 in both places):

With 1000-byte values and target_partitions = 2, the final aggregate fails at -m 24M (Failed to allocate additional 16.8 MB for FinalHashAggregateStream[0]). With 1000-byte values and target_partitions = 1 over 2M rows, it fails at both 24M and 64M.

Expected behavior

The query completes under a memory limit large enough for one group, as the other repeat counts do.

Additional context

Compacting the dictionary of each spilled chunk would be cheap here, but whether it pays off depends on whether the groups share their values. If they do, each chunk writes its own copy and the spill file grows, which is why #25853 doesn't compact every dictionary (array_agg over 100 distinct 200-byte values spilled 371 MB, against 14.7 MB without compaction). Telling the two cases apart needs the number of distinct values referenced per row, which SpilledRowSizes doesn't track for List<Dictionary>.

Related: #25851, #25853, #26711.

Willingness to contribute

I would like to contribute a fix for this bug, but request guidance from the DataFusion community