aokolnychyi commented on code in PR #6304:
URL: https://github.com/apache/iceberg/pull/6304#discussion_r1035346108


##########
core/src/main/java/org/apache/iceberg/util/TableScanUtil.java:
##########
@@ -151,38 +153,68 @@ public static <T extends PartitionScanTask> 
List<ScanTaskGroup<T>> planTaskGroup
     Function<T, Long> weightFunc =
         task -> Math.max(task.sizeBytes(), task.filesCount() * openFileCost);
 
-    Map<Integer, StructProjection> projectionsBySpec = Maps.newHashMap();
+    Map<Integer, StructProjection> groupingKeyProjectionsBySpec = 
Maps.newHashMap();
 
-    // Group tasks by their partition values
-    StructLikeMap<List<T>> tasksByPartition = 
StructLikeMap.create(projectedPartitionType);
+    // group tasks by grouping keys derived from their partition tuples
+    StructLikeMap<List<T>> tasksByGroupingKey = 
StructLikeMap.create(groupingKeyType);
 
     for (T task : tasks) {
       PartitionSpec spec = task.spec();
-      StructProjection projectedStruct =
-          projectionsBySpec.computeIfAbsent(
+      StructProjection groupingKeyProjection =
+          groupingKeyProjectionsBySpec.computeIfAbsent(
               spec.specId(),
-              specId -> StructProjection.create(spec.partitionType(), 
projectedPartitionType));
-      List<T> taskList =
-          tasksByPartition.computeIfAbsent(
-              projectedStruct.copyFor(task.partition()), k -> 
Lists.newArrayList());
+              specId -> StructProjection.create(spec.partitionType(), 
groupingKeyType));
+      List<T> groupingKeyTasks =
+          tasksByGroupingKey.computeIfAbsent(
+              projectGroupingKey(groupingKeyProjection, groupingKeyType, task),
+              groupingKey -> Lists.newArrayList());
       if (task instanceof SplittableScanTask<?>) {
-        ((SplittableScanTask<? extends T>) 
task).split(splitSize).forEach(taskList::add);
+        ((SplittableScanTask<? extends T>) 
task).split(splitSize).forEach(groupingKeyTasks::add);
       } else {
-        taskList.add(task);
+        groupingKeyTasks.add(task);
       }
     }
 
-    // Now apply task combining within each partition
-    return FluentIterable.from(tasksByPartition.values())
-        .transformAndConcat(ts -> toTaskGroupIterable(ts, splitSize, lookback, 
weightFunc))
-        .toList();
+    List<ScanTaskGroup<T>> taskGroups = Lists.newArrayList();
+
+    for (Map.Entry<StructLike, List<T>> entry : tasksByGroupingKey.entrySet()) 
{
+      StructLike groupingKey = entry.getKey();
+      List<T> groupingKeyTasks = entry.getValue();
+      Iterables.addAll(
+          taskGroups,
+          toTaskGroupIterable(groupingKey, groupingKeyTasks, splitSize, 
lookback, weightFunc));
+    }
+
+    return taskGroups;
+  }
+
+  private static StructLike projectGroupingKey(
+      StructProjection groupingKeyProjection,
+      Types.StructType groupingKeyType,
+      PartitionScanTask task) {
+
+    PartitionData groupingKey = new PartitionData(groupingKeyType);
+
+    groupingKeyProjection.wrap(task.partition());

Review Comment:
   Correct, but this would mean I no longer project the key in that method, so 
I'll have to rename it to something like `copyAsPartitionData`. That would make 
the method disconnected from the fact that we project this key from the 
partition, so using `PartitionData` for the key would be weird. On top, 
Spotless formatted the new code in a less readable way, which I can't change.
   
   I see your point but given that I can't change Spotless formatting, do you 
mind if I keep it this way?



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