Fix: Excessive bundle partitioning in AnVIL (#7528)

Author: hannes-ucsc

src/azul/indexer/__init__.py

@@ -580,8 +580,17 @@ def canning_qualifier(cls) -> str:
 class BundlePartition(UUIDPartition):
     """
     A binary partitioning of the UUIDs of outer entities in a bundle.
+
+    >>> BundlePartition.root
+    BundlePartition(prefix_length=0, prefix=0, group=4)
     """
 
+    #: We use the fifth group because the first group may not produce a random
+    #: distribution of the entities in some types of bundles. For example, all
+    #: entity UUIDs in an AnVIL replica bundle share a specifc batch prefix.
+    #:
+    group: int = 4
+
     #: 512 caused timeouts writing contributions, even in the retry Lambda
     #:
     max_partition_size: ClassVar[int] = 256