non-jank idempotent query to populate volume_resource_usage

schema/crdb/crucible-ref-count-records/up08.sql

@@ -2,15 +2,7 @@
    snapshot records to populate the volume resource usage records */
 INSERT INTO volume_resource_usage (
   SELECT
-    (
-      OVERLAY(
-       OVERLAY(
-        MD5(volume.id::TEXT || dataset_id::TEXT || region_id::TEXT || snapshot_id::TEXT || snapshot_addr || volume_references::TEXT)
-        PLACING '4' from 13
-       )
-       PLACING '8' from 17
-      )::uuid
-    ) as usage_id,
+    gen_random_uuid() AS usage_id,
     volume.id AS volume_id,
     'region_snapshot' AS usage_type,
     NULL AS region_id,
@@ -22,10 +14,18 @@ INSERT INTO volume_resource_usage (
   JOIN
     region_snapshot
   ON
-    volume.data like '%' || region_snapshot.snapshot_addr || '%'
+    volume.data like ('%' || region_snapshot.snapshot_addr || '%')
+  LEFT JOIN
+    volume_resource_usage
+  ON
+    volume_resource_usage.volume_id = volume.id AND
+    volume_resource_usage.usage_type = 'region_snapshot' AND
+    volume_resource_usage.region_id IS NULL AND
+    volume_resource_usage.region_snapshot_region_id = region_snapshot.region_id AND
+    volume_resource_usage.region_snapshot_dataset_id = region_snapshot.dataset_id AND
+    volume_resource_usage.region_snapshot_snapshot_id = region_snapshot.snapshot_id
   WHERE
     volume.time_deleted is NULL AND
-    region_snapshot.deleting = false
-)
-ON CONFLICT (usage_id) DO NOTHING
-;
+    region_snapshot.deleting = false AND
+    volume_resource_usage.usage_id IS NULL
+);