Prune propagation scores per valid_time to avoid massive single DELETE

lib/microwaveprop/propagation.ex

@@ -151,13 +151,26 @@ defmodule Microwaveprop.Propagation do
 
   @doc "Remove scores with valid_times older than 1 hour before the earliest current forecast."
   def prune_old_scores do
-    # Keep all forecast hours from the latest run + 1 hour buffer for the previous run
     cutoff = DateTime.add(DateTime.utc_now(), -2, :hour)
 
-    {deleted, _} = Repo.delete_all(from(gs in GridScore, where: gs.valid_time < ^cutoff))
+    # Get distinct old valid_times and delete one at a time to avoid massive single DELETEs
+    old_times =
+      Repo.all(
+        from(gs in GridScore,
+          where: gs.valid_time < ^cutoff,
+          select: gs.valid_time,
+          distinct: gs.valid_time
+        )
+      )
 
-    if deleted > 0 do
+    total =
-      Logger.info("PropagationScores: pruned #{deleted} old scores (valid_time < #{cutoff})")
+      Enum.reduce(old_times, 0, fn vt, acc ->
+        {deleted, _} = Repo.delete_all(from(gs in GridScore, where: gs.valid_time == ^vt))
+        acc + deleted
+      end)
+
+    if total > 0 do
+      Logger.info("PropagationScores: pruned #{total} old scores across #{length(old_times)} valid_times")
     end
   end