fix(job): hopefully fix pg timeouts in Cron::PurgeUnattachedBlobsJob

app/jobs/cron/purge_unattached_blobs_job.rb

@@ -1,9 +1,16 @@
 class Cron::PurgeUnattachedBlobsJob < Cron::CronJob
   self.schedule_expression = "every day at midnight"
 
-  def perform(*args)
-    ActiveStorage::Blob.unattached
-      .where('active_storage_blobs.created_at < ?', 24.hours.ago)
-      .find_each(&:purge_later)
+  def perform
+    # .in_batches { _1.each... } is more efficient in this case that in_batches.each_record or find_each
+    # because it plucks only ids in a preliminary query, then load records with selected columns in batches by ids.
+    # This is faster than other batch strategies, which load at once selected columns with an ORDER BY in the same query, triggering timeouts.
+    ActiveStorage::Blob.unattached.select(:id, :service_name, :created_at).in_batches do |relation|
+      relation.each do |blob|
+        return if blob.created_at > 24.hours.ago # not in where() because it's not an indexed column
+
+        blob.purge_later
+      end
+    end
   end
 end