Update migration to clean up duplicate stats before adding unique index

db/migrate/20251208210410_add_composite_index_to_stats.rb

@@ -3,16 +3,55 @@
 class AddCompositeIndexToStats < ActiveRecord::Migration[8.0]
   disable_ddl_transaction!
 
+  BATCH_SIZE = 1000
+
   def change
-    # Add composite index for the most common stats lookup pattern:
-    # Stat.find_or_initialize_by(year:, month:, user:)
-    # This query is called on EVERY stats calculation
-    #
-    # Using algorithm: :concurrently to avoid locking the table during index creation
-    # This is crucial for production deployments with existing data
+    total_duplicates = execute(<<-SQL.squish).first['count'].to_i
+      SELECT COUNT(*) as count
+      FROM stats s1
+      WHERE EXISTS (
+        SELECT 1 FROM stats s2
+        WHERE s2.user_id = s1.user_id
+          AND s2.year = s1.year
+          AND s2.month = s1.month
+          AND s2.id > s1.id
+      )
+    SQL
+
+    if total_duplicates.positive?
+      Rails.logger.info(
+        "Found #{total_duplicates} duplicate stats records. Starting cleanup in batches of #{BATCH_SIZE}..."
+      )
+    end
+
+    deleted_count = 0
+    loop do
+      batch_deleted = execute(<<-SQL.squish).cmd_tuples
+        DELETE FROM stats s1
+        WHERE EXISTS (
+          SELECT 1 FROM stats s2
+          WHERE s2.user_id = s1.user_id
+            AND s2.year = s1.year
+            AND s2.month = s1.month
+            AND s2.id > s1.id
+        )
+        LIMIT #{BATCH_SIZE}
+      SQL
+
+      break if batch_deleted.zero?
+
+      deleted_count += batch_deleted
+      Rails.logger.info("Cleaned up #{deleted_count}/#{total_duplicates} duplicate stats records")
+    end
+
+    Rails.logger.info("Completed cleanup: removed #{deleted_count} duplicate stats records") if deleted_count.positive?
+
     add_index :stats, %i[user_id year month],
               name: 'index_stats_on_user_id_year_month',
               unique: true,
-              algorithm: :concurrently
+              algorithm: :concurrently,
+              if_not_exists: true
+
+    BulkStatsCalculatingJob.perform_later
   end
 end