FIX: optimise MoveNewSinceToTable (#12136)

* FIX: optimise MoveNewSinceToTable

Avoids shuffling all ids around to the app (only use min / max)
Ensure the query for boundaries is ordered by user_id

db/post_migrate/20210208022739_move_new_since_to_new_table.rb

@@ -7,8 +7,20 @@ class MoveNewSinceToNewTable < ActiveRecord::Migration[6.0]
   def up
     offset = 0
     loop do
-      user_stat_user_ids = DB.query("SELECT user_id FROM user_stats LIMIT #{BATCH_SIZE} OFFSET :offset", offset: offset).map(&:user_id)
-      break if user_stat_user_ids.count.zero?
+      min_id, max_id = DB.query_single(<<~SQL, offset: offset, batch_size: BATCH_SIZE)
+        SELECT min(user_id), max(user_id)
+        FROM (
+          SELECT user_id
+          FROM user_stats
+          ORDER BY user_id
+          LIMIT :batch_size
+          OFFSET :offset
+        ) X
+      SQL
+
+      # will return nil
+      break if !min_id
+
       sql = <<~SQL
         INSERT INTO dismissed_topic_users (user_id, topic_id, created_at)
         SELECT users.id, topics.id, user_stats.new_since
@@ -26,13 +38,15 @@ class MoveNewSinceToNewTable < ActiveRecord::Migration[6.0]
         LEFT JOIN topic_users ON topics.id = topic_users.topic_id AND users.id = topic_users.user_id
         LEFT JOIN dismissed_topic_users ON dismissed_topic_users.topic_id = topics.id AND users.id = dismissed_topic_users.user_id
         WHERE user_stats.new_since IS NOT NULL
-        AND user_stats.user_id IN (:user_stat_user_ids)
+        AND user_stats.user_id >= :min_id
+        AND user_stats.user_id <= :max_id
         AND topic_users.last_read_post_number IS NULL
         AND topics.id IS NOT NULL
         AND dismissed_topic_users.id IS NULL
         ORDER BY topics.created_at DESC
         ON CONFLICT DO NOTHING
       SQL
+
       DB.exec(sql,
                now: DateTime.now,
                last_visit: User::NewTopicDuration::LAST_VISIT,
@@ -40,9 +54,12 @@ class MoveNewSinceToNewTable < ActiveRecord::Migration[6.0]
                default_duration: SiteSetting.default_other_new_topic_duration_minutes,
                min_date: Time.at(SiteSetting.min_new_topics_time).to_datetime,
                private_message: Archetype.private_message,
-               user_stat_user_ids: user_stat_user_ids,
+               min_id: min_id,
+               max_id: max_id,
                max_new_topics: SiteSetting.max_new_topics)
-      offset = offset + BATCH_SIZE
+
+      offset += BATCH_SIZE
+
     end
   end