FIX: Optimize performance of UnescapeEventName migration on large sit…

…es (#284)

On large sites, the migration can take up to 4 hours which is not
something we want. This commit adds the following optimisations:

1. Add an index for notifications `WHERE notifcation_type IN (27, 28)`
1. Increase each batch size to 10000, the smaller the batch size the more
times we have to scan the notifications table.
1. Avoid having to start the scan of the notifications table from id 1
and instead start from the first event notification type.

Follow-up to 2719b9e

db/post_migrate/20220613073844_unescape_event_name.rb

@@ -3,10 +3,14 @@
 class UnescapeEventName < ActiveRecord::Migration[7.0]
   disable_ddl_transaction!
 
+  TEMP_INDEX_NAME = "_temp_discourse_calendar_unescape_event_name_migration"
+
   def up
     # event notifications
-    start = 1
-    limit = DB.query_single("SELECT MAX(id) FROM notifications WHERE notification_type IN (27, 28)").first.to_i
+    DB.exec("CREATE INDEX CONCURRENTLY #{TEMP_INDEX_NAME} ON notifications(id) WHERE notification_type IN (27, 28)")
+    start, limit = DB.query_single("SELECT MIN(id), MAX(id) FROM notifications WHERE notification_type IN (27, 28)")
+
+    return if !start
 
     notifications_query = <<~SQL
       SELECT id, data
@@ -16,27 +20,35 @@ def up
         notification_type IN (27, 28) AND
         data::json ->> 'topic_title' LIKE '%&%'
       ORDER BY id ASC
-      LIMIT 1000
+      LIMIT 10000
     SQL
+
     while true
       if start > limit
         break
       end
+
       max_seen = -1
+
       DB.query(notifications_query, start: start).each do |record|
         id = record.id
+
         if id > max_seen
           max_seen = id
         end
+
         data = JSON.parse(record.data)
         unescaped = CGI.unescapeHTML(data["topic_title"])
         next if unescaped == data["topic_title"]
         data["topic_title"] = unescaped
+
         DB.exec(<<~SQL, data: data.to_json, id: id)
           UPDATE notifications SET data = :data WHERE id = :id
         SQL
       end
-      start += 1000
+
+      start += 10000
+
       if max_seen > start
         start = max_seen + 1
       end
@@ -57,6 +69,8 @@ def up
         UPDATE discourse_post_event_events SET name = :unescaped_name WHERE id = :id
       SQL
     end
+  ensure
+    DB.exec("DROP INDEX IF EXISTS #{TEMP_INDEX_NAME}")
   end
 
   def down