Add progress indicators to MigrateAccountConversations (#9101)

* Add progress indicators to MigrateAccountConversations

* Avoid running expensive query for explain

* Use exec_query instead of execute

db/migrate/20181024224956_migrate_account_conversations.rb

@@ -14,12 +14,29 @@ class MigrateAccountConversations < ActiveRecord::Migration[5.2]
       sleep 1
     end
 
-    local_direct_statuses.find_each do |status|
+    total        = estimate_rows(local_direct_statuses) + estimate_rows(notifications_about_direct_statuses)
+    migrated     = 0
+    started_time = Time.zone.now
+    last_time    = Time.zone.now
+
+    local_direct_statuses.includes(:account, mentions: :account).find_each do |status|
       AccountConversation.add_status(status.account, status)
+      migrated += 1
+
+      if Time.zone.now - last_time > 1
+        say_progress(migrated, total, started_time)
+        last_time = Time.zone.now
+      end
     end
 
-    notifications_about_direct_statuses.find_each do |notification|
+    notifications_about_direct_statuses.includes(:account, mention: { status: [:account, mentions: :account] }).find_each do |notification|
       AccountConversation.add_status(notification.account, notification.target_status)
+      migrated += 1
+
+      if Time.zone.now - last_time > 1
+        say_progress(migrated, total, started_time)
+        last_time = Time.zone.now
+      end
     end
   end
 
@@ -28,16 +45,31 @@ class MigrateAccountConversations < ActiveRecord::Migration[5.2]
 
   private
 
+  def estimate_rows(query)
+    result = exec_query("EXPLAIN #{query.to_sql}").first
+    result['QUERY PLAN'].scan(/ rows=([\d]+)/).first&.first&.to_i || 0
+  end
+
+  def say_progress(migrated, total, started_time)
+    status = "Migrated #{migrated} rows"
+
+    percentage = 100.0 * migrated / total
+    status += " (~#{sprintf('%.2f', percentage)}%, "
+
+    remaining_time = (100.0 - percentage) * (Time.zone.now - started_time) / percentage
+
+    status += "#{(remaining_time / 60).to_i}:"
+    status += sprintf('%02d', remaining_time.to_i % 60)
+    status += ' remaining)'
+
+    say status, true
+  end
+
   def local_direct_statuses
-    Status.unscoped
-          .local
-          .where(visibility: :direct)
-          .includes(:account, mentions: :account)
+    Status.unscoped.local.where(visibility: :direct)
   end
 
   def notifications_about_direct_statuses
-    Notification.joins(mention: :status)
-                .where(activity_type: 'Mention', statuses: { visibility: :direct })
-                .includes(:account, mention: { status: [:account, mentions: :account] })
+    Notification.joins(mention: :status).where(activity_type: 'Mention', statuses: { visibility: :direct })
   end
 end