PERF: Keep track of first unread PM and first unread group PM for user.

This optimization helps to filter away topics so that the joins on
related tables when querying for unread messages is not expensive.

db/migrate/20200902054531_add_first_unread_pm_a_to_group_user.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class AddFirstUnreadPmAToGroupUser < ActiveRecord::Migration[6.0]
+  def up
+    add_column :group_users, :first_unread_pm_at, :datetime, null: false, default: -> { 'CURRENT_TIMESTAMP' }
+
+    execute <<~SQL
+    UPDATE group_users gu
+    SET first_unread_pm_at = gu.created_at
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

db/migrate/20200902082203_add_first_unread_pm_at_to_user_stats.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class AddFirstUnreadPmAtToUserStats < ActiveRecord::Migration[6.0]
+  def up
+    add_column :user_stats, :first_unread_pm_at, :datetime, null: false, default: -> { 'CURRENT_TIMESTAMP' }
+
+    execute <<~SQL
+    UPDATE user_stats us
+    SET first_unread_pm_at = u.created_at
+    FROM users u
+    WHERE u.id = us.user_id
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end

db/post_migrate/20200903045539_add_index_topics_on_timestamps_private.rb

@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+class AddIndexTopicsOnTimestampsPrivate < ActiveRecord::Migration[6.0]
+  disable_ddl_transaction!
+
+  def up
+    execute <<~SQL
+    CREATE INDEX CONCURRENTLY IF NOT EXISTS
+    index_topics_on_timestamps_private
+    ON topics (bumped_at, created_at, updated_at)
+    WHERE deleted_at IS NULL AND archetype = 'private_message'
+    SQL
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end