posts: add partial indexes on is_pending, is_flagged.

db/migrate/20170608043651_add_pending_flagged_indexes_to_posts.rb

@@ -0,0 +1,8 @@
+class AddPendingFlaggedIndexesToPosts < ActiveRecord::Migration
+  def change
+    Post.without_timeout do
+      add_index :posts, :is_pending, where: "is_pending = true"
+      add_index :posts, :is_flagged, where: "is_flagged = true"
+    end
+  end
+end