PERF: Tune indexes to speed up lookup for bounced email.

db/migrate/20180719103905_alter_indexes_on_email_logs.rb

@@ -0,0 +1,12 @@
+class AlterIndexesOnEmailLogs < ActiveRecord::Migration[5.2]
+  def change
+    remove_index :email_logs,
+      name: "index_email_logs_on_user_id_and_created_at",
+      column: [:user_id, :created_at]
+
+    add_index :email_logs, :user_id
+
+    remove_index :email_logs, [:skipped, :created_at]
+    add_index :email_logs, [:skipped, :bounced, :created_at]
+  end
+end