dmails: add indexes on is_read and is_deleted (fix #2886).

db/migrate/20170218104710_add_indexes_to_dmails.rb

@@ -0,0 +1,7 @@
+class AddIndexesToDmails < ActiveRecord::Migration
+  def change
+    execute "set statement_timeout = 0"
+    add_index :dmails, :is_read
+    add_index :dmails, :is_deleted
+  end
+end