add a few useful indexes

These were found missing by watching the log of unindexed queries.

Change-Id: I1823eeb5e5e7dfc4351ced92a5a354aac05d97eb
Reviewed-on: https://gerrit.instructure.com/5261
Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>

db/migrate/20110825131747_add_useful_indexes.rb

@@ -0,0 +1,17 @@
+class AddUsefulIndexes < ActiveRecord::Migration
+  def self.up
+    add_index :courses, :uuid
+    add_index :content_tags, [ :associated_asset_id, :associated_asset_type ], :name => 'index_content_tags_on_associated_asset'
+    add_index :discussion_entries, :parent_id
+    add_index :learning_outcomes, [ :context_id, :context_type ]
+    add_index :role_overrides, :context_code
+  end
+
+  def self.down
+    remove_index :courses, :uuid
+    remove_index :content_tags, :name => 'index_content_tags_on_associated_asset'
+    remove_index :discussion_entries, :parent_id
+    remove_index :learning_outcomes, [ :context_id, :context_type ]
+    remove_index :role_overrides, :context_code
+  end
+end