add index on report snapshots

refs CNVS-5180

Change-Id: I57bb1e049af086fff2d943ec3c70ccb726d11883
Reviewed-on: https://gerrit.instructure.com/26030
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>

db/migrate/20131106161158_add_report_snapshots_index.rb

@@ -0,0 +1,12 @@
+class AddReportSnapshotsIndex < ActiveRecord::Migration
+  tag :postdeploy
+  disable_ddl_transaction!
+
+  def self.up
+    add_index :report_snapshots, [:report_type, :account_id, :created_at], algorithm: :concurrently, name: 'index_on_report_snapshots'
+  end
+
+  def self.down
+    remove_index :report_snapshots, name: 'index_on_report_snapshots'
+  end
+end