Add temporary build stage priority partial index

db/migrate/20180417101040_add_tmp_stage_priority_index_to_ci_builds.rb

+class AddTmpStagePriorityIndexToCiBuilds < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index(:ci_builds, [:stage_id, :stage_idx],
+                         where: 'stage_idx IS NOT NULL', name: 'tmp_build_stage_priority_index')
+  end
+
+  def down
+    remove_concurrent_index_by_name(:ci_builds, 'tmp_build_stage_priority_index')
+  end
+end