Add index for ci_runners.locked, feedback:

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4093#note_12571602
and
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4093#note_12571670

db/migrate/20160620115026_add_index_on_runners_locked.rb

@@ -0,0 +1,12 @@
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddIndexOnRunnersLocked < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def change
+    add_concurrent_index :ci_runners, :locked
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160616084004) do
+ActiveRecord::Schema.define(version: 20160620115026) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -291,6 +291,7 @@ ActiveRecord::Schema.define(version: 20160616084004) do
   end
 
   add_index "ci_runners", ["description"], name: "index_ci_runners_on_description_trigram", using: :gin, opclasses: {"description"=>"gin_trgm_ops"}
+  add_index "ci_runners", ["locked"], name: "index_ci_runners_on_locked", using: :btree
   add_index "ci_runners", ["token"], name: "index_ci_runners_on_token", using: :btree
   add_index "ci_runners", ["token"], name: "index_ci_runners_on_token_trigram", using: :gin, opclasses: {"token"=>"gin_trgm_ops"}