Add index to ci_trigger_requests for commit_id

https://gitlab.com/gitlab-org/gitlab-ce/pipelines.json makes
a number of unindexed slow queries. This index should
speed things up.

changelogs/unreleased/sh-add-index-to-ci-trigger-requests.yml

+---
+title: Add index to ci_trigger_requests for commit_id
+merge_request:
+author:

db/migrate/20170210075922_add_index_to_ci_trigger_requests_for_commit_id.rb

+class AddIndexToCiTriggerRequestsForCommitId < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def change
+    add_concurrent_index :ci_trigger_requests, :commit_id
+  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: 20170206101030) do
+ActiveRecord::Schema.define(version: 20170210075922) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -367,6 +367,8 @@ ActiveRecord::Schema.define(version: 20170206101030) do
     t.integer "commit_id"
   end
+  add_index "ci_trigger_requests", ["commit_id"], name: "index_ci_trigger_requests_on_commit_id", using: :btree
+
   create_table "ci_triggers", force: :cascade do |t|
     t.string "token"
     t.integer "project_id"