Merge branch 'blackst0ne/gitlab-ce-add_index_on_ci_builds_user_id'

This manually merges https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10874
into master after resolving conflicts.

changelogs/unreleased/add_index_on_ci_builds_user_id.yml

+---
+title: Add index on ci_builds.user_id
+merge_request: 10874
+author: blackst0ne

db/migrate/20170424095707_add_index_on_ci_builds_user_id.rb

+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddIndexOnCiBuildsUserId < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :ci_builds, :user_id
+  end
+
+  def down
+    remove_concurrent_index :ci_builds, :user_id if index_exists?(:ci_builds, :user_id)
+  end
+end

db/schema.rb

@@ -242,6 +242,7 @@ ActiveRecord::Schema.define(version: 20170424142900) do
   add_index "ci_builds", ["status"], name: "index_ci_builds_on_status", using: :btree
   add_index "ci_builds", ["token"], name: "index_ci_builds_on_token", unique: true, using: :btree
   add_index "ci_builds", ["updated_at"], name: "index_ci_builds_on_updated_at", using: :btree
+  add_index "ci_builds", ["user_id"], name: "index_ci_builds_on_user_id", using: :btree
 
   create_table "ci_pipelines", force: :cascade do |t|
     t.string "ref"