Add index to find stuck merge requests.

See https://gitlab.com/gitlab-org/gitlab-ce/issues/45608#note_113803790
for query and plan.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/45608.

changelogs/unreleased/ab-45608-stuck-mr-query.yml

+---
+title: Add index to find stuck merge requests.
+merge_request: 22749
+author:
+type: performance

db/migrate/20181101144347_add_index_for_stuck_mr_query.rb

+# frozen_string_literal: true
+class AddIndexForStuckMrQuery < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :merge_requests, [:id, :merge_jid], where: "merge_jid IS NOT NULL and state = 'locked'"
+  end
+
+  def down
+    remove_concurrent_index :merge_requests, [:id, :merge_jid], where: "merge_jid IS NOT NULL and state = '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: 20181031190559) do
+ActiveRecord::Schema.define(version: 20181101144347) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -1294,6 +1294,7 @@ ActiveRecord::Schema.define(version: 20181031190559) do
   add_index "merge_requests", ["created_at"], name: "index_merge_requests_on_created_at", using: :btree
   add_index "merge_requests", ["description"], name: "index_merge_requests_on_description_trigram", using: :gin, opclasses: {"description"=>"gin_trgm_ops"}
   add_index "merge_requests", ["head_pipeline_id"], name: "index_merge_requests_on_head_pipeline_id", using: :btree
+  add_index "merge_requests", ["id", "merge_jid"], name: "index_merge_requests_on_id_and_merge_jid", where: "((merge_jid IS NOT NULL) AND ((state)::text = 'locked'::text))", using: :btree
   add_index "merge_requests", ["latest_merge_request_diff_id"], name: "index_merge_requests_on_latest_merge_request_diff_id", using: :btree
   add_index "merge_requests", ["merge_user_id"], name: "index_merge_requests_on_merge_user_id", where: "(merge_user_id IS NOT NULL)", using: :btree
   add_index "merge_requests", ["milestone_id"], name: "index_merge_requests_on_milestone_id", using: :btree