Add index to optimize loading pipeline charts

The pipeline charts were hitting a database timeout because the SQL
query used `ORDER BY id desc`, which caused the database to scan all
rows instead of only the rows indexed by `project_id`. To fix this
query, we create an index on (project_id, id DESC).

Closes https://gitlab.com/gitlab-org/gitlab/issues/118746

changelogs/unreleased/sh-add-pipeline-index.yml

+---
+title: Add index to optimize loading pipeline charts
+merge_request: 22052
+author:
+type: performance

db/post_migrate/20191218225624_add_index_on_project_id_to_ci_pipelines.rb

+# frozen_string_literal: true
+
+class AddIndexOnProjectIdToCiPipelines < ActiveRecord::Migration[5.2]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+  INDEX_NAME = 'index_ci_pipelines_on_project_id_and_id_desc'
+
+  disable_ddl_transaction!
+
+  def up
+    add_concurrent_index :ci_pipelines, [:project_id, :id], name: INDEX_NAME, order: { id: :desc }
+  end
+
+  def down
+    remove_concurrent_index :ci_pipelines, [:project_id, :id], name: INDEX_NAME, order: { id: :desc }
+  end
+end

db/schema.rb

@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 2019_12_16_183532) do
+ActiveRecord::Schema.define(version: 2019_12_18_225624) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "pg_trgm"
@@ -860,6 +860,7 @@ ActiveRecord::Schema.define(version: 2019_12_16_183532) do
     t.index ["external_pull_request_id"], name: "index_ci_pipelines_on_external_pull_request_id", where: "(external_pull_request_id IS NOT NULL)"
     t.index ["merge_request_id"], name: "index_ci_pipelines_on_merge_request_id", where: "(merge_request_id IS NOT NULL)"
     t.index ["pipeline_schedule_id"], name: "index_ci_pipelines_on_pipeline_schedule_id"
+    t.index ["project_id", "id"], name: "index_ci_pipelines_on_project_id_and_id_desc", order: { id: :desc }
     t.index ["project_id", "iid"], name: "index_ci_pipelines_on_project_id_and_iid", unique: true, where: "(iid IS NOT NULL)"
     t.index ["project_id", "ref", "id"], name: "index_ci_pipelines_on_project_idandrefandiddesc", order: { id: :desc }
     t.index ["project_id", "ref", "status", "id"], name: "index_ci_pipelines_on_project_id_and_ref_and_status_and_id"