Reorganize indexes for the "deployments" table

This removes the need for a default scope that adds a "WHERE project_id
= X" clause. This commit also includes an additional index for
Environment#last_deployment, ensuring this query uses just an index scan
instead of also applying a Filter.

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/36877

app/models/environment.rb

@@ -6,9 +6,7 @@ class Environment < ActiveRecord::Base
 
   belongs_to :project, required: true, validate: true
 
-  has_many :deployments,
-    -> (env) { where(project_id: env.project_id) },
-    dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
+  has_many :deployments, dependent: :destroy # rubocop:disable Cop/ActiveRecordDependent
 
   has_one :last_deployment, -> { order('deployments.id DESC') }, class_name: 'Deployment'
 

changelogs/unreleased/reoganize-deployment-indexes.yml

@@ -0,0 +1,5 @@
+---
+title: Reorganize indexes for the "deployments" table
+merge_request:
+author:
+type: other

db/migrate/20170918222253_reorganize_deployments_indexes.rb

@@ -0,0 +1,28 @@
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class ReorganizeDeploymentsIndexes < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  def up
+    add_index_if_not_exists :deployments, [:environment_id, :iid, :project_id]
+    remove_index_if_exists :deployments, [:project_id, :environment_id, :iid]
+  end
+
+  def down
+    add_index_if_not_exists :deployments, [:project_id, :environment_id, :iid]
+    remove_index_if_exists :deployments, [:environment_id, :iid, :project_id]
+  end
+
+  def add_index_if_not_exists(table, columns)
+    add_concurrent_index(table, columns) unless index_exists?(table, columns)
+  end
+
+  def remove_index_if_exists(table, columns)
+    remove_concurrent_index(table, columns) if index_exists?(table, columns)
+  end
+end

db/migrate/20170918223303_add_deployments_index_for_last_deployment.rb

@@ -0,0 +1,21 @@
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+
+class AddDeploymentsIndexForLastDeployment < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  # Set this constant to true if this migration requires downtime.
+  DOWNTIME = false
+
+  disable_ddl_transaction!
+
+  TO_INDEX = [:deployments, %i[environment_id id]].freeze
+
+  def up
+    add_concurrent_index(*TO_INDEX)
+  end
+
+  def down
+    remove_concurrent_index(*TO_INDEX)
+  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: 20170914135630) do
+ActiveRecord::Schema.define(version: 20170918223303) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -506,7 +506,8 @@ ActiveRecord::Schema.define(version: 20170914135630) do
   end
 
   add_index "deployments", ["created_at"], name: "index_deployments_on_created_at", using: :btree
-  add_index "deployments", ["project_id", "environment_id", "iid"], name: "index_deployments_on_project_id_and_environment_id_and_iid", using: :btree
+  add_index "deployments", ["environment_id", "id"], name: "index_deployments_on_environment_id_and_id", using: :btree
+  add_index "deployments", ["environment_id", "iid", "project_id"], name: "index_deployments_on_environment_id_and_iid_and_project_id", using: :btree
   add_index "deployments", ["project_id", "iid"], name: "index_deployments_on_project_id_and_iid", unique: true, using: :btree
 
   create_table "emails", force: :cascade do |t|