From 05a8f401c20e2a0b35360544aba12905239cc369 Mon Sep 17 00:00:00 2001
From: Yorick Peterse <yorickpeterse@gmail.com>
Date: Tue, 19 Sep 2017 00:37:38 +0200
Subject: 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                          |  4 +---
 .../unreleased/reoganize-deployment-indexes.yml    |  5 ++++
 ...0170918222253_reorganize_deployments_indexes.rb | 28 ++++++++++++++++++++++
 ...03_add_deployments_index_for_last_deployment.rb | 21 ++++++++++++++++
 db/schema.rb                                       |  5 ++--
 5 files changed, 58 insertions(+), 5 deletions(-)
 create mode 100644 changelogs/unreleased/reoganize-deployment-indexes.yml
 create mode 100644 db/migrate/20170918222253_reorganize_deployments_indexes.rb
 create mode 100644 db/migrate/20170918223303_add_deployments_index_for_last_deployment.rb

diff --git a/app/models/environment.rb b/app/models/environment.rb
index 44e39e21442..b6868ccbe8f 100644
--- a/app/models/environment.rb
+++ b/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'
 
diff --git a/changelogs/unreleased/reoganize-deployment-indexes.yml b/changelogs/unreleased/reoganize-deployment-indexes.yml
new file mode 100644
index 00000000000..87734b4fe4b
--- /dev/null
+++ b/changelogs/unreleased/reoganize-deployment-indexes.yml
@@ -0,0 +1,5 @@
+---
+title: Reorganize indexes for the "deployments" table
+merge_request:
+author:
+type: other
diff --git a/db/migrate/20170918222253_reorganize_deployments_indexes.rb b/db/migrate/20170918222253_reorganize_deployments_indexes.rb
new file mode 100644
index 00000000000..139427ed2b9
--- /dev/null
+++ b/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
diff --git a/db/migrate/20170918223303_add_deployments_index_for_last_deployment.rb b/db/migrate/20170918223303_add_deployments_index_for_last_deployment.rb
new file mode 100644
index 00000000000..b91efb86d98
--- /dev/null
+++ b/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
diff --git a/db/schema.rb b/db/schema.rb
index 2d8c33591f0..36f4c88ba70 100644
--- a/db/schema.rb
+++ b/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|
-- 
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