Prevent adding columns to wide tables

Add a cop to prevent adding columns to wide tables. At the moment the
only wide tables are users, projects and ci_builds with more than 50
columns each.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/201933

db/migrate/20180102220145_add_pages_https_only_to_projects.rb

@@ -4,6 +4,6 @@ class AddPagesHttpsOnlyToProjects < ActiveRecord::Migration[4.2]
   DOWNTIME = false
 
   def change
-    add_column :projects, :pages_https_only, :boolean
+    add_column :projects, :pages_https_only, :boolean # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20180228172924_add_include_private_contributions_to_users.rb

@@ -2,6 +2,6 @@ class AddIncludePrivateContributionsToUsers < ActiveRecord::Migration[4.2]
   DOWNTIME = false
 
   def change
-    add_column :users, :include_private_contributions, :boolean
+    add_column :users, :include_private_contributions, :boolean # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20180503141722_add_remote_mirror_available_overridden_to_projects.rb

@@ -6,7 +6,7 @@ class AddRemoteMirrorAvailableOverriddenToProjects < ActiveRecord::Migration[4.2
   disable_ddl_transaction!
 
   def up
-    add_column(:projects, :remote_mirror_available_overridden, :boolean) unless column_exists?(:projects, :remote_mirror_available_overridden)
+    add_column(:projects, :remote_mirror_available_overridden, :boolean) unless column_exists?(:projects, :remote_mirror_available_overridden) # rubocop:disable Migration/AddColumnsToWideTables
   end
 
   def down

db/migrate/20180722103201_add_private_profile_to_users.rb

@@ -5,6 +5,6 @@ class AddPrivateProfileToUsers < ActiveRecord::Migration[4.2]
   DOWNTIME = false
 
   def change
-    add_column :users, :private_profile, :boolean
+    add_column :users, :private_profile, :boolean # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20180814153625_add_commit_email_to_users.rb

@@ -27,7 +27,11 @@ class AddCommitEmailToUsers < ActiveRecord::Migration[4.2]
   # comments:
   # disable_ddl_transaction!
 
+  # rubocop:disable Migration/AddLimitToStringColumns
+  # rubocop:disable Migration/AddColumnsToWideTables
   def change
-    add_column :users, :commit_email, :string # rubocop:disable Migration/AddLimitToStringColumns
+    add_column :users, :commit_email, :string
   end
+  # rubocop:enable Migration/AddLimitToStringColumns
+  # rubocop:enable Migration/AddColumnsToWideTables
 end

db/migrate/20180924190739_add_scheduled_at_to_ci_builds.rb

@@ -4,6 +4,6 @@ class AddScheduledAtToCiBuilds < ActiveRecord::Migration[4.2]
   DOWNTIME = false
 
   def change
-    add_column :ci_builds, :scheduled_at, :datetime_with_timezone
+    add_column :ci_builds, :scheduled_at, :datetime_with_timezone # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20181019032408_add_repositories_table.rb

@@ -9,7 +9,7 @@ class AddRepositoriesTable < ActiveRecord::Migration[4.2]
       t.string :disk_path, null: false, index: { unique: true } # rubocop:disable Migration/AddLimitToStringColumns
     end
 
-    add_column :projects, :pool_repository_id, :bigint
+    add_column :projects, :pool_repository_id, :bigint # rubocop:disable Migration/AddColumnsToWideTables
     add_index :projects, :pool_repository_id, where: 'pool_repository_id IS NOT NULL'
   end
 end

db/migrate/20181116141504_add_encrypted_runners_token_to_projects.rb

@@ -5,7 +5,11 @@ class AddEncryptedRunnersTokenToProjects < ActiveRecord::Migration[4.2]
 
   DOWNTIME = false
 
+  # rubocop:disable Migration/AddColumnsToWideTables
+  # rubocop:disable Migration/AddLimitToStringColumns
   def change
-    add_column :projects, :runners_token_encrypted, :string # rubocop:disable Migration/AddLimitToStringColumns
+    add_column :projects, :runners_token_encrypted, :string
   end
+  # rubocop:enable Migration/AddColumnsToWideTables
+  # rubocop:enable Migration/AddLimitToStringColumns
 end

db/migrate/20181129104854_add_token_encrypted_to_ci_builds.rb

@@ -5,7 +5,11 @@ class AddTokenEncryptedToCiBuilds < ActiveRecord::Migration[5.0]
 
   DOWNTIME = false
 
+  # rubocop:disable Migration/AddColumnsToWideTables
+  # rubocop:disable Migration/AddLimitToStringColumns
   def change
-    add_column :ci_builds, :token_encrypted, :string # rubocop:disable Migration/AddLimitToStringColumns
+    add_column :ci_builds, :token_encrypted, :string
   end
+  # rubocop:enable Migration/AddColumnsToWideTables
+  # rubocop:enable Migration/AddLimitToStringColumns
 end

db/migrate/20181203002526_add_project_bfg_object_map_column.rb

@@ -3,7 +3,11 @@
 class AddProjectBfgObjectMapColumn < ActiveRecord::Migration[5.0]
   DOWNTIME = false
 
+  # rubocop:disable Migration/AddColumnsToWideTables
+  # rubocop:disable Migration/AddLimitToStringColumns
   def change
-    add_column :projects, :bfg_object_map, :string # rubocop:disable Migration/AddLimitToStringColumns
+    add_column :projects, :bfg_object_map, :string
   end
+  # rubocop:enable Migration/AddColumnsToWideTables
+  # rubocop:enable Migration/AddLimitToStringColumns
 end

db/migrate/20190312071108_add_detected_repository_languages_to_projects.rb

@@ -7,6 +7,6 @@ class AddDetectedRepositoryLanguagesToProjects < ActiveRecord::Migration[5.0]
   DOWNTIME = false
 
   def change
-    add_column :projects, :detected_repository_languages, :boolean
+    add_column :projects, :detected_repository_languages, :boolean # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20190419121952_add_bridged_pipeline_id_to_bridges.rb

@@ -10,6 +10,6 @@ class AddBridgedPipelineIdToBridges < ActiveRecord::Migration[5.0]
   DOWNTIME = false
 
   def change
-    add_column :ci_builds, :upstream_pipeline_id, :integer
+    add_column :ci_builds, :upstream_pipeline_id, :integer # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20190715215532_add_project_emails_disabled.rb

@@ -4,6 +4,6 @@ class AddProjectEmailsDisabled < ActiveRecord::Migration[5.2]
   DOWNTIME = false
 
   def change
-    add_column :projects, :emails_disabled, :boolean
+    add_column :projects, :emails_disabled, :boolean # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20190722104947_add_static_object_token_to_users.rb

@@ -9,7 +9,7 @@ class AddStaticObjectTokenToUsers < ActiveRecord::Migration[5.2]
   disable_ddl_transaction!
 
   def up
-    add_column :users, :static_object_token, :string, limit: 255
+    add_column :users, :static_object_token, :string, limit: 255 # rubocop:disable Migration/AddColumnsToWideTables
   end
 
   def down

db/migrate/20190820163320_add_first_last_name_to_user.rb

@@ -7,8 +7,10 @@ class AddFirstLastNameToUser < ActiveRecord::Migration[5.2]
   # Set this constant to true if this migration requires downtime.
   DOWNTIME = false
 
+  # rubocop:disable Migration/AddColumnsToWideTables
   def change
     add_column(:users, :first_name, :string, null: true, limit: 255)
     add_column(:users, :last_name, :string, null: true, limit: 255)
   end
+  # rubocop:enable Migration/AddColumnsToWideTables
 end

db/migrate/20190911115056_add_projects_max_pages_size.rb

@@ -4,6 +4,6 @@ class AddProjectsMaxPagesSize < ActiveRecord::Migration[5.2]
   DOWNTIME = false
 
   def change
-    add_column :projects, :max_pages_size, :integer
+    add_column :projects, :max_pages_size, :integer # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20190911115207_add_projects_max_artifacts_size.rb

@@ -4,6 +4,6 @@ class AddProjectsMaxArtifactsSize < ActiveRecord::Migration[5.2]
   DOWNTIME = false
 
   def change
-    add_column :projects, :max_artifacts_size, :integer
+    add_column :projects, :max_artifacts_size, :integer # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20190912223232_add_role_to_users.rb

@@ -7,6 +7,6 @@ class AddRoleToUsers < ActiveRecord::Migration[5.2]
   DOWNTIME = false
 
   def change
-    add_column :users, :role, :smallint
+    add_column :users, :role, :smallint # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20190924124627_add_pull_mirror_branch_prefix_to_projects.rb

@@ -4,6 +4,6 @@ class AddPullMirrorBranchPrefixToProjects < ActiveRecord::Migration[5.2]
   DOWNTIME = false
 
   def change
-    add_column :projects, :pull_mirror_branch_prefix, :string, limit: 50
+    add_column :projects, :pull_mirror_branch_prefix, :string, limit: 50 # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20191003161031_add_mark_for_deletion_to_projects.rb

@@ -4,8 +4,10 @@ class AddMarkForDeletionToProjects < ActiveRecord::Migration[5.2]
   # Set this constant to true if this migration requires downtime.
   DOWNTIME = false
 
+  # rubocop:disable Migration/AddColumnsToWideTables
   def change
     add_column :projects, :marked_for_deletion_at, :date
     add_column :projects, :marked_for_deletion_by_user_id, :integer
   end
+  # rubocop:enable Migration/AddColumnsToWideTables
 end

db/migrate/20191017094449_add_remove_source_branch_after_merge_to_projects.rb

@@ -8,7 +8,7 @@ class AddRemoveSourceBranchAfterMergeToProjects < ActiveRecord::Migration[5.1]
   DOWNTIME = false
 
   def up
-    add_column :projects, :remove_source_branch_after_merge, :boolean
+    add_column :projects, :remove_source_branch_after_merge, :boolean # rubocop:disable Migration/AddColumnsToWideTables
   end
 
   def down

db/migrate/20191115114032_add_processed_to_ci_builds.rb

@@ -4,6 +4,6 @@ class AddProcessedToCiBuilds < ActiveRecord::Migration[5.2]
   DOWNTIME = false
 
   def change
-    add_column :ci_builds, :processed, :boolean
+    add_column :ci_builds, :processed, :boolean # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20191129144630_add_resource_group_id_to_ci_builds.rb

@@ -3,6 +3,7 @@
 class AddResourceGroupIdToCiBuilds < ActiveRecord::Migration[5.2]
   DOWNTIME = false
 
+  # rubocop:disable Migration/AddColumnsToWideTables
   def up
     unless column_exists?(:ci_builds, :resource_group_id)
       add_column :ci_builds, :resource_group_id, :bigint
@@ -12,6 +13,7 @@ class AddResourceGroupIdToCiBuilds < ActiveRecord::Migration[5.2]
       add_column :ci_builds, :waiting_for_resource_at, :datetime_with_timezone
     end
   end
+  # rubocop:enable Migration/AddColumnsToWideTables
 
   def down
     if column_exists?(:ci_builds, :resource_group_id)

db/migrate/20191208110214_add_suggestion_commit_message_to_projects.rb

@@ -4,6 +4,6 @@ class AddSuggestionCommitMessageToProjects < ActiveRecord::Migration[5.2]
   DOWNTIME = false
 
   def change
-    add_column :projects, :suggestion_commit_message, :string, limit: 255
+    add_column :projects, :suggestion_commit_message, :string, limit: 255 # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20191223124940_add_scheduling_type_to_ci_builds.rb

@@ -6,6 +6,6 @@ class AddSchedulingTypeToCiBuilds < ActiveRecord::Migration[5.2]
   DOWNTIME = false
 
   def change
-    add_column :ci_builds, :scheduling_type, :integer, limit: 2
+    add_column :ci_builds, :scheduling_type, :integer, limit: 2 # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20200103195205_add_autoclose_referenced_issues_to_projects.rb

@@ -4,6 +4,6 @@ class AddAutocloseReferencedIssuesToProjects < ActiveRecord::Migration[5.2]
   DOWNTIME = false
 
   def change
-    add_column :projects, :autoclose_referenced_issues, :boolean
+    add_column :projects, :autoclose_referenced_issues, :boolean # rubocop:disable Migration/AddColumnsToWideTables
   end
 end

db/migrate/20200304085423_add_user_type.rb

@@ -7,7 +7,7 @@ class AddUserType < ActiveRecord::Migration[6.0]
 
   def up
     with_lock_retries do
-      add_column :users, :user_type, :integer, limit: 2
+      add_column :users, :user_type, :integer, limit: 2 # rubocop:disable Migration/AddColumnsToWideTables
     end
   end
 

db/post_migrate/20190424134256_drop_projects_ci_id.rb

@@ -23,7 +23,7 @@ class DropProjectsCiId < ActiveRecord::Migration[5.1]
 
   def down
     unless column_exists?(:projects, :ci_id)
-      add_column :projects, :ci_id, :integer
+      add_column :projects, :ci_id, :integer # rubocop:disable Migration/AddColumnsToWideTables
     end
 
     unless index_exists?(:projects, :ci_id)

db/post_migrate/20190511144331_remove_users_support_type.rb

@@ -17,7 +17,7 @@ class RemoveUsersSupportType < ActiveRecord::Migration[5.1]
   end
 
   def down
-    add_column :users, :support_bot, :boolean
+    add_column :users, :support_bot, :boolean # rubocop:disable Migration/AddColumnsToWideTables
 
     add_concurrent_index :users, :support_bot
     add_concurrent_index :users, :state,

db/post_migrate/20191015154408_drop_merge_requests_require_code_owner_approval_from_projects.rb

@@ -15,7 +15,7 @@ class DropMergeRequestsRequireCodeOwnerApprovalFromProjects < ActiveRecord::Migr
   end
 
   def down
-    add_column :projects, :merge_requests_require_code_owner_approval, :boolean
+    add_column :projects, :merge_requests_require_code_owner_approval, :boolean # rubocop:disable Migration/AddColumnsToWideTables
 
     add_concurrent_index(
       :projects,

rubocop/cop/migration/add_columns_to_wide_tables.rb

+# frozen_string_literal: true
+
+require_relative '../../migration_helpers'
+
+module RuboCop
+  module Cop
+    module Migration
+      # Cop that prevents adding columns to wide tables.
+      class AddColumnsToWideTables < RuboCop::Cop::Cop
+        include MigrationHelpers
+
+        MSG = '`%s` is a wide table with several columns, addig more should be avoided unless absolutely necessary.' \
+              ' Consider storing the column in a different table or creating a new one.'.freeze
+
+        BLACKLISTED_METHODS = %i[
+          add_column
+          add_column_with_default
+          add_reference
+          add_timestamps_with_timezone
+        ].freeze
+
+        def on_send(node)
+          return unless in_migration?(node)
+
+          method_name = node.children[1]
+          table_name = node.children[2]
+
+          return unless offense?(method_name, table_name)
+
+          add_offense(node, location: :selector, message: format(MSG, table_name.value))
+        end
+
+        private
+
+        def offense?(method_name, table_name)
+          wide_table?(table_name) &&
+            BLACKLISTED_METHODS.include?(method_name)
+        end
+
+        def wide_table?(table_name)
+          table_name && table_name.type == :sym &&
+            WIDE_TABLES.include?(table_name.value)
+        end
+      end
+    end
+  end
+end

rubocop/migration_helpers.rb

@@ -6,7 +6,7 @@ module RuboCop
       plan_limits
     ].freeze
 
-    # Blacklisted table due to:
+    # Blacklisted tables due to:
     #   - size in GB (>= 10 GB on GitLab.com as of 02/2020)
     #   - number of records
     BLACKLISTED_TABLES = %i[
@@ -44,6 +44,15 @@ module RuboCop
        web_hook_logs
     ].freeze
 
+    # Blacklisted tables due to:
+    #   - number of columns (> 50 on GitLab.com as of 03/2020)
+    #   - number of records
+    WIDE_TABLES = %i[
+      users
+      projects
+      ci_builds
+    ].freeze
+
     # Returns true if the given node originated from the db/migrate directory.
     def in_migration?(node)
       dirname(node).end_with?('db/migrate', 'db/geo/migrate') || in_post_deployment_migration?(node)

spec/rubocop/cop/migration/add_columns_to_wide_tables_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+require 'rubocop'
+
+require_relative '../../../../rubocop/cop/migration/add_columns_to_wide_tables'
+
+describe RuboCop::Cop::Migration::AddColumnsToWideTables do
+  include CopHelper
+
+  let(:cop) { described_class.new }
+
+  context 'outside of a migration' do
+    it 'does not register any offenses' do
+      expect_no_offenses(<<~RUBY)
+        def up
+          add_column(:users, :another_column, :string)
+        end
+      RUBY
+    end
+  end
+
+  context 'in a migration' do
+    before do
+      allow(cop).to receive(:in_migration?).and_return(true)
+    end
+
+    context 'with wide tables' do
+      it 'registers an offense when adding a column to a wide table' do
+        offense = '`projects` is a wide table with several columns, addig more should be avoided unless absolutely necessary. Consider storing the column in a different table or creating a new one.'
+
+        expect_offense(<<~RUBY)
+          def up
+            add_column(:projects, :another_column, :integer)
+            ^^^^^^^^^^ #{offense}
+          end
+        RUBY
+      end
+
+      it 'registers an offense when adding a column with default to a wide table' do
+        offense = '`users` is a wide table with several columns, addig more should be avoided unless absolutely necessary. Consider storing the column in a different table or creating a new one.'
+
+        expect_offense(<<~RUBY)
+          def up
+            add_column_with_default(:users, :another_column, :boolean, default: false)
+            ^^^^^^^^^^^^^^^^^^^^^^^ #{offense}
+          end
+        RUBY
+      end
+
+      it 'registers an offense when adding a reference' do
+        offense = '`ci_builds` is a wide table with several columns, addig more should be avoided unless absolutely necessary. Consider storing the column in a different table or creating a new one.'
+
+        expect_offense(<<~RUBY)
+          def up
+            add_reference(:ci_builds, :issue, :boolean, index: true)
+            ^^^^^^^^^^^^^ #{offense}
+          end
+        RUBY
+      end
+
+      it 'registers an offense when adding timestamps' do
+        offense = '`projects` is a wide table with several columns, addig more should be avoided unless absolutely necessary. Consider storing the column in a different table or creating a new one.'
+
+        expect_offense(<<~RUBY)
+          def up
+            add_timestamps_with_timezone(:projects, null: false)
+            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ #{offense}
+          end
+        RUBY
+      end
+
+      it 'register no offense when using other method' do
+        expect_no_offenses(<<~RUBY)
+          def up
+            add_concurrent_index(:projects, :new_index)
+          end
+        RUBY
+      end
+    end
+
+    context 'with a regular table' do
+      it 'registers no offense for notes' do
+        expect_no_offenses(<<~RUBY)
+          def up
+            add_column(:notes, :another_column, :boolean)
+          end
+        RUBY
+      end
+    end
+  end
+end