Add index on namespaces lower(name) for UsersController#exists

changelogs/unreleased/index-namespaces-lower-name.yml

+---
+title: Add index on namespaces lower(name) for UsersController#exists
+merge_request:
+author:
+type: performance

db/migrate/20171220191323_add_index_on_namespaces_lower_name.rb

+class AddIndexOnNamespacesLowerName < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+  DOWNTIME = false
+  INDEX_NAME = 'index_on_namespaces_lower_name'
+
+  disable_ddl_transaction!
+
+  def up
+    return unless Gitlab::Database.postgresql?
+
+    disable_statement_timeout
+    if Gitlab::Database.version.to_f >= 9.5
+      # Allow us to hot-patch the index manually ahead of the migration
+      execute "CREATE INDEX CONCURRENTLY IF NOT EXISTS #{INDEX_NAME} ON namespaces (lower(name));"
+    else
+      execute "CREATE INDEX CONCURRENTLY #{INDEX_NAME} ON namespaces (lower(name));"
+    end
+  end
+
+  def down
+    return unless Gitlab::Database.postgresql?
+
+    disable_statement_timeout
+    if Gitlab::Database.version.to_f >= 9.2
+      execute "DROP INDEX CONCURRENTLY IF EXISTS #{INDEX_NAME};"
+    else
+      execute "DROP INDEX IF EXISTS #{INDEX_NAME};"
+    end
+  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: 20171219121201) do
+ActiveRecord::Schema.define(version: 20171220191323) do
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"

lib/tasks/migrate/setup_postgresql.rake

@@ -6,6 +6,7 @@ require Rails.root.join('db/migrate/20161212142807_add_lower_path_index_to_route
 require Rails.root.join('db/migrate/20170317203554_index_routes_path_for_like')
 require Rails.root.join('db/migrate/20170724214302_add_lower_path_index_to_redirect_routes')
 require Rails.root.join('db/migrate/20170503185032_index_redirect_routes_path_for_like')
+require Rails.root.join('db/migrate/20171220191323_add_index_on_namespaces_lower_name.rb')
 desc 'GitLab | Sets up PostgreSQL'
 task setup_postgresql: :environment do
@@ -15,4 +16,5 @@ task setup_postgresql: :environment do
   IndexRoutesPathForLike.new.up
   AddLowerPathIndexToRedirectRoutes.new.up
   IndexRedirectRoutesPathForLike.new.up
+  AddIndexOnNamespacesLowerName.new.up
 end