Add index for project_features.container_registry_access_level

The index can replace the existing
index_project_feature_usages_on_project_id. This commit does
not remove the existing index though, since the existing (and new)
index is a unique index and we don't want there to be no index
for any amount of time.

Changelog: performance

db/post_migrate/20210606143426_add_index_for_container_registry_access_level.rb

+# frozen_string_literal: true
+
+class AddIndexForContainerRegistryAccessLevel < ActiveRecord::Migration[6.1]
+  include Gitlab::Database::SchemaHelpers
+  include Gitlab::Database::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  INDEX = 'index_project_features_on_project_id_include_container_registry'
+
+  def up
+    if index_exists_by_name?('project_features', INDEX)
+      Gitlab::AppLogger.warn "Index not created because it already exists (this may be due to an aborted migration or similar): table_name: project_features, index_name: #{INDEX}"
+      return
+    end
+
+    begin
+      disable_statement_timeout do
+        execute "CREATE UNIQUE INDEX CONCURRENTLY #{INDEX} ON project_features " \
+          'USING btree (project_id) INCLUDE (container_registry_access_level)'
+      end
+    rescue ActiveRecord::StatementInvalid => ex
+      raise "The index #{INDEX} couldn't be added: #{ex.message}"
+    end
+
+    create_comment(
+      'INDEX',
+      INDEX,
+      'Included column (container_registry_access_level) improves performance of the ContainerRepository.for_group_and_its_subgroups scope query'
+    )
+  end
+
+  def down
+    remove_concurrent_index_by_name('project_features', INDEX)
+  end
+end

db/schema_migrations/20210606143426

+1f99d446428ddac2a0fa7d64bdce9fc300bf02e88c35cdb3d726c501641e721d
\ No newline at end of file

db/structure.sql

@@ -24144,6 +24144,10 @@ CREATE UNIQUE INDEX index_project_features_on_project_id ON project_features USI
 
 CREATE INDEX index_project_features_on_project_id_bal_20 ON project_features USING btree (project_id) WHERE (builds_access_level = 20);
 
+CREATE UNIQUE INDEX index_project_features_on_project_id_include_container_registry ON project_features USING btree (project_id) INCLUDE (container_registry_access_level);
+
+COMMENT ON INDEX index_project_features_on_project_id_include_container_registry IS 'Included column (container_registry_access_level) improves performance of the ContainerRepository.for_group_and_its_subgroups scope query';
+
 CREATE INDEX index_project_features_on_project_id_ral_20 ON project_features USING btree (project_id) WHERE (repository_access_level = 20);
 
 CREATE INDEX index_project_group_links_on_group_id ON project_group_links USING btree (group_id);