wp_upgrade()

function wp_upgrade() {
	global $wp_current_db_version, $wp_db_version, $wpdb;

	$wp_current_db_version = __get_option('db_version');

	// We are up-to-date. Nothing to do.
	if ( $wp_db_version == $wp_current_db_version )
		return;

	if ( ! is_blog_installed() )
		return;

	wp_check_mysql_version();
	wp_cache_flush();
	pre_schema_upgrade();
	make_db_current_silent();
	upgrade_all();
	if ( is_multisite() && is_main_site() )
		upgrade_network();
	wp_cache_flush();

	if ( is_multisite() ) {
		$site_id = get_current_blog_id();

		if ( $wpdb->get_row( $wpdb->prepare( "SELECT blog_id FROM {$wpdb->blog_versions} WHERE blog_id = %d", $site_id ) ) ) {
			$wpdb->query( $wpdb->prepare( "UPDATE {$wpdb->blog_versions} SET db_version = %d WHERE blog_id = %d", $wp_db_version, $site_id ) );
		} else {
			$wpdb->query( $wpdb->prepare( "INSERT INTO {$wpdb->blog_versions} ( `blog_id` , `db_version` , `last_updated` ) VALUES ( %d, %d, NOW() );", $site_id, $wp_db_version ) );
		}
	}

	/**
	 * Fires after a site is fully upgraded.
	 *
	 * @since WP-3.9.0
	 *
	 * @param int $wp_db_version         The new $wp_db_version.
	 * @param int $wp_current_db_version The old (current) $wp_db_version.
	 */
	do_action( 'wp_upgrade', $wp_db_version, $wp_current_db_version );
}