pre_schema_upgrade example

// 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() ) {
            update_site_meta( get_current_blog_id(), 'db_version', $wp_db_version );
            update_site_meta( get_current_blog_id(), 'db_last_updated', microtime() );
        }