search hidden meta_keys – with _underscore ?

Hi,

Sorry for the delay.
Please add this code in your function.php to list all meta keys.

/**
 * Select all meta keys including those start with _(underscore)
 */
function wpes_meta_keys_query($query) {
    global $wpdb;
    $query = "select DISTINCT meta_key from $wpdb->postmeta ORDER BY meta_key ASC";
    return $query;
}
add_filter('wpes_meta_keys_query', 'wpes_meta_keys_query');