Call to undefined function update_sql() on update

Running update.php results in the following error:

An AJAX HTTP error occurred. HTTP Result Code: 200 Debugging information follows. Path: http://example.com/update.php?id=20&op=do StatusText: OK ResponseText: Fatal error: Call to undefined function update_sql() in /.../sites/all/modules/tagadelic/tagadelic.install on line 16

It's caused by this intent to clear the cache:

update_sql("DELETE FROM {cache} WHERE cid LIKE 'tagadelic_cache_%'");