Sql optimise nested set final

Sql optimise nested set final
we have a different field name for "alias" in the assets table wich is name so
we need to add
```
$tablenamefield=($this->_tbl == '#__assets') ? 'name' : 'alias';
```
and change the select accordingly
```
->select('group_concat(p.'.$tablenamefield.' SEPARATOR "/")')
```