better count sql statement on SELECT DISTINCT involked

Issue Summary

If I involke pager_query() with 'SELECT DISTINCT u.uid FROM {users}', it will be replaced by 'SELECT count(*) FROM {users}' when counting users. Of course, it is just an example sql statement here. But the counting statement would be better be replaced to 'SELECT COUNT(DISTINCT(u.uid)) FROM {users}'.

Currently, the only way getting the wanted result is to passing extra $count parameters. But it would be better pager_query could do as following:
If 'SELECT DISTINCT' was found
$count_query = 'SELECT COUNT(DISTINCT(first_field))'...