My pager disappears when I use hook_views_pre_execute
if query exec is too long (for pagination views must return total count of rows at all pages and correspond it to rows per page ) pagination not returned by views. 

nid of nodes returns from one table in database and this queries is very simple &amp; fast

another fields (cck, ubercart, stock) returned from another db tables and it can be too long. for this views just returns first page to prevent timeout