Very simple script for debugging queries

debugging-main-query.php

<?php

/**
 * Will print to the page the main query used for that page. It will do this only for admins.
 * This can be put in your theme's functions.php, but should be removed when its no longer needed.
 */

add_action( 'posts_request', 'debug_post_request', 10, 2 );
function debug_post_request( $r, $query ){
      if( current_user_can( 'manage_options' ) && $query->is_main_query() ){       
            echo '<pre>';
            print_r( $query );
            echo '</pre>';
       }
       return $r;
}
?>