ServiceNow getRowCount versus GlideAggregate (best practice)

 see developer site

and docs 

and my article on passing a gr to a ga

 

Use GlideAggregate for Simple Record Counting

If you need to count rows, you have two options: the getRowCount() method from GlideRecord, or GlideAggregate. Using GlideRecord to count rows can cause scalability issues as tables grow over time, because it retrieves every record with the query and then counts them. GlideAggregate gets its result from built-in database functionality, which is much quicker and doesn't suffer from the scalability issues that GlideRecord does.

Bad example:

/*
 * countInactiveIncidents - return the number of closed incidents
 *
 * @param - none
 * @returns integer - number of records found
 *
 */
function countInactiveIncidents() {

     var inc = new GlideRecord('incident');

     inc.addInactiveQuery();
     inc.query();

     var count = inc.getRowCount();
     gs.print(count + ' inactive incidents found');

     return count;
}

Good example:

/*
 * countInactiveIncidents - return the number of closed incidents
 *
 * @param - none
 * @returns integer - number of records found
 *
 */
function countInactiveIncidents() {

     var inc = new GlideAggregate('incident');

     inc.addAggregate('COUNT')
     inc.addInactiveQuery();
     inc.query();

     var count = 0;
     if (inc.next())
          count = inc.getAggregate('COUNT');

     gs.print(count + ' inactive incidents found');

     return count;
}