Calculate extent for feature layer

Question

I have a polygon layer consisting of 58-60 thousand records.I am displaying small number of records(2-3 thousand) using "setDefinitionExpression" expression .When I add layer at the same time i have added zoom to layer functionality but the problem is since the data is spread across in large area of map when i zoom to layer it set the extent of the entire layer rather then only the records in which it is published and since polygon are very small the feature are not visible.Is i possible to get the extent of feature which are only returned after setDefinitionExpression .I have used Example.but the looping through 2-5 thousand records is big overhead, is there any other way to do this...???