how to make Hits.getTotal() return the exact number of hits

[Jason]

hi guys,by default,Hits.getTotal() returns only an estimate of the total number 
of hits,I want the exact total number of hits,so I I use the function 
setTotalIsExact(true),after that,totalIsExact() returns true,but getTotal() 
still returns an estimate of number,what should I do to get the exact number? 
thanks in advance.