Saving loading time at counting rows

I had this question, which I didn't really know where to ask, so I thought to begin at this mailing list. Very basicly said, I count the rows of a table which had approx 50000 or more rows. The problem is, there is a notable difference in loading time with other pages. Is there a technique to estimate the number of rows instead of exactly couting them? So that it saves loading time. For example, when you search with google, you get an estimate of the number of results, how do they do this? Maybe this is not a question for the PHP Mailing list, but if not where is a better place to ask this? regards, Joep