how to statistics categories amount

[lutan]

if I search a keyword likes 'computer' in a shopping website.
the result may contains.
total:   
          (1000) products .
categories:
pc        (500) products .
notebook  (300) products .
server    (200) products .
 
so how do get each categories 'amount.
I try to search many times on onec user search,
but it is so slow.
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