paging through results with fetchmany()

I return a set of results, which I break into multiple pages.  This is how I have implemented, but I feel that it is too expensive in terms of execution.  Is there anyway to tell PgSQL to start fetching from a particular number? 
 
c.execute(Query)
c.fetchmany( (pagenum-1)*NumPerPage) # skip previous results
c.fetchmany(NumPerPage) # fetch for page
 
Uzo


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