Kenneth Marshall <ktm(at)rice(dot)edu> writes:
> On Thu, Dec 23, 2010 at 10:42:26PM +0800, Li Jie wrote:
>> But in the last query that sorts on "id", since the query selects all the columns for output, the actual sorted size is the same, and the only difference is the comparison cost. The query sorting on two columns needs to do twice the comparison. Am I right?
> I think you are right. Sorry for the confusion.
I doubt the cost of comparing two integers is the issue here; rather
it's more likely one of how many merge passes were needed. You could
find out instead of just speculating by turning on trace_sort and
comparing the log outputs.
regards, tom lane