Re: Support loser tree for k-way merge

Hi Heikki,

> What is the worst case scenario for the loser tree, where the heap is 
> faster? How big is the difference?

In tuplesort_heap_replace_top(), it has 2 comparisons each level, but it can early return
if the parent less than both child.

In tuplesort_loser_tree_adjust(), it has 1 comparison each level, but it can't early return.

So on specific data, the heap may be better than the loser tree. But I think the possibility
is very small.

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Regards,
ChangAo Chen