Re: PoC: Partial sort

On Thu, Feb 13, 2014 at 1:54 AM, Marti Raudsepp <marti@juffo.org> wrote:

> I think the 1st patch now has a bug in initial_cost_mergejoin; you
> still pass the "presorted_keys" argument to cost_sort, making it
> calculate a partial sort cost, but generated plans never use partial
> sort. I think 0 should be passed instead. Patch attached, needs to be
> applied on top of partial-sort-basic-1 and then reverse-applied on
> partial-sort-merge-1.
>

It doesn't look so for me. Merge join doesn't find partial sort especially.
But if path with some presorted pathkeys will be accidentally selected then
partial sort will be used. See create_mergejoin_plan function. So, I think
this cost_sort call is relevant to create_mergejoin_plan. If we don't want
partial sort to be used in such rare cases then we should revert it from
both places. However, I doubt that it does any overhead, so we can leave it
as is.

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With best regards,
Alexander Korotkov.