Horton

Re: Memory usage during sorting

Robert Haas <robertmhaas@gmail.com>

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From: Robert Haas <robertmhaas@gmail.com>
To: Greg Stark <stark@mit.edu>
Cc: Tom Lane <tgl@sss.pgh.pa.us>, Jeff Janes <jeff.janes@gmail.com>, Hitoshi Harada <umi.tanuki@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>
Date: 2012-03-20T16:17:31Z
Lists: pgsql-hackers

Commits

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  1. Improve performance of our private version of qsort. Per recent testing,

  2. Further performance improvements in sorting: reduce number of comparisons

On Tue, Mar 20, 2012 at 7:44 AM, Greg Stark <stark@mit.edu> wrote:
> On Tue, Mar 20, 2012 at 1:57 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>> That was a long time ago, of course, but I have some vague recollection
>> that keeping next-run tuples in the current heap achieves a net savings
>> in the total number of comparisons needed to heapify both runs.
>
> Offhand I wonder if this is all because we don't have the O(n) heapify
> implemented.

I'm pretty sure that's not the problem.  Even though our heapify is
not as efficient as it could be, it's plenty fast enough.  I thought
about writing a patch to implement the better algorithm, but it seems
like a distraction at this point because the heapify step is such a
small contributor to overall sort time.  What's taking all the time is
the repeated siftup operations as we pop things out of the heap.

-- 
Robert Haas
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