On 12/28/14, 2:45 PM, Peter Geoghegan wrote:
On Sun, Dec 28, 2014 at 12:37 PM, Jeff Davis <pg...@j-davis.com> wrote:
Do others have similar numbers? I'm quite surprised at how little
work_mem seems to matter for these plans (HashJoin might be a different
story though). I feel like I made a mistake -- can someone please do a
sanity check on my numbers?

I have seen external sorts that were quicker than internal sorts
before. With my abbreviated key patch, under certain circumstances
external sorts are faster, while presumably the same thing is true of
int4 attribute sorts today. Actually, I saw a 10MB work_mem setting
that was marginally faster than a multi-gigabyte one that fit the
entire sort in memory. It probably has something to do with caching
effects dominating over the expense of more comparisons, since higher
work_mem settings that still resulted in an external sort were slower
than the 10MB setting.

I was surprised by this too, but it has been independently reported by
Jeff Janes.

I haven't tested for external faster than internal in a while, but I've 
certainly seen this effect before. Generally, once you get beyond a certain 
size (maybe 100MB?) you run the risk of a tapesort being faster than an 
internal sort.
Jim Nasby, Data Architect, Blue Treble Consulting
Data in Trouble? Get it in Treble! http://BlueTreble.com

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