On 27/01/14 18:00, Simon Riggs wrote:
On 27 January 2014 17:44, Pavel Stehule <pavel.steh...@gmail.com> wrote:
This topic is interesting - we found very bad performance with hashing large
tables with high work_mem. MergeJoin with quicksort was significantly
faster.
I've seen this also.
I didn't deeper research - there is a possibility of virtualization
overhead.
I took measurements and the effect was repeatable and happened for all
sizes of work_mem, but nothing more to add.
FWIW my current list-based internal merge seems to perform worse at
larger work-mem, compared to quicksort. I've been starting to wonder
if the rate of new ram-chip page opens is an issue (along with the
more-usually considered cache effects). Any random-access workload
would be affected by this, if it really exists.
--
Cheers,
Jeremy
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