Re: Add min and max execute statement time in pg_stat_statement

On 01/22/2014 08:28 PM, KONDO Mitsumasa wrote:
> (2014/01/22 22:26), Robert Haas wrote:
>> On Wed, Jan 22, 2014 at 3:32 AM, KONDO Mitsumasa
>> <kondo(dot)mitsumasa(at)lab(dot)ntt(dot)co(dot)jp> wrote:
>>>> OK, Kondo, please demonstrate benchmarks that show we have <1% impact
>>>> from this change. Otherwise we may need a config parameter to allow
>>>> the calculation.
>>>
>>> OK, testing DBT-2 now. However, error range of benchmark might be 1%
>>> higher.
>>> So I show you detail HTML results.
>>
>> To see any impact from spinlock contention, I think you're pretty much
>> going to need a machine with >32 cores, I think, and lots of
>> concurrency. pgbench -S is probably a better test than DBT-2, because
>> it leaves out all the writing, so percentage-wise more time will be
>> spent doing things like updating the pgss hash table.
> Oh, thanks to inform me. I think essential problem of my patch has
> bottle neck in sqrt() function and other division caluculation. I will
> replcace sqrt() function in math.h to more faster algorithm. And
> moving unneccessary part of caluculation in LWlocks or other locks. It
> might take time to improvement, so please wait for a while.
>

Umm, I have not read the patch, but are you not using Welford's method?
Its per-statement overhead should be absolutely tiny (and should not
compute a square root at all per statement - the square root should
only be computed when the standard deviation is actually wanted, e.g.
when a user examines pg_stat_statements) See for example
<http://www.johndcook.com/standard_deviation.html>

cheers

andrew