On 12/21/2014 02:50 PM, Andrew Dunstan wrote:
>
> On 12/21/2014 02:12 PM, Tom Lane wrote:
>> Andrew Dunstan <andrew@dunslane.net> writes:
>>> On 12/21/2014 01:23 PM, Alvaro Herrera wrote:
>>>> The point, I think, is that without atomic instructions you have to
>>>> hold
>>>> a lock while incrementing the counters.
>>> Hmm, do we do that now?
>> We already have a spinlock mutex around the counter adjustment code, so
>> I'm not sure why this discussion is being held.
>
> Because Peter suggested we might be able to use atomics. I'm a bit
> dubious that we can for min and max anyway.
>
>>
>>> I would like someone more versed in numerical analysis than me to
>>> tell me how safe using sum of squares actually is in our case.
>> That, on the other hand, might be a real issue. I'm afraid that
>> accumulating across a very long series of statements could lead
>> to severe roundoff error in the reported values, unless we use
>> a method chosen for numerical stability.
>>
>>
>
>
> Right.
>
> The next question along those lines is whether we need to keep a
> running mean or whether that can safely be calculated on the fly. The
> code at <http://www.johndcook.com/blog/standard_deviation/> does keep
> a running mean, and maybe that's required to prevent ill conditioned
> results, although I'm quite sure I see how it would. But this isn't my
> area of expertise.
>
I played it safe and kept a running mean, and since it's there and
useful in itself I exposed it via the function, so there are four new
columns, min_time, max_time, mean_time and stddev_time.
I'll add this to the upcoming commitfest.
cheers
andrew