On 24/10/13 12:00, Gavin Flower wrote:
> On 24/10/13 11:26, Peter Geoghegan wrote:
>> On Wed, Oct 23, 2013 at 2:57 PM, Gavin Flower
>> <GavinFlower@archidevsys.co.nz> wrote:
>>> Looks definitely bimodal in the log version, very clear!
>>>
>>> Yes, I feel that having a 32 log binary binned histogram (as Alvaro
>>> Herrera
>>> suggested) would be very useful.
>> I'm having a hard time imagining how you'd actually implement that.
>> For example, this:
>>
>> https://wiki.postgresql.org/wiki/Aggregate_Histogram
>>
>> requires that a "limit" be specified ahead of time. Is there a
>> principled way to increase or decrease this kind of limit over time,
>> and have the new buckets contents "spill into each other"?
>>
> To smplify things, I'm using 5 buckets, but 32 would be better.
> Assume first bucket width is 1ms.
>
> bucket range
> 0 x =< 1ms
> 1 1ms < x =< 2ms
> 2 2ms < x =< 4ms
> 3 4ms < x =< 8ms
> 5 8ms < x
>
>
> If the size of the first bucket changed, then implicitly the histogram
> would be restarted. As there is no meaningful way of using any data
> from the existing histogram - even if the size of the first bucket was
> a power of 2 greater than the old one (here things are fine, until you
> try and apportion the data in the last bucket!).
>
>
> Cheers,
> Gavin
>
>
Arghhhhh!
Just realized, that even if the size of the first bucket was a power of
2 greater than the old one, then you can't meaningfully use any of the
old data in any of the old buckets (this is 'obvious; but somewhat messy
to explain!)
Cheers,
Gavin
