Re: Add min and max execute statement time in pg_stat_statement

On Wed, Oct 23, 2013 at 3:26 PM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:

> On Wed, Oct 23, 2013 at 2:57 PM, Gavin Flower
> <GavinFlower(at)archidevsys(dot)co(dot)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"?
>

If you are doing a log scale in the bucket widths, 32 buckets covers a huge
range, so I think you could get away without having the ability to rescale
at all. For example just have the bottom bucket mean <=1ms, and the top
bucket mean > 12.42 days (rather than between 12.42 and 24.85 days). But
it should be possible to rescale if you really want to. If a bucket >32 is
needed, just add bucket[2] into bucket[1], slide buckets 3..32 down one
each, and initialize a new bucket 32, and bump the factor that says how
many times this shift has been done before.

Cheers,

Jeff