Re: btree: implement dynamic prefix truncation (was: Improving btree performance through specializing by key shape, take 2)

st 1. 11. 2023 v 11:32 odesílatel Matthias van de Meent <
boekewurm+postgres(at)gmail(dot)com> napsal:

> On Wed, 1 Nov 2023 at 07:47, Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
> wrote:
> >
> > Hi
> >
> > út 31. 10. 2023 v 22:12 odesílatel Matthias van de Meent <
> boekewurm+postgres(at)gmail(dot)com> napsal:
> >> This patch was originally suggested at [0], but it was mentioned that
> >> they could be pulled out into it's own thread. Earlier, the
> >> performance gains were not clearly there for just this patch, but
> >> after further benchmarking this patch stands on its own for
> >> performance: it sees no obvious degradation of performance, while
> >> gaining 0-5% across various normal indexes on the cc-complete sample
> >> dataset, with the current worst-case index shape getting a 60%+
> >> improved performance on INSERTs in the tests at [0].
> >
> >
> > +1
>
> Thanks for showing interest.
>
> > This can be nice functionality. I had a customer with a very slow index
> scan - the main problem was a long common prefix like prg010203xxxx.
>
> I'll have to note that this patch doesn't cover cases where e.g. text
> attributes have large shared prefixes, but are still unique: the
> dynamic prefix compression in this patch is only implemented at the
> tuple attribute level; it doesn't implement type aware dynamic prefix
> compression inside the attributes. So, a unique index on a column of
> int32 formatted like '%0100i' would not materially benefit from this
> patch.
>
> While would certainly be possible to add some type-level prefix
> truncation in the framework of this patch, adding that would require
> significant code churn in btree compare operators, because we'd need
> an additional return argument to contain a numerical "shared prefix",
> and that is not something I was planning to implement in this patch.
>

Thanks for the explanation.

Pavel

> Kind regards,
>
> Matthias van de Meent
> Neon (https://neon.tech)
>