Re: Pre-alloc ListCell's optimization

* Stephen Frost (sfrost(at)snowman(dot)net) wrote:
> * Tom Lane (tgl(at)sss(dot)pgh(dot)pa(dot)us) wrote:
> > ISTM the first thing we'd need to have before
> > we could think about this rationally is some measurements about the
> > frequencies of different List lengths in a typical workload.
>
> I agree, that'd be a good thing to have. I'll look into measuring that.

Ok, it took me, uh, a little while to get around to this, but:

http://tamriel.snowman.net/~sfrost/list_histgram.svg

Is what our list lengths look like for the regression tests. We could
do a pg_bench run, but it looks like Tom's right here- the vast majority
of our lists are small. Highlights:

63% are 1-element
25% are 2-element

Lists of 4 or fewer elements are 97%
Lists of 8 or fewer elements are 99%

So, when it comes to palloc() reduction, this patch would eliminate 99%
of palloc's due to lists. For the regression tests, we're talking about
reducing 893,206 palloc calls to only 1.

Thanks,

Stephen