Re: [HACKERS] [PATCH] Generic type subscripting

> On Wed, Dec 02, 2020 at 12:58:51PM -0500, Tom Lane wrote:
> So ... one of the things that's been worrying me about this patch
> from day one is whether it would create a noticeable performance
> penalty for existing use-cases. I did a small amount of experimentation
> about that with the v35 patchset, and it didn't take long at all to
> find that this:
>
> --- cut ---
> create or replace function arraytest(n int) returns void as
> $$
> declare
> a int[];
> begin
> a := array[1, 1];
> for i in 3..n loop
> a[i] := a[i-1] - a[i-2];
> end loop;
> end;
> $$
> language plpgsql stable;
>
> \timing on
>
> select arraytest(10000000);
> --- cut ---
>
> is about 15% slower with the patch than with HEAD. I'm not sure
> what an acceptable penalty might be, but 15% is certainly not it.
>
> I'm also not quite sure where the cost is going. It looks like
> 0001+0002 aren't doing much to the executor except introducing
> one level of subroutine call, which doesn't seem like it'd account
> for that.

I've tried to reproduce that, but get ~2-4% slowdown (with a pinned
backend, no turbo etc). Are there any special steps I've probably
missed? At the same time, I remember had conducted this sort of tests
before when you and others raised the performance degradation question
and the main part of the patch was already more or less stable. From
what I remember the numbers back then were also rather small.