Re: POC: converting Lists into arrays

Hi,

On 2019-02-25 11:59:06 -0800, Peter Geoghegan wrote:
> On Mon, Feb 25, 2019 at 10:59 AM Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> > Because it involves touching ten times more code (and that's a very
> > conservative estimate). Excluding changes in pg_list.h + list.c,
> > what I posted touches approximately 600 lines of code (520 insertions,
> > 644 deletions to be exact). For comparison's sake, there are about
> > 1800 uses of foreach in the tree, each of which would require at least
> > 3 changes to replace (the foreach itself, the ListCell variable
> > declaration, and at least one lfirst() reference in the loop body).
>
> If we knew that the list code was the bottleneck in a handful of
> cases, then I'd come down on Robert's side here. It would then be
> possible to update the relevant bottlenecked code in an isolated
> fashion, while getting the lion's share of the benefit. However, I
> don't think that that's actually possible. The costs of using Lists
> everywhere are real and measurable, but it's also highly distributed.
> At least, that's my recollection from previous discussion from several
> years back. I remember talking about this with Andres in early 2016.

It's distributed, but not *that* distributed. The largest source of
"cost" at execution time used to be all-over expression evaluation, but
that's gone now. That was a lot of places, but it's not outside of reach
of a targeted change. Now it's targetlist handling, which'd have to
change together with plan time, where it's a large issue.

> > So we've already blown past 5000 lines worth of changes if we want to
> > do it another way ... and that's just *one* component of the List API.
>
> If you want to stop third party code from compiling, you can find a
> way to do that without really changing your approach. Nothing stops
> you from changing some symbol names minimally, and then making
> corresponding mechanical changes to all of the client code within the
> tree. Third party code authors would then follow this example, with
> the expectation that it's probably going to be a totally mechanical
> process.
>
> I'm not necessarily advocating that approach. I'm simply pointing out
> that a compromise is quite possible.

That breaks extension code using lists unnecessarily though. And given
that there's semantic change, I don't htink it's an entirely mechanical
process.

Greetings,

Andres Freund