Re: index prefetching

On Thu, Jul 24, 2025 at 7:52 PM Tomas Vondra <tomas(at)vondra(dot)me> wrote:
> Yeah, I forgot about that. Should be fixed in the v2. Admittedly I don't
> know that much about nbtree internals, so this is mostly copy pasting
> from verify_nbtree.

As long as the scan only moves to the right (never the left), and as
long as you don't forget about P_IGNORE() pages, everything should be
fairly straightforward. You don't really need to understand things
like page deletion, and you'll never need to hold more than a single
buffer lock at a time, provided you stick to the happy path.

I've taken a quick look at v2, and it looks fine to me. It's
acceptable for the purpose that you have in mind, at least.

> Yeah, probably. And we'll probably test on such uniform data sets, or at
> least we we'll start with those. But at some point I'd like to test with
> some of these "weird" indexes too, if only to test how well the prefetch
> heuristics adjusts the distance.

That makes perfect sense. I was just providing context.

> I have a very good reason why I didn't do it that way. I was lazy. But
> v2 should be doing that, I think.

I respect that. That's why I framed my feedback as "it'll be less
effort to just do it than to explain why you haven't done so". :-)

> Yeah, this interface seems useful. I suppose it'll be handy when looking
> at an index scan, to get stats from the currently loaded batches. In
> principle you get that from v3 by filtering, but it might be slow on
> large indexes. I'll try doing that in v3.

Cool.

--
Peter Geoghegan