On Wed, Jul 16, 2025 at 3:00 PM Tomas Vondra <to...@vondra.me> wrote: > Yes, sounds like a fair summary.
Cool. > Perhaps, although I don't quite see why the simpler patch couldn't > address some of those problems (within the limit of a single leaf page, > of course). I don't think there's anything that's prevent collecting the > "details" somewhere (e.g. in the IndexScanDesc), and querying it from > the callbacks. Or something like that. That is technically possible. But ISTM that that's just an inferior version of the "complex" patch, that duplicates lots of things across index AMs. > I understand you may see the "one leaf page" as a limitation of various > optimizations, and that's perfectly correct, ofc. I also saw it as a > crude limitation of how "bad" the things can go. I'm not opposed to some fairly crude mechanism that stops the prefetching from ever being too aggressive based on index characteristics. But the idea of exclusively relying on leaf page boundaries to do that for us doesn't even seem like a good stopgap solution. On average, the cost of accessing leaf pages is relatively insignificant. But occasionally, very occasionally, it's the dominant cost. I don't think that you can get away with making a static assumption about how much leaf page access costs matter -- it doesn't average out like that. I think that you need at least a simple dynamic approach, that mostly doesn't care too much about how many leaf pages we've read, but occasionally makes heap prefetching much less aggressive in response to the number of leaf pages the scan needs to read being much higher than is typical. I get the impression that you're still of the opinion that the "simple" approach might well have the best chance of success. If that's still how you view things, then I genuinely don't understand why you still see things that way. That perspective definitely made sense to me 6 months ago, but no longer. Do you imagine that (say) Thomas will be able to add pause-and-resume to the read stream interface some time soon, at which point the regressions we see with the "simple" patch (but not the "complex" patch) go away? -- Peter Geoghegan
