Robert Haas <robertmh...@gmail.com> wrote: > The trouble is, AFAICS, that you can't bound M very well without > scanning the whole table. I mean, it's bounded by theoretical > limit, but that's it. What would the theoretical limit be? (black size - page header size - minimum size of one tuple) / item pointer size? So, on an 8KB page, somewhere in the neighborhood of 1350? Hmm. If that's right, that would mean a 1% random sample would need 13.5 probes per page, meaning there wouldn't tend to be a lot of pages missed. Still, the technique for getting a random sample seems sound, unless someone suggests something better. Maybe we just want to go straight to a seqscan to get to the pages we want to probe rather than reading just the ones on the "probe list" in physical order? -Kevin
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