On Thu, Oct 2, 2014 at 8:56 PM, Josh Berkus <josh@agliodbs.com> wrote:
> Yes, it's only intractable if you're wedded to the idea of a tiny,
> fixed-size sample. If we're allowed to sample, say, 1% of the table, we
> can get a MUCH more accurate n_distinct estimate using multiple
> algorithms, of which HLL is one. While n_distinct will still have some
> variance, it'll be over a much smaller range.
I've gone looking for papers on this topic but from what I read this
isn't so. To get any noticeable improvement you need to read 10-50% of
the table and that's effectively the same as reading the entire table
-- and it still had pretty poor results. All the research I could find
went into how to analyze the whole table while using a reasonable
amount of scratch space and how to do it incrementally.
--
greg
